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PubMed1985

Development of the rhesus monkey retina. I. Emergence of the inner plexiform layer and its synapses.

The emergence and differentiation of the inner plexiform layer (IPL) and the establishment of its synapses were analyzed in retinae of 18 rhesus monkeys ranging in age from the 55th embryonic day (E55) to 10 years. The IPL becomes recognizable by E65 as a thin acellular zone consisting of immature neurites and growth cones scattered within large extracellular spaces. In each specimen, apposing paired membrane specializations were classified as junctions without synaptic vesicles, conventional synapses, ribbon synapses, or gap junctions. Initially, at E65, the IPL consists of variety of immature cell processes that are interconnected exclusively by junctions without synaptic vesicles, at a density of 4.7/100 microns2. By E73, the IPL becomes more distinct and wider and contains 7.8 such junctions/100 microns2. Conventional synapses develop by the addition of vesicles to initially vesicle-free junctions. The first conventional synapses appear at E78. They increase in density from 1.5 to 3.2/100 microns2 between E78 and E84 and reach a density of 7.9/100 microns2 by E99. A rapid burst in synaptogenesis occurs in the IPL between E99 and E114; a density of 16.5/100 microns2 is reached, mainly due to accretion of conventional synapses. Ribbon synapses first become recognizable at E99, almost 3 weeks after the emergence of conventional synapses. By E114 they account for about 7% of all synaptic contacts in the IPL. The rate of synaptogenesis slows down during the last quarter of gestation; the adult level of about 24 contacts/100 microns2 is reached between E130 and E149. Of these, 72.2% are of conventional type, 15.4% are ribbon synapses, and 12.4% are junctions without vesicles. However, in the adult the density of junction without vesicles is only about one-half that found at E149. Gap junctions are absent during the initial and rapid phases of synaptogenesis; they appear abruptly, between E130 and E149, only after the density of chemical synapses in the IPL has reached the adult level. In the rhesus monkey, synaptogenesis begins several weeks later in the IPL than in its primary targets--the dorsal lateral geniculate nucleus and the superior colliculus (Hendrickson and Rakic, '77; Cooper and Rakic, '83). However, the rapid increase in density of conventional synapses in the IPL coincides with the segregation of retinal projections from right and left eyes in the geniculate nucleus (Rakic, '76) and with the elimination of the large surplus of retinal ganglion cell axons (Rakic and Reley, '83).

Animals↗
PubMed1987

Small vesicle bouton synapses on the distal half of the lateral dendrite of the goldfish Mauthner cell: freeze-fracture and thin section study.

To understand principles of synaptic integration, it is necessary to define the types of synapses on a particular neuron and their distribution. Thin sectioning and double replica freeze-fracture techniques were employed to characterize the small vesicle bouton (SVB) synapses on the distal half of the Mauthner (M) cell lateral dendrite, which probably mediate a remote dendritic inhibition. Three morphologically distinct SVB synapses, types A, B, and C, were found. These three SVB synapses form roughly 90% of the synapses on the distal half of the lateral dendrite, with types A and B being most common. The SVB A synapse is characterized by mostly oval and round synaptic vesicles, a discrete presynaptic active zone with a highly variable shape, and a postsynaptic active zone with no apparent particle aggregate in either the E or P face. At the SVB B synapse, most of the synaptic vesicles are flat. A very high particle density is present throughout the presynaptic P face, and vesicle attachment sites are dispersed over much of the presynaptic membrane. Postsynaptic P face particle aggregates are subjacent to the presynaptic vesicle attachment sites, and are often large and anastomosing. The SVB C synapse is characterized by synaptic vesicle profiles that vary from flattened to round. The SVB C cytoplasm was unclouded by the flocculent material that characterized SVBs A and B. The presynaptic active zones at the SVB C synapse are discrete, and macular or oblong. No particle aggregates are apparent in the postsynaptic active zone. Small, macular particle aggregates were found in nonactive zone regions of the postsynaptic E face of all three types of SVBs. Small subsurface cisterns were also observed underlying the M cell membrane at all three types of SVB synapses. Neither the postsynaptic E face aggregates nor the subsurface cisterns were ever observed directly subjacent to presynaptic active zones, but were often seen adjacent to active zones. Short, straight rows of particles and short cylinders were often seen in both pre- and postsynaptic surrounding zone regions of SVB A and C synapses. These structures are thought to represent tight junctions.

