Heterogeneity of synaptic vesicles in the squid giant fibre system

Four synaptic contacts in the squid giant fibre system (from the afferent boutons on the first order giant cell in the brain through the giant synapses in the palliovisceral lobe and the stellate ganglion), as well as neuromuscular junctions in the fin, contain populations of agranular (electron transparent) synaptic vesicles. A classification of these vesicle populations is possible on the basis of the average vesicle diameter and the reactivity with zinc iodide-osmium (ZIO). We distinguish 4 synapses with significantly different average vesicle sizes, 3 synapses with ZIO-positive vesicles and two synapses with ZIO-negative vesicles. In two consecutive links of the giant fibre system there are always structurally different synaptic contacts.     

Synaptic organization in the pacemaker nucleus of a medium-frequency weakly electric fish,Eigenmannia sp

The medullary command nucleus (MCN) of the medium-frequency weakly electric fish,Eigenmannia sp., contains two types of neurones, namely large and small cells, which are embedded in a neuropile of large and small myelinated fibers. Using serial semi-thin and ultra-thin sectioning, combined with HRP labelling established that both cell types possess rich dendritic arborization and large myelinated axons. Only the axons of the large cells leave the nucleus and these contribute the unique output of the MCN. Axon branching has been observed only in the axons of small cells and their collaterals show an exclusively intranuclear course. Two types of synaptic terminals have been found on large as well as on small cells: (1) large club endings forming both gap (electronic) junctions and polarized chemical synapses, which often appear at the same junction constituting morphologically mixed synapses; and (2) small bouton-like terminals forming exclusively chemical synaptic contacts. No differences between the two neuron types could be detected with respect to the arrangement of the synaptic contacts: club endings and small bouton-like terminals synapse on dendritic processes as well as on perikarya, while the unmyelinated initial segments were always found to be free of synaptic contacts. Large and small cells were found to be simultaneously connected by the same club ending or small bouton-like terminal: in the case of club endings by means of gap junctions and chemical synapses, whereas in the case of boutons by chemical synapses only. Club endings sometimes form gap junctions with each other. The possible role of these unusual synaptic connections in local synchronization is suggested. Club endings originate from the large axons of small cells, while small bouton-like terminals originate from the fine myelinated fibers of extranuclear origin. InEigenmannia, small cells, being connected to large cells as well as to each other by axo-somatic and axodendritic synapses, can be considered as the pacemaker cells of the MCN whereas large cells are relay cells. Small bouton-like terminals may convey exogeneous impulses towards the MCN exerting modulatory effects at both pacemaker and relay cell levels. The greater variety of ultrastructural correlates established in the MCN ofEigenmannia, in comparison withSternarchus⁵ (see also ref. 16), suggests increased modulation possibilities in the former fish's EOD behaviour.     

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish,Sternarchus albifrons

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 μm in diameter) and large neurons (approx. 30 μm in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called “relay cells” of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to “pacemaker cells”. The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the electromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and Sv-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.     

Three-dimensional organization of cell adhesion junctions at synapses and dendritic spines in area CA1 of the rat hippocampus

Recent work has emphasized the role of adhesion molecules in synaptic plasticity, including long-term potentiation in the hippocampus. Such adhesion molecules are concentrated in junctions that are characterized by dense thickenings on both sides of the junction and are called puncta adhaerentia (PA). Reconstruction from serial electron microscopy was used to determine the location and size of PA in the stratum radiatum of hippocampal area CA1, where many of the previous functional studies have been performed. PAs were found at the edges of synapses on 33% of dendritic spines. The areas occupied by PA were variable across different types of synapses, occupying 0.010 ± 0.005 μm² at macular synapses and 0.034 ± 0.031 μm² at perforated synapses. Another zone, called a vesicle-free transition zone (VFTZ), was identified. Like the PA, this zone also had no presynaptic vesicles and was located at the edges of synapses; however, unlike the PA, the presynaptic thickening was less than the postsynaptic thickening. Together, 45% of spine synapses had PA and/or VFTZ occupying 23 ± 11% of the total junctional area between axons and spines. PA also occurred at nonsynaptic sites involving neuronal as well as glial elements. Most (64%) of these PAs occurred between nonsynaptic portions of dendritic spines and neighboring astrocytic processes. Smooth endoplasmic reticulum was often apposed to one or both sides of the synaptic and the nonsynaptic PA. These findings provide further data as a structural basis for understanding the roles of cell adhesion junctions in hippocampal synaptic function and plasticity. J. Comp. Neurol. 393:58–68, 1998. © 1998 Wiley-Liss, Inc.     

