Perforated and non-perforated synapses in rat neocortex: three-dimensional reconstructions

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

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

Galactic cosmic radiation exposure causes multifaceted neurocognitive impairments

Technological advancements have facilitated the implementation of realistic, terrestrial-based complex 33-beam galactic cosmic radiation simulations (GCR Sim) to now probe central nervous system functionality. This work expands considerably on prior, simplified GCR simulations, yielding new insights into responses of male and female mice exposed to 40-50 cGy acute or chronic radiations relevant to deep space travel. Results of the object in updated location task suggested that exposure to acute or chronic GCR Sim induced persistent impairments in hippocampus-dependent memory formation and reconsolidation in female mice that did not manifest robustly in irradiated male mice. Interestingly, irradiated male mice, but not females, were impaired in novel object recognition and chronically irradiated males exhibited increased aggressive behavior on the tube dominance test. Electrophysiology studies used to evaluate synaptic plasticity in the hippocampal CA1 region revealed significant reductions in long-term potentiation after each irradiation paradigm in both sexes. Interestingly, network-level disruptions did not translate to altered intrinsic electrophysiological properties of CA1 pyramidal cells, whereas acute exposures caused modest drops in excitatory synaptic signaling in males. Ultrastructural analyses of CA1 synapses found smaller postsynaptic densities in larger spines of chronically exposed mice compared to controls and acutely exposed mice. Myelination was also affected by GCR Sim with acutely exposed mice exhibiting an increase in the percent of myelinated axons; however, the myelin sheathes on small calibur (< 0.3 mm) and larger (> 0.5 mm) axons were thinner when compared to controls. Present findings might have been predicted based on previous studies using single and mixed beam exposures and provide further evidence that space-relevant radiation exposures disrupt critical cognitive processes and underlying neuronal network-level plasticity, albeit not to the extent that might have been previously predicted.

3D Synaptic Organization of the Rat CA1 and Alterations Induced by Cocaine Self-Administration

The hippocampus plays a key role in contextual conditioning and has been proposed as an important component of the cocaine addiction brain circuit. To gain knowledge about cocaine-induced alterations in this circuit, we used focused ion beam milling/scanning electron microscopy to reveal and quantify the three-dimensional synaptic organization of the neuropil of the stratum radiatum of the rat CA1, under normal circumstances and after cocaine-self administration (SA). Most synapses are asymmetric (excitatory), macular-shaped, and in contact with dendritic spine heads. After cocaine-SA, the size and the complexity of the shape of both asymmetric and symmetric (inhibitory) synapses increased but no changes were observed in the synaptic density. This work constitutes the first detailed report on the 3D synaptic organization in the stratum radiatum of the CA1 field of cocaine-SA rats. Our data contribute to the elucidation of the normal and altered synaptic organization of the hippocampus, which is crucial for better understanding the neurobiological mechanisms underlying cocaine addiction.

Glutamatergic Dysfunction and Synaptic Ultrastructural Alterations in Schizophrenia and Autism Spectrum Disorder: Evidence from Human and Rodent Studies

The correlation between dysfunction in the glutamatergic system and neuropsychiatric disorders, including schizophrenia and autism spectrum disorder, is undisputed. Both disorders are associated with molecular and ultrastructural alterations that affect synaptic plasticity and thus the molecular and physiological basis of learning and memory. Altered synaptic plasticity, accompanied by changes in protein synthesis and trafficking of postsynaptic proteins, as well as structural modifications of excitatory synapses, are critically involved in the postnatal development of the mammalian nervous system. In this review, we summarize glutamatergic alterations and ultrastructural changes in synapses in schizophrenia and autism spectrum disorder of genetic or drug-related origin, and briefly comment on the possible reversibility of these neuropsychiatric disorders in the light of findings in regular synaptic physiology.