Animals↗
PubMed1988

Number of parallel fiber synapses on an individual Purkinje cell in the cerebellum of the rat.

In the present study, stereological techniques applied to electron micrographs of the molecular layer of the rat cerebellum have been used to estimate the number of parallel fiber synapses on the dendritic tree of a single Purkinje cell. Quantitative features of the parallel fiber to Purkinje cell dendritic spine synapses and of the parallel fibers were investigated as a preliminary to estimating the number of synapses. Parallel fiber to Purkinje cell synapses are flattened disclike structures with a mean axial ratio of 14.7 and a mean diameter of 319 microns in fixed tissue. The density of synapses in our fixed material was 8.17 x 10(8) per microliters of molecular layer. Determination of the length density of the synapses per unit area of micrograph indicated a synapse density of 8.03 x 10(8) per microliters. These densities give a total number of synapses per Purkinje cell of 1.74 x 10(5) and 1.71 x 10(5), respectively. Estimation of the number of parallel fiber varicosities and of varicosity length gave a density of 9.31 x 10(8) varicosities per microliters of molecular layer and determining the mean number of parallel fiber to Purkinje cell synapses per varicosity gave a synapse density of 9.82 x 10(8) per microliters, equivalent to 2.09 x 10(5) per Purkinje cell. The reasons why this estimate is likely to be too high are discussed. We conclude that there are some 175,000 parallel fiber synapses on an individual Purkinje cell dendritic tree in the cerebellar cortex of the rat, considerably more than previously reported.

Animals↗
PubMed1994

Ultrastructural plasticity of the dentate gyrus granule cells following recurrent limbic seizures: II. Alterations in somatic synapses.

Hilus lesion-induced recurrent limbic seizures cause a dramatic increase in the numbers of somatic spines on dentate gyrus granule cells in the adult rat. Somatic spines are maximally increased 3 h after the initiation of seizures at which time many of these spines form synapses. The present quantitative electron microscopic study assessed the numbers and types of synapses present on the granule cell perikarya and somatic spines of control and experimental seizure rats with the goal of determining if newly elaborated somatic spines arise at the site of pre-existing synapses or are associated with new innervation. Experimental rats were sacrificed 5 h after hilar lesion placement (or 3 h after seizure onset). In both control and hilus-lesioned (HL) rats, 15-20% of the somatic spines could be seen to form synaptic contacts within a single plane of section; these synapses were almost exclusively of the asymmetric type. With the increased incidence of spines in experimental-seizure rats, there was a 6.25-fold greater number of spine synapses in HL versus control rats. There was, in addition, a 60% decrease in the number of asymmetric synapses occurring directly on the granule cell perikarya but no change in the total (spine plus somatic) number of asymmetric synapses. Although few asymmetric synapses were associated with spines in control tissue, 60-70% of asymmetric synapses were associated with spines in experimental-seizure tissue. In addition, in hilus lesion rats symmetric somatic synapses were increased by 20% on cells in deep stratum granulosum resulting in a dissolution of the superficial-to-deep innervation gradient present in the untreated rat. These findings support the conclusion that spines induced by seizure activity form at the site of pre-existing asymmetric synapses on the granule cells and demonstrate that brief seizure episodes can rapidly induce marked changes in innervation patterns in the adult brain.

Animals↗
PubMed2001

Immunoelectron microscopic localization of the neural recognition molecules L1, NCAM, and its isoform NCAM180, the NCAM-associated polysialic acid, beta1 integrin and the extracellular matrix molecule tenascin-R in synapses of the adult rat hippocampus.