Ultrastructure and synaptology of the nucleus ambiguus in the rat: The compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 ± 0.4 S. D. μm in diameter). Motoneuronal somatic profiles have 0–5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 × 0.7 μm in the transverse plane and are up to 3.0 μm long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (>90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injections of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis. © 1995 Wiley-Liss, Inc.     

An electron microscopic study of primary afferent terminals from slowly adapting type I receptors in the cat

Primary afferent fibers transmitting impulses from slowly adapting (SA) Type I receptors in the glabrous skin of the hind paw of the cat were injected intraaxonally in the spinal cord with horseradish peroxidase (HRP). At the light microscopic level, terminal arborizations were observed in the medial dorsal horn extending up to 6 mm rostrocaudally in and near the seventh lumbar segment. Boutonlike swellings labelled with HRP were distributed in clusters in Rexed's laminae III-VI. There was a tendency for the most dorsal clusters from an individual fiber to be located rostrally and for the most ventral clusters to be located caudally. At the electron microscopic level, a combination of morphometric analysis and serial reconstruction revealed the following: (1) All the boutons labelled with HRP contained predominantly clear, round synaptic vesicles, 40–50 nm in diameter. (2) Labelled boutons (n = 75) had cross-sectional longest dimensions of 1.72 ± 0.53 μm (Mean ± S.D.), perimeters of 4.95 ± 1.52 μn, and areas of 1.18 ± 0.59 μm². Their shapes in section varied from rounded to elongated forms. (3) The sizes of labelled boutons decreased significantly and linearly with depth from lamina IV to VI. The shapes of the bouton cross sections also became rounder with depth in the dorsal horn. (4) About 72% of synaptic contacts associated with HRP-filled boutons were with dendritic spines and shafts; most of these synapses were of the asymmetric type. (5) The remainder (28%) of the appositions were synapselike contacts between labelled boutons and unlabelled structures containing flattened or pleomorphic vesicles, and occasional dense-cored vesicles. (6) We observed no unequivocal axosomatic contacts made by labelled boutons. (7) The lengths of synaptic appositions with dendritic spines (0.46 ± 0.20 μm) or with dendritic shafts (0.51 ± 0.18 μm) were significantly greater than the synapselike contacts with vesicle-containing unlabelled structures (0.29 ± 0.09 μm). (8) Complex neuropilar organization was occasionally seen with labelled boutons as central elements, although simpler organizations were much more common. In summary, HRP-labelled fibers ended predominantly in boutons containing clear, round vesicles forming axospinous and axodendritic synapses. This is consistent with physiological observations of postsynaptic excitation of dorsal horn cells by the primary afferent fibers from SA Type I receptors in the glabrous skin.     

