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.

A Quantitative Study on the Distribution of Mitochondria in the Neuropil of the Juvenile Rat Somatosensory Cortex.

Mitochondria play a key role in energy production and calcium buffering, among many other functions. They provide most of the energy required by neurons, and they are transported along axons and dendrites to the regions of higher energy demands. We have used focused ion beam milling and scanning electron microscopy (FIB/SEM) to obtain stacks of serial sections from the somatosensory cortex of the juvenile rat. We have estimated the volume fraction occupied by mitochondria and their distribution between dendritic, axonal, and nonsynaptic processes. The volume fraction of mitochondria increased from layer I (4.59%) to reach its maximum in layer IV (7.74%) and decreased to its minimum in layer VI (4.03%). On average, 44% of mitochondrial volume was located in dendrites, 15% in axons and 41% in nonsynaptic elements. Given that dendrites, axons, and nonsynaptic elements occupied 38%, 23%, and 39% of the neuropil, respectively, it can be concluded that dendrites are proportionally richer in mitochondria with respect to axons, supporting the notion that most energy consumption takes place at the postsynaptic side. We also found a positive correlation between the volume fraction of mitochondria located in neuronal processes and the density of synapses.

Layer-specific alterations to CA1 dendritic spines in a mouse model of Alzheimer's disease.

Why memory is a particular target for the pathological changes in Alzheimer's Disease (AD) has long been a fundamental question when considering the mechanisms underlying this disease. It has been established from numerous biochemical and morphological studies that AD is, at least initially, a consequence of synaptic malfunction provoked by Amyloid ß (Aß) peptide. APP/PS1 transgenic mice accumulate Aß throughout the brain, and they have therefore been employed to investigate the effects of Aß overproduction on brain circuitry and cognition. Previous studies show that Aß overproduction affects spine morphology in the hippocampus and amygdala, both within and outside plaques (Knafo et al., (2009) Cereb Cortex 19:586-592; Knafo et al., (in press) J Pathol). Hence, we conducted a detailed analysis of dendritic spines located in the stratum oriens and stratum radiatum of the CA1 hippocampal subfield of APP/PS1 mice. Three-dimensional analysis of 18,313 individual dendritic spines revealed a substantial layer-specific decrease in spine neck length and an increase in the frequency of spines with a small head volume. Since dendritic spines bear most of the excitatory synapses in the brain, changes in spine morphology may be one of the factors contributing to the cognitive impairments observed in this AD model.

Gender differences in human cortical synaptic density.

Certain cognitive functions differ in men and women, although the anatomical and functional substrates underlying these differences remain unknown. Because neocortical activity is directly related with higher brain function, numerous studies have focused on the cerebral cortex when searching for possible structural correlates of cognitive gender differences. However, there are no studies on possible gender differences at the synaptic level. In the present work we have used stereological and correlative light and electron microscopy to show that men have a significantly higher synaptic density than women in all cortical layers of the temporal neocortex. These differences may represent a microanatomical substrate contributing to the functional gender differences in brain activity.

3D Analysis of the Synaptic Organization in the Entorhinal Cortex in Alzheimer's Disease.

The entorhinal cortex (EC) is especially vulnerable in the early stages of Alzheimer's disease (AD). In particular, cognitive deficits have been linked to alterations in the upper layers of EC. In the present report, we examined Layers II and III from eight human brain autopsies (four subjects with no recorded neurologic alterations and four AD cases). We used stereological methods to assess cortical atrophy of the EC and possible changes in the volume occupied by different cortical elements (neuronal and glial cell bodies; blood vessels; and neuropil). We performed 3D ultrastructural analyses of synapses using focused ion beam/scanning electron microscopy (FIB/SEM) to examine possible alterations related to AD. At the light microscope level, we found a significantly lower volume fraction occupied by neuronal bodies in Layer III and a higher volume fraction occupied by glial cell bodies in Layer II in AD cases. At the ultrastructural level, we observed that (1) there was a significantly lower synaptic density in both layers in AD cases; (2) synapses were larger and more complex in Layer II in AD cases; and (3) there was a greater proportion of small and simple synapses in Layer III in AD cases than in control individuals. These structural differences may play a role in the anatomic basis for the impairment of cognitive functions in AD.

Widespread changes in dendritic spines in a model of Alzheimer's disease.

The mechanism by which dementia occurs in patients with Alzheimer's disease (AD) is not known. We assessed changes in hippocampal dendritic spines of APP/PS1 transgenic mice that accumulate amyloid beta throughout the brain. Three-dimensional analysis of 21,507 dendritic spines in the dentate gyrus, a region crucial for learning and memory, revealed a substantial decrease in the frequency of large spines in plaque-free regions of APP/PS1 mice. Plaque-related dendrites also show striking alterations in spine density and morphology. However, plaques occupy only 3.9% of the molecular layer volume. Because large spines are considered to be the physical traces of long-term memory, widespread decrease in the frequency of large spines likely contributes to the cognitive impairments observed in this AD model.

3D Electron Microscopy Study of Synaptic Organization of the Normal Human Transentorhinal Cortex and Its Possible Alterations in Alzheimer's Disease.