We have investigated the possibility that morphologically different excitatory glutamatergic synapses of the "trisynaptic circuit" in the adult rodent hippocampus, which display different types of long-term potentiation (LTP), may express the immunoglobulin superfamily recognition molecules L1 and NCAM, the extracellular matrix molecule tenascin-R, and the extracellular matrix receptor constituent beta1 integrin in a differential manner. The neural cell adhesion molecules L1, NCAM (all three major isoforms), NCAM180 (the largest major isoform with the longest cytoplasmic domain), beta1 integrin, polysialic acid (PSA) associated with NCAM, and tenascin-R were localized by pre-embedding immunostaining procedures in the CA3/CA4 region (mossy fiber synapses) and in the dentate gyrus (spine synapses) of the adult rat hippocampus. Synaptic membranes of mossy fiber synapses where LTP is expressed presynaptically did not show detectable levels of immunoreactivity for any of the molecules/epitopes studied. L1, NCAM, and PSA, but not NCAM180 or beta1 integrin, were detectable on axonal membranes of fasciculating mossy fibers. In contrast to mossy fiber synapses, spine synapses in the outer third of the molecular layer of the dentate gyrus, which display postsynaptic expression mechanisms of LTP, were both immunopositive and immunonegative for NCAM, NCAM180, beta1 integrin, and PSA. Those spine synapses postsynaptically immunoreactive for NCAM or PSA also showed immunoreactivity on their presynaptic membranes. NCAM180 was not detectable presynaptically in spine synapses. L1 could not be found in spine synapses either pre- or postsynaptically. Also, the extracellular matrix molecule tenascin-R was not detectable in synaptic clefts of all synapses tested, but was amply present between fasciculating axons, axon-astrocyte contact areas, and astrocytic gap junctions. Differences in expression of the membrane-bound adhesion molecules at both types of synapses may reflect the different mechanisms for induction and/or maintenance of synaptic plasticity.

Animals↗
PubMed1995

Synapses between cones and diffuse bipolar cells of a primate retina.

The photoreceptor synapses of three representative cells of the six types of diffuse bipolar cell of the rhesus macaque monkey's retina are described at 3.5-4.0 mm eccentricity. Bipolar cell DB3 was found to be postsynaptic to 11 cones at 155 basal synapses; about 70% of these were triad associated. Bipolar cell DB4 as postsynaptic to eight cones at 52 ribbon synapses; in addition it was found also to make an average of two or three basal (non-ribbon) synapses per cone (total 23). The DB5 bipolar cell type had 57 invaginating synapses with seven cones. It too had basal synapses, but only two with each of three cones. The diffuse invaginating bipolar cell described by Mariani (1981) is identified as a member of the DB5 category. Dendrites of cone bipolar cell types which have axons ending in the a-layer of the inner plexiform layer make only basal synapses with the cone pedicle. Those so far investigated are the flat midget bipolar cell and the DB2 and DB3 flat diffuse bipolar cells. All bipolar cells whose axons terminate in the b-layer of the inner plexiform layer are postsynaptic at the ribbon synapses of the cone pedicles. They now appear to fall into two groups. Those whose dendrites are exclusively postsynaptic at the ribbons; these are the blue cone and invaginating midget bipolar cells. And the diffuse bipolar cell DB4, that has both ribbon and basal synapses in a ratio of about 2.3:1. It is uncertain into which category cell DB5 should be placed; its basal synapses are so few the cell could be anomalous. It now seems that at least one primate bipolar cell type may be like those of other vertebrates in having, as defined ultrastructurally, two different kinds of synaptic connection with its cones. The results are discussed in the context of a brief review of the photoreceptor synapses of other mammalian bipolar cells.

Animals↗
PubMed1991

Distribution of synapses on fast and slow pyramidal tract neurons in the cat. An electron microscopic study.