Properties of unitary IPSPs evoked by anatomically identified basket cells in the rat hippocampus

Hippocampal pyramidal cells receive GABA-mediated synaptic input from several distinct interneurons. In order to define the effect of perisomatic synapses, intracellular recordings were made with biocytin-containing microelectrodes from synaptically connected inhibitory and pyramidal cell pairs in subfields CA1 and CA3 of the rat hippocampus. Subsequent physiological analysis was restricted to the category of cells, here referred to as basket cells (n= 14), which had an efferent synaptic target profile (n= 282 synaptic contacts) of predominantly somatic (48.2%) and proximal dendritic synapses (45.0%). Electron microscopic analysis revealed that in two instances identified postsynaptic pyramidal cells received a total of 10 and 12 labelled basket cell synapses respectively. At an average membrane potential of -57.8 ± 4.6 mV, unitary inhibitory postsynaptic potentials (IPSPs; n= 24) had a mean amplitude of 450 ± 238 μV, a 10–90% rise time of 4.6 ± 3.2 ms and, measured at half-amplitude, a mean duration of 31.6 ± 18.2 ms. In most instances (n= 19) the IPSP decay could be fitted with a single exponential with a mean time constant of 32.4 ± 18.0 ms. Unitary basket cell-evoked IPSPs fluctuated widely in amplitude, ranging from the level of detectability to <2 mV. The response reversal of IPSPs (n= 5) was extrapolated to be at -74.9 ± 6.0 mV. Averages of unitary IPSPs had a mean calculated conductance of 0.95 ± 0.29 nS, ranging from 0.52 to 1.16 nS. Unitary basket cell IPSPs (n= 3) increased in amplitude by 26.3 ± 19.9% following bath application of the GABA_B receptor antagonist CGP 35845A (1–4 μM), whereas subsequent addition of the GABA^A receptor antagonist bicuculline (10–13 μM) reduced the IPSP amplitude to 13.5 ± 3.1% of the control response. Rapid presynaptic trains of basket cell action potentials resulted in the summation of up to four postsynaptic responses (n= 5). However, any increase in the rate of tonic firing (2- to 10-fold) led to a <50% reduction of the postsynaptic response amplitude. At depolarized membrane potentials, averaged IPSPs could be followed by a distinct depolarizing overshoot or postinhibitory facilitation (n= 4). At firing threshold, pyramidal cells fired postinhibitory rebound-like action potentials, the latter in close temporal overlap with the depolarizing overshoot. In conclusion, hippocampal basket cells have been identified as one source of fast, GABA_A receptor-evoked perisomatic inhibition. Unitary events are mediated by multiple synaptic release sites, thus providing an effective mechanism to avoid total transmission failures.     

Specificity of identified central synapses in the embryonic cockroach: Appropriate connections form before the onset of spontaneous afferent activity

The mechanisms by which neurons recognize the appropriate postsynaptic cells remain largely unknown. A useful approach to this problem is to use a system with a few identifiable neurons that form highly specific synaptic connections. We studied the development of synapses between two identified cercal sensory afferents and two giant interneurons (GIs) in the embryonic cockroach Periplaneta americana. By 46% of embryonic development, the axons of the filiform hair sensory neurons have entered the terminal ganglionic neuropil and grow alongside the GI primary dendrites, although they do not form synapses. From 50% of development, the GI dendrites grow outward from the center of the neuropil to contact the presynaptic axons and their branches. The sensory neurons begin to spike at 52% of development, and, from 55% of development, these action potentials evoked excitatory postsynaptic potentials in the GIs. Synaptic contacts were first seen at this time. The pattern of synaptic connections was highly specific from the outset. GI2 had strong input from the medial (M) afferent and had almost negligible input from the lateral (L) afferent, whereas GI3 had input from both. This specificity was present before bursts of spontaneous activity began in the sensory neurons at 59% of development. GI2 filopodia selectively formed synaptic contacts with the M axon rather than the L axon. The few contacts made by GI2 with the L axon had a normal morphology but fewer presynaptic densities. Filopodial insertions were not involved in selective synapse formation. In this system, highly specific synaptic recognition appears to be activity independent. © 1996 Wiley-Liss, Inc.     