Cortical Presynaptic Boutons Progressively Engulf Spinules as They Mature

Despite decades of discussion in the neuroanatomical literature, the role of the synaptic "spinule" in synaptic development and function remains elusive. Canonically, spinules are finger-like projections that emerge from postsynaptic spines and can become enveloped by presynaptic boutons. When a presynaptic bouton encapsulates a spinule in this manner, the membrane apposition between the spinule and surrounding bouton can be significantly larger than the membrane interface at the synaptic active zone. Hence, spinules may represent a mechanism for extrasynaptic neuronal communication and/or may function as structural "anchors" that increase the stability of cortical synapses. Yet despite their potential to impact synaptic function, we have little information on the percentages of developing and adult cortical bouton populations that contain spinules, the percentages of these cortical spinule-bearing boutons (SBBs) that contain spinules from distinct neuronal/glial origins, or whether the onset of activity or cortical plasticity are correlated with increased prevalence of cortical SBBs. Here, we employed 2D and 3D electron microscopy to determine the prevalence of spinules in excitatory presynaptic boutons at key developmental time points in the primary visual cortex (V1) of female and male ferrets. We find that the prevalence of SBBs in V1 increases across postnatal development, such that ∼25% of excitatory boutons in late adolescent ferret V1 contain spinules. In addition, we find that a majority of spinules within SBBs at later developmental time points emerge from postsynaptic spines and adjacent boutons/axons, suggesting that synaptic spinules may enhance synaptic stability and allow for axo-axonal communication in mature sensory cortex.

3D Ultrastructural Study of Synapses in the Human Entorhinal Cortex

The entorhinal cortex (EC) is a brain region that has been shown to be essential for memory functions and spatial navigation. However, detailed three-dimensional (3D) synaptic morphology analysis and identification of postsynaptic targets at the ultrastructural level have not been performed before in the human EC. In the present study, we used Focused Ion Beam/Scanning Electron Microscopy to perform a 3D analysis of the synapses in the neuropil of medial EC in layers II and III from human brain autopsies. Specifically, we studied synaptic structural parameters of 3561 synapses, which were fully reconstructed in 3D. We analyzed the synaptic density, 3D spatial distribution, and type (excitatory and inhibitory), as well as the shape and size of each synaptic junction. Moreover, the postsynaptic targets of synapses could be clearly determined. The present work constitutes a detailed description of the synaptic organization of the human EC, which is a necessary step to better understand the functional organization of this region in both health and disease.

Three-dimensional synaptic organization of the human hippocampal CA1 field

The hippocampal CA1 field integrates a wide variety of subcortical and cortical inputs, but its synaptic organization in humans is still unknown due to the difficulties involved studying the human brain via electron microscope techniques. However, we have shown that the 3D reconstruction method using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) can be applied to study in detail the synaptic organization of the human brain obtained from autopsies, yielding excellent results. Using this technology, 24,752 synapses were fully reconstructed in CA1, revealing that most of them were excitatory, targeting dendritic spines and displaying a macular shape, regardless of the layer examined. However, remarkable differences were observed between layers. These data constitute the first extensive description of the synaptic organization of the neuropil of the human CA1 region.

A unifying theory for the pathoetiologic mechanism of tardive dyskinesia

INTRODUCTION

Chronic treatment with dopamine D2 receptor antagonists has been proposed to lead to dopamine receptor supersensitivity. Frequently, this is conceptualized as upregulation or changes in the structure or function of the post-synaptic D2 receptor. However, the measured 1.4-fold increase in D2 receptor density and the lack of actual receptor supersensitivity are probably inadequate to explain outcomes such as tardive dyskinesia (TD) and dopamine supersensitivity psychosis.

HYPOTHESIS

Recent data suggest that TD may result from a combination of presynaptic, synaptic, and postsynaptic changes.

DISCUSSION

Presynaptic increase in dopamine release occurs when super-therapeutic blockade of postsynaptic D2 receptors results in excess synaptic unbound dopamine which ultimately ends up being reuptaken by the presynaptic neuron through the dopamine transporter. The increased availability of recycled dopamine results in higher vesicular dopamine concentrations. Since the quantity of neurotransmitter released (known as quanta) is determined by the number of presynaptic neurotransmitter vesicles, the increase in the number (concentration) of dopamine molecules in the vesicles results in a higher concentration of synaptic dopamine with successive depolarization events. Synaptic changes such as the appearance of perforated synapses which is an early step in new synapse formation have been shown in animal models of TD. Finally, postsynaptic increases in D2 receptor expression without demonstration of increased sensitivity or potency has been demonstrated.

CONCLUSION

TD likely develops due to changes across the synapse and terminology such as 'dopamine receptor supersensitivity' can be misleading. 'Synaptic upregulation' may be a more correct term.