Distributions of synapses on various portions of fast and slow pyramidal tract neurons (PTNs) in cat motor cortex were studied with electron microscopy. PTNs were identified by their antidromic invasion following stimulation of the medullary pyramid and were classified into fast and slow PTNs according to conduction velocities of their axons. Two fast and two slow PTNs were intracellularly labeled and, by systematic sampling, electron micrographs from various portions of these neurons were examined to compare the distributions of different types of synapses. It was found that most synapses formed on apical and basal dendrites of fast PTNs were with the dendritic shafts. In slow PTNs, while synapses on apical dendrites were mostly axospinous, about 70% of the sampled synapses on basal dendrites of slow PTNs were established with the dendritic shafts. Virtually all synapses on apical dendrites of slow PTNs belonged to asymmetrical type and most of the synapses sampled from basal dendrites of fast PTNs were also asymmetrical. On the other hand, about 29% of the synapses found on apical dendrites of fast PTNs were symmetrical and a trend was observed for this type of synapses to increase their number with increasing proximity to the cell body. Over 28% of the synapses on basal dendrites of slow PTNs were also symmetrical and seemed to be mainly distributed in layer VI. All synapses formed on the soma were symmetrical both for the fast and slow PTNs.

Animals↗
PubMed1992

Structural maturation of synapses in the rat superior cervical ganglion continues beyond four weeks of age.

We have examined the morphology of preganglionic synapses in the rat superior cervical ganglion (SCG) at 10 days, 4 weeks and 1 year. Between 10 days and 4 weeks the mean thickness of the postsynaptic density (PSD) increased from 45.9 +/- 0.1 nm to 52.1 +/- 1.7 nm (P = 0.017), the mean length of the PSD (0.41 +/- 0.02 microns) did not change, and the distribution of synapses on the neuronal surface changed with a decrease in the proportion of somatic and an increase in the proportion of dendritic spine synapses. Since both synapse elimination and synapse formation are occurring during this period several mechanisms may contribute to these changes. However, between 4 weeks and 1 year, when there is no net change in the number of synapses, the mean length of the PSD increased to 0.53 +/- 0.02 microns (P = 0.001), there was no change in either the mean thickness of the PSD or the distribution of the synapses but the proportion of concave ('smile') synapses increased. A comparison with previous developmental studies of synapses in cerebral cortex of rat and chicken indicate that both the nature and the rate of synapse maturation can vary between different populations of synapses.

Aging↗
PubMed2004

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

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

Animals↗
PubMed1999

Activity-dependent formation of perforated synapses in cultured hippocampal neurons.

The study investigated the formation of perforated synapses in rat hippocampal cell cultures. Perforated synapses are defined by their discontinuous postsynaptic densities (PSDs) and are believed to occur in parallel with changes in synaptic activity and possibly also synaptic efficacy. Several in vivo studies have demonstrated an increase in the frequency of perforated synapses induced by development and environmental stimulation as well as long-term potentiation (LTP). Also in in vitro brain slices, LTP was associated with an elevated number of perforated spine synapses. Our study demonstrated for the first time that the formation of perforated synapses can be induced by a short-term increase in spontaneous neural activity in a hippocampal cell culture model. Stimulation with the GABAA-antagonist picrotoxin (PTX) induced a significant increase in the percentage of perforated synapses. This strong increase was blocked when APV was added together with PTX, indicating that the formation of perforated synapses depended on the activation of NMDA receptors. We also showed that inhibition of the tissue type plasminogen activator (tPA-stop/PAI-1) significantly interfered with the activity-induced increase in perforated synapses. This implies that the proteolytic activities of tPA might be involved in steps which are downstream from the NMDA receptor-mediated synaptic plasticity leading to structural changes at synaptic contacts. In contrast, even long-term inhibition of electrical network activity by tetrodotoxin had no effect on the number of perforated synapses, but almost completely abolished the formation of spine synapses. These results indicate that a short-term increase in neural activity via NMDA receptors and a proteolytic cascade involving tPA lead to the formation of perforated synapses.

2-Amino-5-phosphonovalerate↗
PubMed2003

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: evidence for multiquantal release.