Cytoarchitecture,fiber connections,and ultrastructure of nucleus isthmi in a teleost (Navodon modestus) with a special reference to degenerating isthmic afferents from optic tectum and nucleus pretectalis

Cytoarchitecture and fiber connections of the nucleus isthmi in a teleost (Navodon modestus) were studied by means of Nissl, Bodian, toluidine blue, Golgi, and Fink-Heimer methods. Synaptic terminals were classified by the ultrastructural characteristics, and their origins were determined by electron microscopic degeneration experiments. The nucleus isthmi is composed of an outer cellular area or shell and an inner noncellular area or core. The shell covers anterior, dorsal, and ventral aspects of the core. The cell bodies in the shell are oval (15 × 20 μm) with an anteroposterior long axis, and have many somatic spines. Spines are also seen on the initial segment of the axon. Primary dendrites extend postermedially and branch out in the core. The core contains thin and thick myelinated fibers, which originate in the optic tectun and in the nucleus pretectalis, respectively. At least two types of axons terminal were distinguished in the nucleus isthmi: S type, containing spherical vesciles, and F type, containing flattened vesicles. S terminals are derived from thin myelinated fibers and are only seen in the core where they form asymmetric synapses with dendrites. Frequently a portion of the S terminal membrane near the usual synaptic cleft is in close apposition with the membrane of an adjacent small dendrite or spine. F terminals, which derived from thick myelinated fibers, make symmetric synaptic contacts with both cell bodies in the shell and dendrites in the core. S terminals degenerate after ipsilateral ablation of the optic tectum, whereas F terminals degenerate after destruction of the nucleus pretectalis.     

Sex differences in the number of synaptic junctions in the binocular area of the rat visual cortex

We had found that the binocular area of the visual cortex is larger in volume and has more neurons in male than in female rats. The present study examined whether the number of synaptic junctions in this area is sexually dimorphic. Ten littermate pairs of 90-day-old (socially housed) Long-Evans hooded rats were used. Synaptic junctions were counted and their lengths were measured on electron micrographs taken from layers II–III of the binocular visual cortex. There were no sex differences in the numerical density of synaptic junctions, the number of synaptic junctions per neuron, or the length of synaptic junctions within any synaptic category or of all synapses combined. Sex differences were found in the total number of synaptic junctions and in several categories (asymmetric synapses, spine synapses, asymmetric spine synapses): male rats had more synaptic junctions than female rats because of the larger volume of layers II--III in the binocular area of male rats. These data indicate that neurons in the binocular visual cortex of both male and female rats receive a characteristic number of synaptic junctions, but the greater number of neurons in the binocular area of male rats results in more synaptic junctions in the area. © 1995 Wiley-Liss, Inc.     

Freeze-fracture study of the large myelinated club ending synapse on the goldfish Mauthner cell: special reference to the quantitative analysis of gap junctions

The large myelinated club endings (LMCEs) of primary eighth nerve afferents form mixed synapses on the lateral dendrite of the giant Mauthner cell. The double replica freeze-fracture technique was employed to examine the intramembrane fine structure of these LMCE synapses. Morphological correlates of both chemical and electrical transmission were found at the LMCE synapses. Electrical synaptic junctions, or gap junctions, were located over much (10-20%) of the synaptic contact. These were seen in both pre-and postsynaptic membrane as tightly packed P face particle aggregates and corresponding aggregates of E face pits. Specializations characteristic of chemical synaptic junctions were most prominent at the periphery of the synaptic contact. These specializations consisted of postsynaptic E face particle aggregates which were subjacent to presynaptic active zones. The active zones were distinguishable as regions with an increased density of large particles and vesicle attachment sites represented by P face depressions and E face protuberances. Quantitative analysis of gap junction particle (connexon) number at five LMCEs revealed 24,000-106,000 connexons per LMCE. Comparison with data from electrophysiological studies of single LMCEs indicates that only a small fraction of the connexon channels are open at any given time during electrotonic transmission at an LMCE synapse.     