Study of the Size and Shape of Synapses in the Juvenile Rat Somatosensory Cortex with 3D Electron Microscopy

Changes in the size of the synaptic junction are thought to have significant functional consequences. We used focused ion beam milling and scanning electron microscopy (FIB/SEM) to obtain stacks of serial sections from the six layers of the rat somatosensory cortex. We have segmented in 3D a large number of synapses (n = 6891) to analyze the size and shape of excitatory (asymmetric) and inhibitory (symmetric) synapses, using dedicated software. This study provided three main findings. Firstly, the mean synaptic sizes were smaller for asymmetric than for symmetric synapses in all cortical layers. In all cases, synaptic junction sizes followed a log-normal distribution. Secondly, most cortical synapses had disc-shaped postsynaptic densities (PSDs; 93%). A few were perforated (4.5%), while a smaller proportion (2.5%) showed a tortuous horseshoe-shaped perimeter. Thirdly, the curvature was larger for symmetric than for asymmetric synapses in all layers. However, there was no correlation between synaptic area and curvature.

Protective effect of tetrahydroxy stilbene glucoside on learning and memory by regulating synaptic plasticity

Damage to synaptic plasticity induced by neurotoxicity of amyloid-beta is regarded to be one of the pathological mechanisms of learning and memory disabilities in Alzheimer's disease patients. This study assumed that the damage of amyloid-beta to learning and memory abilities was strongly associated with the changes in the Fyn/N-methyl-D-aspartate receptor 2B (NR2B) expression. An APP695V7171 transgenic mouse model of Alzheimer's disease was used and treatment with tetrahydroxy-stilbene glucoside was administered intragastrically. Results showed that intragastric administration of tetrahydroxy-stilbene glucoside improved the learning and memory abilities of the transgenic mice through increasing NR2B receptors and Fyn expression. It also reversed parameters for synaptic interface structure of gray type I. These findings indicate that tetrahydroxy stilbene glucoside has protective effects on the brain, and has prospects for its clinical application to improve the learning and memory abilities and treat Alzheimer's disease.

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

Neurons and especially their synapses often project long thin processes that can invaginate neighboring neuronal or glial cells. These "invaginating projections" can occur in almost any combination of postsynaptic, presynaptic, and glial processes. Invaginating projections provide a precise mechanism for one neuron to communicate or exchange material exclusively at a highly localized site on another neuron, e.g., to regulate synaptic plasticity. The best-known types are postsynaptic projections called "spinules" that invaginate into presynaptic terminals. Spinules seem to be most prevalent at large very active synapses. Here, we present a comprehensive review of all kinds of invaginating projections associated with both neurons in general and more specifically with synapses; we describe them in all animals including simple, basal metazoans. These structures may have evolved into more elaborate structures in some higher animal groups exhibiting greater synaptic plasticity. In addition to classic spinules and filopodial invaginations, we describe a variety of lesser-known structures such as amphid microvilli, spinules in giant mossy terminals and en marron/brush synapses, the highly specialized fish retinal spinules, the trophospongium, capitate projections, and fly gnarls, as well as examples in which the entire presynaptic or postsynaptic process is invaginated. These various invaginating projections have evolved to modify the function of a particular synapse, or to channel an effect to one specific synapse or neuron, without affecting those nearby. We discuss how they function in membrane recycling, nourishment, and cell signaling and explore how they might change in aging and disease.

Tetrahydroxystilbene glucoside improves the learning and memory of amyloid-β(₁₋₄₂)-injected rats and may be connected to synaptic changes in the hippocampus

The aim of this study was to evaluate the protective effects of 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG), an active component extracted from Polygonum multiflorum, on learning/memory deficits in Alzheimer's disease (AD). We randomly divided 24 male Sprague-Dawley rats among 4 groups: (i) the sham-operated group (control); (ii) sham-operated group also treated with TSG (sham+TSG); (iii) beta amyloid treated group (Aβ); and (iv) Aβ treatment group also treated with TSG (Aβ+TSG). Rats in the Aβ and Aβ+TSG groups were treated with Aβ₁₋₄₂ intracerebroventricularly, whereas the control and sham+TSG groups were given phosphate-buffered saline. Rats in the sham+TSG and Aβ+TSG groups were then treated intragastrically with TSG (50 mg·(kg body mass)⁻¹·day⁻¹) for 4 weeks, and rats in the Aβ and control groups were treated with saline. The results from Morris water maze tests, electron microscopy, real-time polymerase chain reaction, and Western blotting demonstrated that Aβ₁₋₄₂ induced impairment in learning and memory, degeneration in synaptic structures, and downregulation of Src and NR2B at the gene and protein level, respectively. These alterations were reversed by the administration of TSG, suggesting that TSG exerts anti-AD properties by protecting synaptic structure and function. TSG-induced upregulation of Src and NR2B may be responsible for this process.