Most of our knowledge about transmission at central synapses has been obtained by studying populations of synapses, but some important properties of synapses can be determined only by studying them individually. An important issue is whether a presynaptic action potential causes, at most, a single vesicle to be released, or whether multiquantal transmission is possible. Previous work in the CA1 region has shown that the response to stimulation of a single axon can be highly variable, apparently because it is composed of a variable number of quantal elements ( approximately 5 pA in amplitude). These quantal events have a low coefficient of variation (CV). Because the number of synaptic contacts involved is not known, the response could be because of uniquantal transmission at a varying number of synapses, or to multliquantal transmission at a single synapse. The former predicts that the CV at individual synapses should be small. We have used optical methods to measure the N-methyl-D-aspartate receptor-mediated Ca(2+) elevation at single active synapses. Our main finding is that the amplitude of nonfailure responses could be highly variable, having a CV as large as 0.63. In one fortuitous experiment, the optically studied synapse was the only active synapse, and we could therefore measure both its N-methyl-D-aspartate (NMDA) receptor- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated signals. At this synapse, both signals varied over a 10-fold range and were highly correlated. These results strongly suggest that transmission at single CA1 synapses can be multiquantal. Furthermore, the individual quantal response is very far from saturation, allowing the effective summation of many quanta. The existence of multiquantal release has important implications for defining synaptic strength and understanding the mechanisms of synaptic plasticity.

Animals↗
PubMedDate not supplied

Quantitative distribution of GABA-immunopositive and -immunonegative neurons and synapses in the monkey striate cortex (area 17).

The number of GABA-immunoreactive [GABA(+)] neurons and synapses was determined in functionally distinct subregions delineated as rich and poor in cytochrome oxidase (CO) in the visual cortex of adult macaque monkeys. The average numerical density (number per unit volume, Nv) of GABA(+) neurons and synapses was not significantly different between the CO-rich and -poor regions. Twenty percent of the total number of cortical neurons and 17% of the synapses were GABA(+). On average, each visual cortical neuron receives 3900 synapses, 660 of them being GABA(+). The latter were distributed on the target cell in a pattern that predicts the site of GABA influences in cortex. The major targets of GABA(+) synapses were dendritic shafts, comprising nearly two-thirds of the postsynaptic elements. About every fourth and every eighth GABA(+) synapse was devoted to dendritic spines and to neuronal somata, respectively. Axon initial segments, although the exclusive targets of GABA(+) cells, comprise less than 0.1% of structures postsynaptic to GABA(+) boutons. From this distribution, we estimate that in each cubic millimeter of striate cortex there were about 20 million GABA(+) synapses on dendritic spines, 47 million on dendritic trunks, 9 million on somata, and fewer than 0.1 million on axon initial segments. The sites of influences of GABA-immunonegative [GABA(-)] synapses were different in that they target mainly dendritic spines and dendritic trunks. About two-thirds of GABA(-) synapses were on dendritic spines, and the remainder were devoted to dendritic trunks. Only a minute fraction innervate somata. We estimate that in 1 mm3 of striate cortex there were about 235 million GABA(-) synapses on spines, 133 million on dendrites, and about 2 million on somata. The proportions of GABA(+) neurons and synapses and their target distribution did not appreciably differ from those of the visual cortex of the cat even though the numerical density of neurons was 2.5 times higher in the monkey.

Animals↗
PubMed2004

Localized expression of amphiphysin Ir, a retina-specific variant of amphiphysin I, in the ribbon synapse and its functional implication.

In the vertebrate retina, presynaptic terminals of photoreceptors and bipolar cells form ribbon synapses and release neurotransmitter continuously. Endocytic machinery in the ribbon synapse is likely to differ from that in conventional synapses because of the much higher rate of synaptic vesicle recycling. However, protein components of the ribbon synapse identified so far are quite similar to those of the conventional synapses. Recently we identified amphiphysin I splice variants, termed amphiphysin Ir, that are transcribed specifically in retina from the authentic amphiphysin I gene [Y. Terada et al. (2002) FEBS Lett., 519, 185-190]. Amphiphysin I is a nerve terminal-enriched protein, and involved in synaptic vesicle endocytosis as heterodimer with amphiphysin II, an isoform of amphiphysin I. We report here that the retina-specific amphiphysin Ir is expressed exclusively in the ribbon synapse and not in conventional synapses. This is the first endocytosis-related, ribbon synapse-specific protein identified in the retina. By immunoprecipitation and double-immunolabelling, amphiphysin Ir was shown to be associated not only with amphiphysin II, but also with dynamin, clathrin and alpha-adaptin that are involved in synaptic vesicle recycling. The results suggest that endocytosis of the synaptic vesicle membrane in retinal ribbon synapses proceeds through a pathway similar to the one that is used in conventional synapses, although amphiphysin Ir is substituted for amphiphysin I. Amphiphysin Ir may play an essential role in the avid endocytic activity of ribbon synapses by associating with yet unknown protein partner(s) through its large insertional domain, which is absent from the conventional amphiphysin I.