Horizontal cell electrical coupling in the giant danio: synaptic modulation by dopamine and synaptic maintenance by calcium

Electrical synapses, and their structural manifestation, gap junctions, are critical elements of retinal circuitry. These synapses are subject to both rapid modulation and slower structural changes by physiological signals which mediate changes in the adaptational state of the retina. The electrical synapses of fish retinal horizontal cells are an excellent preparation for in vitro studies of electrical synapses. We have examined the rapid modulation of electrical coupling by dopamine and effects on the expression and maintenance of electrical synapses by cell calcium in pairs of horizontal cells isolated from retinas of the giant danio (Danio aquipinnatus). We report that rapid modulation by dopamine reduces junctional conductance by modifying gap junction channel gating, while maintaining cells in reduced calcium medium, and lowering; intracellular calcium concentration, results in the loss of electrical coupling. The effects of calcium on synaptic maintenance may be related to structural changes observed in horizontal cell electrical synapses during light adaptation.     

Synaptogenesis and its relation to growth of the postsynaptic cell: A quantitative study of the developing mauthner neuron of the axolotl

We describe the relation between growth and branching of an identified dendrite and the formation of synapses on its surface during a 3 1/2-day period early in development. We studied the lateral dendrite and the adjacent lateral perikaryon of the Mauthner cell (M-cell) during embryonic stages 39–43 in the axolotl Ambystoma mexicanum. Reconstructions from light micrographs of serial sections through the cell revealed that during this interval the dendrite elongates rapidly, and large numbers of ventrally directed branches are formed. Samples of the same material by electron microscopy showed that large numbers of synaptic contacts appear during the same interval. We quantitatively estimated changes in local synapse densities (the number of contacts/100 μm² of M-cell surface) and local surface areas of the M-cell and found that synapses were most densely clustered, and accumulated most rapidly, on regions of the cell that were rapidly expanding. These data are in accord with previous evidence from work in this and in other systems that synaptic contacts induce local growth of dendrites. Furthermore, the data are consistent with a proposal that outgrowth of new dendritic branches is induced or stabilized by synapses in a concentration-dependent fashion.     

Cone synapses in mammalian retinal rod bipolar cells

Some mammalian rod bipolar cells (RBCs) can receive excitatory chemical synaptic inputs from both rods and cones (DBC_R2), but anatomical evidence for mammalian cone‐RBC contacts has been sparse. We examined anatomical cone‐RBC contacts using neurobiotin (NB) to visualize individual mouse cones and standard immuno‐markers to identify RBCs, cone pedicles and synapses in mouse and baboon retinas. Peanut agglutinin (PNA) stained the basal membrane of all cone pedicles, and mouse cones were positive for red/green (R/G)‐opsin, whereas baboon cones were positive for calbindin D‐28k. All synapses in the outer plexiform layer were labeled for synaptic vesicle protein 2 (SV2) and PSD (postsynaptic density)‐95, and those that coincided with PNA resided closest to bipolar cell somas. Cone‐RBC synaptic contacts were identified by: (a) RBC dendrites deeply invaginating into the center of cone pedicles (invaginating synapses), (b) RBC dendritic spines intruding into the surface of cone pedicles (superficial synapses), and (c) PKCα immunoreactivity coinciding with synaptic marker SV2, PSD‐95, mGluR6, G protein beta 5 or PNA at cone pedicles. One RBC could form 0‐1 invaginating and 1‐3 superficial contacts with cones. 20.7% and 38.9% of mouse RBCs contacted cones in the peripheral and central retina (p < .05, n = 14 samples), respectively, while 34.4% (peripheral) and 48.5% (central) of cones contacted RBCs (p > .05). In baboon retinas (n = 4 samples), cone‐RBC contacts involved 12.2% of RBCs (n = 416 cells) and 22.5% of cones (n = 225 cells). This suggests that rod and cone signals in the ON pathway are integrated in some RBCs before reaching AII amacrine cells.     