Pansynaptic enlargement at adult cortical connections strengthened by experience

Behavioral experience alters the strength of neuronal connections in adult neocortex. These changes in synaptic strength are thought to be central to experience-dependent plasticity, learning, and memory. However, it is not known how changes in synaptic transmission between neurons become persistent, thereby enabling the storage of previous experience. A long-standing hypothesis is that altered synaptic strength is maintained by structural modifications to synapses. However, the extent of synaptic modifications and the changes in neurotransmission that the modifications support remain unclear. To address these questions, we recorded from pairs of synaptically connected layer 2/3 pyramidal neurons in the barrel cortex and imaged their contacts with high-resolution confocal microscopy after altering sensory experience by whisker trimming. Excitatory connections strengthened by experience exhibited larger axonal varicosities, dendritic spines, and interposed contact zones. Electron microscopy showed that contact zone size was strongly correlated with postsynaptic density area. Therefore, our findings indicate that whole synapses are larger at strengthened connections. Synaptic transmission was both stronger and more reliable following experience-dependent synapse enlargement. Hence, sensory experience modified both presynaptic and postsynaptic function. Our findings suggest that the enlargement of synaptic contacts is an integral part of long-lasting strengthening of cortical connections and, hence, of information storage in the neocortex.

Changes in synaptic morphology accompany actin signaling during LTP

Stabilization of long-term potentiation (LTP) is commonly proposed to involve changes in synaptic morphology and reorganization of the spine cytoskeleton. Here we tested whether, as predicted from this hypothesis, induction of LTP by theta-burst stimulation activates an actin regulatory pathway and alters synapse morphology within the same dendritic spines. TBS increased severalfold the numbers of spines containing phosphorylated (p) p21-activated kinase (PAK) or its downstream target cofilin; the latter regulates actin filament assembly. The PAK/cofilin phosphoproteins were increased at 2 min but not 30 s post-TBS, peaked at 7 min, and then declined. Double immunostaining for the postsynaptic density protein PSD95 revealed that spines with high pPAK or pCofilin levels had larger synapses (+60-70%) with a more normal size frequency distribution than did neighboring spines. Based on these results and simulations of shape changes to synapse-like objects, we propose that theta stimulation markedly increases the probability that a spine will enter a state characterized by a large, ovoid synapse and that this morphology is important for expression and later stabilization of LTP.

Olfactory learning-induced morphological modifications in single dendritic spines of young rats

Learning-related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague-Dawley rats. We have previously shown that olfactory discrimination rule-learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning-induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser-scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule-learning enhance the cortical network ability to enter into a 'learning mode', in which memories of new odours can be acquired rapidly and efficiently.

Remodeling of synaptic membranes after induction of long-term potentiation

Several morphological changes of synapses have been reported to be associated with the induction of long-term potentiation (LTP) in the CA1 hippocampus, including an transient increase in the proportion of synapses with perforated postsynaptic densities (PSDs) and a later occurrence of multiple spine boutons (MSBs) in which the two spines arise from the same dendrite. To investigate the functional significance of these modifications, we analyzed single sections and reconstructed 134 synapses labeled via activity using a calcium precipitation approach. Analyses of labeled spine profiles showed changes of the spine head area, PSD length, and proportion of spine profiles containing a coated vesicle that reflected variations in the relative proportion of different types of synapses. Three-dimensional reconstruction indicated that the increase of perforated spine profiles observed 30 min after LTP induction essentially resulted from synapses exhibiting segmented, completely partitioned PSDs. These synapses had spine head and PSD areas approximately three times larger than those of simple synapses. They contained coated vesicles in a much higher proportion than that of any other type of synapse and exhibited large spinules associated with the PSD. Also the MSBs with two spines arising from the same dendrite that were observed 1-2 hr after LTP induction included a spine that was smaller and a PSD that was smaller than those of simple synapses. These results support the idea that LTP induction is associated with an enhanced recycling of synaptic membrane and that this process could underlie the formation of synapses with segmented PSDs and eventually result in the formation of a new, immature spine.