Animals↗
PubMed2006

Structural stabilization of CNS synapses during postnatal development in rat cortex.

CNS synapses are produced rapidly upon pre- and post-synaptic recruitment. However, their composition is known to change during development and we reasoned that this may be reflected in the gross biochemical properties of synapses. We found synaptic structure in adult cortical synaptosomes to be resistant to digestion with trypsin in the presence and absence of calcium ions, contrasting with previous observations. We evaluated the divalent cation dependence and trypsin sensitivities of synapses using synaptosomes from different developmental stages. In contrast to adult synapses, at postnatal day (P) 10 EDTA treatment eliminated approximately 60% of the synapses, and trypsin and EDTA, together, eliminated all junctions. Trypsinization in the presence of calcium eliminated approximately 60% of the junctions at P10. By P35, all synapses were calcium independent, whereas full trypsin resistance was not attained until P49. To compare the calcium dependence and trypsin sensitivity of synapses in another region of the adult brain, we examined synapses from adult (P50) hippocampus. Adult hippocampus maintained a population of synapses that resembled that of P35 cortex. Our results show that synapses are modified over a long time period in the developing cortex. We propose a model in which the addition of synergistic calcium-dependent and -independent adhesive systems stabilize synapses.

Animals↗
PubMed1986

Incoming synapses and size of small granule-containing cells in a rat sympathetic ganglion after post-ganglionic axotomy.

A quantitative ultrastructural study has been made of the reaction of the incoming synapses of small granule-containing cells after axotomy of the major post-ganglionic branches of the superior cervical ganglion of the young adult rat. These cells are intrinsic and interneurone-like in this ganglion, receiving a preganglionic input and giving outgoing synapses to principal post-ganglionic neurones. Unlike their outgoing synapses, which are lost after post-ganglionic axotomy (Case & Matthews, 1986), the incoming synapses of the small granule-containing cells in axotomized ganglia increased in incidence post-operatively. The increase first became clearly evident 5-7 days post-operatively and was greater, being both more sustained and progressive, after bilateral than after unilateral axotomy. After bilateral axotomy the incidence of incoming synapses rose to more than four times that of normal ganglia and was still elevated at 128 days post-operatively, but was within normal limits at 390 days. After a unilateral lesion, increases of similar extent and time course to those in the axotomized ganglia were seen in the incoming synapses of small granule-containing cells in the uninjured contralateral ganglia. The incoming synapses of the small granule-containing cells are multifocal, i.e. show several points or active foci of synaptic specialization. The increase in synapses expressed itself both through an increased incidence of these synaptic active foci per nerve terminal and through an increase in the number of presynaptic nerve terminal profiles associated with the cells. Control observations indicated that the increase in synapses was not due to surgical stress, nor was it attributable solely to post-operative ageing. The nerve terminals which were presynaptic to the small granule-containing cells post-operatively were all of preganglionic origin: no incoming synapses or presynaptic nerve terminals remained at 2 days after a preganglionic denervation of axotomized or contralateral ganglia, at whatever stage this was performed throughout the range of survival intervals. There was some evidence that the synapses had increased by sprouting, including terminal sprouting, of the preganglionic nerve fibres. In the shorter term there was an increase in the proportion of small nerve terminal profiles. In the longer term the mean size of the terminal profiles increased, and very large terminals of unusual form were seen. After post-ganglionic axotomy, and in particular after a bilateral lesion, the small granule-containing cells became hypertrophied.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Voltage-dependent properties of electrical synapses formed between identified leech neurones in vitro.