Pre and postsynaptic roles for Drosophila CASK

CASK (‘calcium/calmodulin-dependent serine protein kinase’), also known in Drosophila as ‘Caki’ or ‘Camguk/CMG’, and in C. elegans as ‘Lin-2’, is thought to play an important role in cell–cell junction formation and at synapses in particular. To understand the role of CASK in synapse formation and function, we functionally and morphologically analyzed Drosophila embryonic and larval glutamatergic neuromuscular junctions (NMJs) after pan-cellular and tissue-specific manipulation of CASK expression. Our results show that Drosophila CASK is associated with both pre and postsynaptic membranes. Loss of presynaptic CASK led to less evoked synaptic transmission, fewer spontaneous synaptic events, and reduced synaptic vesicle cycling. These changes were accompanied by a reduction in the number of synapses but no change in overall NMJ size. Loss of postsynaptic CASK, on the other hand, caused reduced spontaneous synaptic current amplitudes and smaller glutamate-gated currents. These changes were accompanied by loss of postsynaptic glutamate receptors, but the receptor loss was subtype-specific: Only receptors containing GluRIIA subunits were lost in CASK mutants. Receptors containing GluRIIB were unaffected.     

Observations on the abdominal stretch receptor and the fine structure of associated axo-dendritic synapses and neuromuscular junctions in homarus

The abdominal stretch receptor of lobster has been examined. Fine structural details of the receptor nerves are presented and neurophysiological correlations are suggested. Both axo-dendritic synapses and neuromuscular junctions were examined. Axo-dendritic synapses are of one type only, possessing elongated, or flattened, presynaptic vesicles approximately 200 Å in diameter and 400–600 Å long. In contrast two distinct types of neuromuscular junctions were observed. The first possesses spherical synaptic vesicles 500–900 Å in diameter. The second possesses synaptic vesicles of dimensions similar to those of axo-dendritic synapses. It is suggested that synapses, both axo-dendritic and neuromuscular, with elongated or flattened vesicles, are inhibitory, and that neuromuscular junctions with spherical vesicles are excitatory.     

Synaptic deficit in the temporal cortex of partial trisomy 16 (Ts65Dn) mice

Down syndrome results from triplication of human chromosome 21. The distal end of mouse chromosome 16 shares a large region of genetic homology with the Down syndrome ‘critical region' of human chromosome 21. Therefore, a partially trisomic mouse (Ts65Dn) that possesses a triplication of the distal region of chromosome 16 has been developed as a putative model for Down syndrome. Ts65Dn mice display learning and memory deficits. However, despite the importance of preserved synaptic integrity for learning and memory, the ultrastructure of neural connectivity has not yet been studied in Ts65Dn mice. Therefore, the density and apposition zone length of synapses in the temporal cortex of aged Ts65Dn mice (n=4) were compared with those in diploid controls (n=4), using quantitative electron microscopy. There were significantly less (30%) asymmetric synapses in the temporal cortex of Ts65Dn mice than in controls (t=−5.067; p=0.023). However, there was no significant difference between the mean density of symmetric synapses in Ts65Dn mice and control mice. In addition, the mean synaptic apposition lengths of both asymmetric (15%; t=9.812, p<0.0001) and symmetric (11%; t=5.582; p<0.0001) synapses were significantly larger in Ts65Dn mice than in controls. These results suggest that excitatory synapses are preferentially affected in Ts65Dn mice and that there is an attempt to compensate for the deficit of asymmetric synapses by increasing the contact zone area of existing synapses. The results may also reveal the morphological basis for the learning and memory deficits observed in Ts65Dn mice and have a bearing on the cognitive deficits in Down syndrome in old age.     