Age-changes of brain synapses and synaptic plasticity in response to an enriched environment

Numerical synaptic density and synaptic vesicle density in rat frontal cortex were examined by electron microscopy as a function of age. The density of axospinous synapses, a major population of synapses, was found to peak at age 1 month, and to gradually decrease with aging. The synaptic vesicle density in axospinous synapses was shown to rapidly increase to a peak during the first 3 weeks and then decrease to the adult level, which remained unchanged in senescence. The time course of synaptic changes in aging is presented in this study. In a previous report (Saito et al. [1994] J. Neurosci. Res. 39:57-62), we showed that enriched rearing conditions restored the age-related decrease of synaptophysin contents. This might be due to increased numerical synaptic density or enhanced packing density of synaptic vesicles in synapses. The results of the present study support the latter explanation; that is, synaptic vesicle contents were increased without changes in synaptic density. Synaptic plasticity induced by environmental stimulation is shown to relate with synaptic strengthening, but not with the formation of new synapses.

Intramembranous structure of synaptic membranes with special reference to spinules in the rat sensorimotor cortex

The intramembranous structure of the synaptic contact zone at presynaptic and postsynaptic membranes in the rat sensorimotor cortex was examined by means of the freeze-etching technique. In axospinous synapses, the synaptic contact zone is characterized by perforated and nonperforated aggregates of intramembranous particles at the extracellular half or E-face of the postsynaptic membrane. On some perforated synaptic contact zones, both synaptic membranes are marked by so called spinules. These invaginations of the postsynaptic membrane and the parallel presynaptic membrane into the axon terminal are situated at the particle free zones among the postsynaptic E-face intramembranous particle aggregates or in close proximity to it. Intramembranous characteristics of the spinules at both freeze-etched faces of presynaptic and postsynaptic membranes and their density of perforated axospinous synapses were analysed. The results are discussed in terms of plasticity at the synaptic contact zone of the axospinous synapses of the sensorimotor cortex in the rat.

Sequential changes in the synaptic structural profile following long-term potentiation in the rat dentate gyrus: I. The intermediate maintenance phase

Changes in synaptic structure have been reported following the induction of long-term potentiation (LTP). The structure of synapses during the intermediate maintenance of LTP has yet to be fully characterized in chronically implanted freely moving animals. The present study examined synapses in the middle third of the molecular layer (MML) of the rat dentate gyrus following repeated high frequency tetanization of the perforant path. Synapses from both 1) the ipsilateral inner third of the dentate molecular layer (IML), which was not directly stimulated during the induction of LTP, as well as 2) implanted, nonstimulated animals, served as controls. LTP was induced over a 4-h period, and the animals were sacrificed 24 h after the final stimulation of the LTP group. Ultrastructural quantification included the total number of synapses, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic contact. Although LTP was not associated with an overall increase in synaptic number, there was a significant increase in the proportion of presynaptically concave-shaped synapses. Further, the concave synapses in the LTP tissue were found to be significantly smaller than control concave synapses. There was also a significant increase in the number of perforated concave synapses which exceeded the overall increase in concave synapses, and occurred despite the lack of a general increase in perforated synapses. It was concluded that this specific structural profile, observed at 24 h postinduction, may help support the potentiated response observed at this stage of LTP maintenance.

Intramembranous structure of the postsynaptic membrane in the rat sensorimotor cortex

Intramembranous structure of the postsynaptic membrane of the axodendritic synapses in the rat sensorimotor cortex was examined by means of freeze-etching technique. Perforated and non-perforated aggregates of particles were found at the extracellular half (E-face) of the postsynaptic membrane. To study correlation between membrane structure and synaptic plasticity we compared size and particle packing density in both type aggregates of particles at the E-face of the postsynaptic dendritic spine membrane. The results are discussed in terms of plasticity on the synaptic contact zone (SCZ) at the postsynaptic membrane of the excitatory axospinous synapses.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Development of human lateral geniculate nucleus: an electron microscopic study