1. The voltage-dependent properties of rectifying and non-rectifying electrical synapses formed between identified leech neurones were quantified during their regeneration in vitro. 2. Junctional conductance increased with time in culture. This was evaluated by making comparisons between cell pairs maintained in vitro for differing amounts of time, as well as by taking repeated measurements from a single cell pair at different time intervals. 3. Non-rectifying electrical synapses were formed between certain identified neurones of the same type. Thus, Leydig cells cultured with Leydig cells established non-rectifying electrical connections, as did Retzius cells, longitudinal motoneurones (L cells) and anterior pagoda (AP) cells, each paired with its own cell type. 4. Rectifying synapses developed when sensory neurones (P cells or N cells) were paired with the other neurones mentioned above that form non-rectifying connections between themselves. The cell combinations examined were L cell-P cell. Leydig cell-N cell, and AP cell-P cell. The direction of current flow across these rectifying synapses was consistently from the sensory neurone to the other cell in the pair. 5. Non-rectifying connections early in the process of synapse regeneration (1-3 days) showed non-linearities greater than those observed in established non-rectifying synapses. There was a subtle, but clear, voltage dependence even at the later stages of synapse formation (4-18 days). 6. In contrast to non-rectifying connections, rectifying synapses formed between cells at early times in culture showed less voltage dependence than those observed at later times. 7. The marked non-linearities of the non-rectifying connections at early stages in synapse formation along with the reduced voltage dependence of the rectifying connections within the same time period revealed unexpected similarities between the two. At the early stages of synapse formation, the two types of electrical synapse were essentially indistinguishable for one direction of junctional current.

Animals↗
PubMed2005

Target-specific short-term dynamics are important for the function of synapses in an oscillatory neural network.

Short-term dynamics such as facilitation and depression are present in most synapses and are often target-specific even for synapses from the same type of neuron. We examine the dynamics and possible functions of two synapses from the same presynaptic neuron in the rhythmically active pyloric network of the spiny lobster. Using simultaneous recordings, we show that the synapses from the lateral pyloric (LP) neuron to the pyloric dilator (PD; a member of the pyloric pacemaker ensemble) and the pyloric constrictor (PY) neurons both show short-term depression. However, the postsynaptic potentials produced by the LP-to-PD synapse are larger in amplitude, depress less, and recover faster than those produced by the LP-to-PY synapse. The main function of the LP-to-PD synapse is to slow down the pyloric rhythm. However, in some cases, it slows down the rhythm only when it is fast and has no effect or to speeds up when it is slow. In contrast, the LP-to-PY synapse functions to delay the activity of the PY neuron; this delay increases as the cycle period becomes longer. Using a computational model, we show that the short-term dynamics of synaptic depression observed for each of these synapses are tailored to their individual functions and that replacing the dynamics of either synapse with the other would disrupt these functions. Together, the experimental and modeling results suggest that the target-specific features of short-term synaptic depression are functionally important for synapses efferent from the same presynaptic neuron.

Animals↗
PubMed2004

Silent synapses in the immature visual cortex: layer-specific developmental regulation.

Central glutamatergic synapses are thought to initially form as immature, so-called silent synapses showing exclusively N-methyl-d-aspartate receptor-mediated synaptic transmission. Postsynaptic insertion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors during further development leads to a conversion into functional, mature synapses. Here, we tested the hypothesis that, according to the "inside first-outside last" pattern of neocortical layer formation and synaptogenesis, pyramidal cells in the superficial layers might show a higher fraction of silent synapses compared with pyramidal cells in the deep layers. We performed an electrophysiological analysis of glutamatergic synapses in acute rat visual cortex slices during postnatal development. In layer VI pyramidal neurons the incidence of silent synapses was high during the first postnatal week and strongly declined during further development. Surprisingly, in superficial cortical plate pyramidal neurons (immature layers II/III), the fraction of silent synapses was initially very low and increased up to the second postnatal week. Thereafter, a similar decline as found in layer VI pyramidal neurons was observed. Thus the developmental regulation of silent synapses was clearly different in pyramidal neurons from different neocortical layers. The almost complete absence of silent synapses at early stages in layer II/III pyramidal neurons indicates that an initially formed subset of synapses is constitutively functional. This might be important to enable spontaneous activity and latter activity-dependent maturation of synapses.

Animals↗