Synaptogenesis in hemimegalencephaly: the numerical density of asymmetric and symmetric synapses in the cerebral cortex

Specimens of cerebral cortex were prepared for electron microscopy from cortical resections performed for the treatment of intractable seizures in four cases of hemimegalencephaly (HME). Morphometric analyses were performed to determine mean cortical thickness, the numerical density of synapses (N _V, contacts per mm³) and the number of synapses in a column of cortex beneath 1 mm² of pial surface. The N _V were calculated separately for asymmetric and symmetric synapses as well as for axospinous, axodendritic and axosomatic contacts. Four cases of Rasmussen’s encephalitis served as controls, with tissue being sampled from a region distant to the site of the inflammatory lesion without obvious necrosis. The N _V of synapses did not differ significantly between HME cases and controls. The proportions or asymmetric and symmetric synapses were similar in both groups, as were the proportions of axospinous, axodendritic and axosomatic contacts. However, there was a significant increase in mean cortical thickness in HME cases (130%, P < 0.05). Consequently, there was a significant increase in the total number of synapses in a column of cortex (126%, P < 0.05). In HME the cerebral cortex is characterized by synaptic dysgenesis. Although synaptic density per unit volume of tissue appears relatively normal, the increased thickness and volume of the cerebral cortex provides for an increase in the total number of synapses in a given cytoarchitectonic area. Received: 31 October 1995 / Revised, Accepted: 5 February 1996     

Synaptic circuitry of identified neurons in the antennal lobe of Drosophila melanogaster

In Drosophila melanogaster olfactory sensory neurons (OSNs) establish synapses with projection neurons (PNs) and local interneurons within antennal lobe (AL) glomeruli. Substantial knowledge regarding this circuitry has been obtained by functional studies, whereas ultrastructural evidence of synaptic contacts is scarce. To fill this gap, we studied serial sections of three glomeruli using electron microscopy. Ectopic expression of a membrane‐bound peroxidase allowed us to map synaptic sites along PN dendrites. Our data prove for the first time that each of the three major types of AL neurons is both pre‐ and postsynaptic to the other two types, as previously indicated by functional studies. PN dendrites carry a large proportion of output synapses, with approximately one output per every three input synapses. Detailed reconstructions of PN dendrites showed that these synapses are distributed unevenly, with input and output sites partially segregated along a proximal–distal gradient and the thinnest branches carrying solely input synapses. Moreover, our data indicate synapse clustering, as we found evidence of dendritic tiling of PN dendrites. PN output synapses exhibited T‐shaped presynaptic densities, mostly arranged as tetrads. In contrast, output synapses from putative OSNs showed elongated presynaptic densities in which the T‐bar platform was supported by several pedestals and contacted as many as 20 postsynaptic profiles. We also discovered synaptic contacts between the putative OSNs. The average synaptic density in the glomerular neuropil was about two synapses/µm³. These results are discussed with regard to current models of olfactory glomerular microcircuits across species. J. Comp. Neurol. 524:1920–1956, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.     

Retinal terminals in the goldfish optic tectum: Identification and characterization

Retinal terminal profiles in the goldfish optic tectum were identified electron microscopically after (1) labeling with horseradish peroxidase and (2) in the early stages of degeneration in short-term eye enucleates. All labeled terminals shared certain common morphological characteristics which were identical to those of a population of terminals in normal tecta. Terminals of this type disappeared 30 days after enucleation of the contralateral eye. Retinal terminal presynaptic profiles were characterized by (1) round and oval synaptic vesicles; (2) mitochondria with irregular, randomly oriented cristae, large intracristal spaces, dilated membrane spaces, and primarily light matrices; (3) a wide range in profile area, 0.06–6.82 μm²; (4) large numbers of synaptic vesicles per profile area 168± 33 synaptic vesicles per μm²; (5) asymmetric synapses; and (6) multiple synaptic contacts (1.46 ± 0.73 per terminal profile). The postsynaptic elements included both dendritic and, less commonly, pleomorphic vesicle-containing profiles. The majority of postsynaptic dendritic profiles were small (0.01–0.40 μm²). Serial synaptic contacts were occasionally seen. The combination of vesicular and mitochondrial morphology (1 and 2 above) was necessary and sufficient to establish the retinal origin of a terminal, but use of such criteria would underestimate the number of retinotectal terminals by omitting those which did not have a mitochondrion in the plane of section. The number of such terminals was calculated from independent measurements, and the total number of retinal terminal profiles per area of neuropil was estimated.