A qualitative and quantitative ultrastructural study has been carried out on lateral geniculate nuclei (LGN) of 21 human fetuses ranging in gestational age from 13-14 to 34-35 weeks. At the early age period of 13-15 weeks, LGN is characterized by immature cells with indented nuclei possessing multiple nucleoli and by a sparse neuropil. During the subsequent age periods studied progressive maturational changes lead to neurons having round nuclei with a single nucleolus and well-developed cytoplasmic organelles as well as to an elaborate neuropil. Synaptic contacts which are seen for the first time at the age of 13-14 weeks are of retinogeniculate type. They show features of immature synapses and are located mainly on the juxtasomatic parts of dendrites. With increasing gestational age, the synapses increase in size, maturity, types and complexity; an acquisition of complex synaptic arrangement (triad) occurs by 20-21 weeks. Excitatory synapses appear earlier than do the inhibitory ones. Formation of retinogeniculate contacts precedes that of the corticogeniculate type. The synaptic density and total synapse number show a progressive increase with increasing gestational age. The age period of 15-20 weeks of gestation is marked by presence of organelles suggestive of a high rate of metabolic activity, significant increase in synaptogenesis, presence of transient contacts on soma and large number of free postsynaptic membrane densities (PSD). The period thereby represents a critical period in the development of synapses in LGN. The numerical values obtained by by the age of 34-35 weeks are still low as compared to the values reported for other areas of brain.

Ultrastructural and morphometric features of the acetylcholine innervation in adult rat parietal cortex: an electron microscopic study in serial sections

This study was aimed at characterizing the ultrastructural morphology of the normal acetylcholine (ACh) innervation in adult rat parietal cortex. After immunostaining with a monoclonal antibody against purified rat brain choline acetyltransferase (ChAT), more than 100 immunoreactive axonal varicosities (terminals) from each layer of the Par 1 area were photographed and examined in serial thin sections across their entire volume. These varicosities were relatively small, averaging 0.6 micron in diameter, 1.6 microns 2 in surface, and 0.12 micron 3 in volume. In every layer, a relatively low proportion exhibited a synaptic membrane differentiation (10% in layer I, 14% in II-III, 11% in IV, 21% in V, 14% in VI), for a I-VI average of 14%. These synaptic junctions were usually single, symmetrical (> 99%), and occupied a small portion of the surface of varicosities (< 3%). A majority were found on dendritic branches (76%), some on spines (24%), and none on cell bodies. On the whole, the ACh junctional varicosities were significantly larger than their nonjunctional counterparts, and both synaptic and nonsynaptic varicosities could be observed on the same fiber. A subsample of randomized single thin sections from these whole varicosities yielded similar values for size and synaptic frequency as the result of a stereological extrapolation. Also analyzed in single sections, the microenvironment of the ChAT-immunostained varicosities appeared markedly different from that of unlabeled varicosity profiles randomly selected from their vicinity, mainly due to a lower incidence of synaptically targeted dendritic spines. Thus, the normal ACh innervation of adult rat parietal cortex is predominantly nonjunctional (> 85% of its varicosities), and the composition of the microenvironment of its varicosities suggests some randomness in their distribution at the microscopic level. It is unlikely that these ultrastructural characteristics are exclusive to the parietal region. Among other functional implications, they suggest that this system depends predominantly on volume transmission to exert its modulatory effects on cortical activity.

Social environment affects synaptic structure in the glomerulus of the accessory olfactory bulb of the hamster

The accessory olfactory bulb (AOB) is the primary center of the vomeronasal chemosensory system. The effects of differential rearing on the numerical density and size of synapse as well as neuronal size were examined in the AOB of adult hamster. At 30 days from birth, male littermates were assigned to one of three experimental conditions. (1) the IC (isolated condition), where a male animal was housed alone, (2) the NC (neighbor condition), where one male was separated from two females by wire shields, and (3) the SC (social condition), where two males and two females were housed together. After 2 months of differential rearing, the AOBs of male littermates from each experimental set were prepared for morphological examination. The lengths of synaptic contact zones and the density of synapses in the glomeruli of the AOB as well as the area of somata of mitral/tufted cells were measured with an image analyzer. The synaptic contact zone was longer in the SC compared with both the IC and the NC while there was no difference in the synaptic density among the three groups. The somal area of mitral/tufted cells was larger in both the SC and the NC compared with the IC. These results indicate that the exposure to different rearing conditions induces differential morphological changes in both synapses and somata in the AOB of adult hamster.

Structure and plasticity of newly formed adult synapses: a morphometric study in the rat hippocampus

Increasing evidence suggests that synaptic structure represents a plastic feature of the neuron, although the plastic nature of newly formed and existing adult synapses has not yet been fully characterized. Following ipsilateral entorhinal cortical lesions, the rat dentate gyrus offers an excellent model for studying synaptogenesis and plasticity in the adult central nervous system. Unilateral entorhinal lesions were performed in young adult male rats. Synaptic counts and structural features were quantified at 3, 6, 10, 15, and 30 days post-lesion. The lesions resulted in an 88% synaptic loss in the denervated dentate middle molecular layer, which was followed by a period of rapid synaptogenesis. Synaptic element size decreased during the period of maximal synaptogenesis, which was associated with a peak in the presence of non-vesicular and perforated synapses. Following this period, synapses showed a gradual increase in the size of their pre- and postsynaptic elements. These data support the suggestion that newly formed adult synapses have smaller synaptic components than existing adult synapses (resembling synapses seen during development), and increase in size over time with usage. The results are discussed in terms of synaptic structural development and plasticity in the adult central nervous system.

Effects of differential rearing on the structure of perforated synapses in the granule cell layer of the rat's accessory olfactory bulb

The effects of differential rearing on synaptic morphology in the granule cell layer of the accessory olfactory bulb (AOB) were examined in adult rats. Forty-day-old male rats were housed in one of three ways: individually (isolated condition, IC); with four males per cage (unisexual condition, UC); or with two males and two females per cage (social condition, SC). After 2 months, the animals were killed for electron microscopy. Two types of synapses were classified: (1) perforated synapses, which are characterized by discontinuities in their postsynaptic thickenings, and (2) non-perforated synapses. The length of the synaptic contact zone as well as the area and the length of the perimeter of both the pre- and postsynaptic terminals were measured in each observed synapse. For perforated synapses, the length of the synaptic contact zone was significantly greater in the UC and the SC than in the IC. For the presynaptic terminals of perforated synapses, the area was greater in the UC and the SC than in the IC, and the length of the perimeter was greater in the UC than in the IC. In contrast, there was no difference in the area and the length of the perimeter among the three groups for postsynaptic terminals. For non-perforated synapses, there was no statistically significant difference in either the area or the length of the perimeter of pre- and postsynaptic terminals among the three experimental groups, although the length of the synaptic contact zone was greater in the SC than in the IC.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphological characteristics of perforated synapses in the latter stages of synaptogenesis in rat neocortex: stereological and three-dimensional approaches

Over recent years attention has been focused on perforated synapses (PSs), on account of their possible involvement in synaptic plasticity in the nervous system. The mechanism invoked has involved the conversion of conventional (non-perforated) synapses into perforated ones, with the latter subsequently splitting to form new daughter synapses. Data challenging this mechanism have emerged from studies using unbiased stereological and three-dimensional reconstruction procedures in early synaptogenesis and also in early- to mid-adulthood. The present study is an attempt to complement these earlier studies by concentrating on the latter part of synaptogenesis and early adulthood in rats (7-60 days postnatal). Using the disector method, the frequency of perforated synapses increases with age from 12% at seven days to 33% at 60 days. Reconstruction of the perforated synapses has led to the description of three sub-types, while quantitation of the reconstructions has highlighted major differences between perforated and non-perforated synapses over this developmental period. For instance, the postsynaptic density of perforated synapses increases in size with age, but remains static in the non-perforated variety; in addition, it is 2-3 times larger in perforated synapses. The proportion of the synaptic contact zone occupied by the postsynaptic density increases in perforated synapses with increasing age, but is static in non-perforated synapses. No evidence has been found to support a perforated synapse splitting model, since perforated synapses are present from early in synaptogenesis, the frequency of non-perforated synapses reaches a peak prior to, but not following, that of perforated synapses, and the size of the postsynaptic density of non-perforated synapses remains constant throughout the period of the study.

Pre- and postnatal development of the primary visual cortex of the common marmoset. II. Formation, remodelling, and elimination of synapses as overlapping processes

During ontogenesis changes in the numerical density of synapses are usually assumed to depend essentially on variations in the formation of synapses. Only the final adjustment to adult synapse densities is thought to include the elimination of synapses in some brain regions of certain species. Here, we focus attention on quantitative aspects of synapse elimination throughout development of area 17 of marmoset monkeys (Callithrix jacchus). Mature synapses, various precursor forms, and indicators of lysosomal degradation of synapses were quantitatively analysed by electron microscopy and morphometric methods. A total number of about 135 x 10(9) synapses was calculated for area 17 in each adult hemisphere corresponding to a volume density of 600 x 10(6) synapses/mm3. At 3 months of age, the respective values were 508 x 10(9)/area and 1,159 x 10(6)/mm3, while at birth these values were 69 x 10(9)/area and 328 x 10(6)/mm3. Consequently, at least three out of four synapses are eliminated between 3 months and adulthood. However, the real number of synapses being eliminated during development is probably much larger if the time course of lysosomal degradation is additionally taken into account. The frequency of lysosomes in presynaptic endings is highest before net-elimination of synapses occurs, i.e., between 1 and 3 months. This suggests that lysosomal degradation is not directly responsible for the majority of synapses removed during ontogenesis but apparently represents a second mechanism for synapse remodelling and elimination. Thus, it appears from this study that remodelling and elimination of synapses are quantitatively as important as their formation, and accompany synaptogenesis from its very onset onwards.

Effect of differential rearing on synapses and soma size in rat medial amygdaloid nucleus

The effects of differential rearing on synaptic morphology and neuronal size were examined in the medial amygdaloid nucleus (MAN) of adult rats. Forty-day-old male rats were housed in one of three ways: individually (isolated condition, IC); with four males per cage (unisexual condition, UC); or with two males and two females per cage (social condition, SC). After 2 months, the animals were prepared for electron and light microscopy. Although there was no statistically significant difference in number of synapses per unit volume in MAN, the number of perforated (P) synapses, which are characterized by discontinuities in the postsynaptic density, were significantly greater in the UC and the SC than in the IC. The length of synaptic contact zone of P synapses was also longer in both the UC and the SC compared with the IC, whereas the length of nonperforated synapses was longer only in the SC. Soma area was also larger in the SC compared with the IC. These results demonstrate that exposure to different rearing conditions, in which the pheromonal environment can be substantially different, can induce striking morphological changes in both synapses and neurons in the MAN of adult rats.

Perforated synapses are present during synaptogenesis in rat neocortex

It has been known for some time that perforated synapses increase in number and size with increasing age. Initially, these trends were used to support the concept that nonperforated synapses enlarge until an optimal size is reached, at which point they perforate and may subsequently split. More recent stereological and three-dimensional reconstruction investigations, however, suggest that this may not be the case and that perforated and nonperforated synapses constitute separate synaptic populations. In order to test the separate population hypothesis, synapses have been studied ultrastructurally in the parietal cortex of rats aged 19 and 20 days gestation, and 1 and 4 days postnatal. By examining synapses serially, and also by studying three-dimensional reconstructions, it has been demonstrated that perforated synapses are present at each of these ages. Some are relatively simple in organization, resembling previously described perforated synapses at 14 days of age, although others appear to consist of two or more separate PSD components. These findings demonstrate that perforated synapses are present from early on in synaptogenesis and that developing perforated synapses may have distinct characteristics that cast light on their developmental course.

Frequency of occurrence of perforated synapses in developing rat neocortex

The frequency of occurrence of perforated synapses in the molecular layer of rat parietal cortex has been determined using the disector method. Perforated and non-perforated synapses were examined in electron micrographs, all quantitative procedures being carried out on adjacent sections, as required by the disector procedure. The numerical densities of perforated, non-perforated and all synapses were determined at nine ages, ranging from 0.5 to 22 months of age. The frequencies of perforated synapses were subsequently calculated. These ranged from 18.1% at 0.5 months to a peak of 37.2% at 10 months of age, and remained at around 34% up to 19 months. A frequency of 26.7% was found at 22 months of age. The mean projected height of the total perforation increased from 66 nm at 0.5 months to 133 nm at 22 months of age. These results suggest that perforated synapses are present throughout the life-span of the animal, and increase in complexity with increasing age.

Perforated synapses and plasticity. A developmental overview

Against a background of existing models relating perforated synapses to synaptic plasticity, the numerical density and frequency of perforated synapses in rat neocortex have been assessed from 1 d to 22 mo of age using the disector procedure, and changes in their morphology were assessed using 3-D computer reconstructions. The data point toward perforated and nonperforated synapses being separate synaptic populations from early in development, and with perforated synapses playing a part in the maintenance of neuronal postsynaptic density surface area from mid-adulthood onwards. This suggests that they play a crucial role in synaptic plasticity, although its nature may be different from that postulated by most recent workers.