The transentorhinal cortex (TEC) is an obliquely oriented cortex located in the medial temporal lobe and, together with the entorhinal cortex, is one of the first affected areas in Alzheimer's disease (AD). One of the most widely accepted hypotheses is that synaptopathy (synaptic alterations and loss) represents the major structural correlate of the cognitive decline observed in AD. However, very few electron microscope (EM) studies are available; the most common method to estimate synaptic density indirectly is by counting, at the light microscopic level, immunoreactive puncta using synaptic markers. To investigate synaptic morphology and possible alterations related to AD, a detailed three-dimensional (3D) ultrastructural analysis using focused ion beam/scanning EM (FIB/SEM) was performed in the neuropil of Layer II of the TEC in human brain samples from non-demented subjects and AD patients. Evaluation of the proportion and shape of asymmetric synapses (AS) and symmetric synapses (SS) targeting spines or dendritic shafts was performed using 3D reconstructions of every synapse. The 3D analysis of 4722 synapses revealed that the preferable targets were spine heads for AS and dendritic shafts for SS, both in control and AD cases. However, in AD patients, we observed a reduction in the percentage of synapses targeting spine heads. Regarding the shape of synapses, in both control cases and AD samples, the vast majority of synapses had a macular shape, followed by perforated or horseshoe-shaped synapses, with fragmented synapses being the least frequent type. Moreover, comparisons showed an increased number of fragmented AS in AD patients.

Three-dimensional analysis of synapses in the transentorhinal cortex of Alzheimer's disease patients.

Synaptic dysfunction or loss in early stages of Alzheimer's disease (AD) is thought to be a major structural correlate of cognitive dysfunction. Early loss of episodic memory, which occurs at the early stage of AD, is closely associated with the progressive degeneration of medial temporal lobe (MTL) structures of which the transentorhinal cortex (TEC) is the first affected area. However, no ultrastructural studies have been performed in this region in human brain samples from AD patients. In the present study, we have performed a detailed three-dimensional (3D) ultrastructural analysis using focused ion beam/scanning electron microscopy (FIB/SEM) to investigate possible synaptic alterations in the TEC of patients with AD. Surprisingly, the analysis of the density, morphological features and spatial distribution of synapses in the neuropil showed no significant differences between AD and control samples. However, light microscopy studies showed that cortical thickness of the TEC was severely reduced in AD samples, but there were no changes in the volume occupied by neuronal and glial cell bodies, blood vessels, and neuropil. Thus, the present results indicate that there is a dramatic loss of absolute number of synapses, while the morphology of synaptic junctions and synaptic spatial distribution are maintained. How these changes affect cognitive impairment in AD remains to be elucidated.

Quantitative analysis of parvalbumin-immunoreactive cells in the human epileptic hippocampus.

Hippocampal sclerosis is the most frequent pathology encountered in mesial temporal structures resected from patients with intractable temporal lobe epilepsy and it mainly involves hippocampal neuronal loss and gliosis. These alterations are accompanied by changes in the expression of a variety of molecules in the surviving neurons, as well as axonal reorganization in both excitatory and inhibitory circuits. The alteration of a subpopulation of GABAergic interneurons that expresses the calcium binding protein parvalbumin (PV) is thought to be a key factor in the epileptogenic process. We investigated the distribution and density of parvalbumin-immunoreactive (PV-ir) neurons in surgically resected hippocampal tissue from epileptic patients with and without sclerosis. Using quantitative stereological methods, we show for the first time that there is no correlation between total neuronal loss and PV-ir neuronal loss in any of the hippocampal fields. We also observed higher values of the total neuronal density in the sclerotic subiculum, which is accompanied by a lower density of PV-ir when compared with non-sclerotic epileptic and autopsy hippocampi. These findings suggest that, the apparently normal subiculum from sclerotic patients also shows unexpected changes in the density and proportion of PV-ir neurons.

Microanatomy of the dysplastic neocortex from epileptic patients.

Focal cortical dysplasia (FCD) is a pathology that is characterized by the abnormal development of the neocortex. Indeed, a wide range of abnormalities in the cortical mantle have been associated with this pathology, including cytoarchitectonic alterations and the presence of dysmorphic neurons, balloon cells and ectopic neurons in the white matter. FCD is commonly associated with epilepsy, and hence we have studied the ultrastructure of cortical tissue resected from three subjects with intractable epilepsy secondary to cortical dysplasia to identify possible alterations in synaptic circuitry, using correlative light and electron microscopic methods. While the balloon cells found in this tissue do not appear to receive synaptic contacts, the ectopic neurons in the white matter were abnormally large and were surrounded by hypertrophic basket formations immunoreactive for the calcium-binding protein parvalbumin. Furthermore, these basket formations formed symmetrical (inhibitory) synapses with both the somata and the proximal portion of the dendrites of these giant ectopic neurons. A quantitative analysis revealed that in the dysplastic tissue, the density of excitatory and inhibitory synapses was different from that of the normal adjacent cortex. Both increases and decreases in synaptic density were observed, as well as changes in the proportion of excitatory and inhibitory synapses. However, we could not establish a common pattern of changes, either in the same patients or between different patients. These results suggest that cortical dysplasia leads to multiple changes in excitatory and inhibitory synaptic circuits. We discuss the possible relationship between these alterations and epilepsy, bearing in mind the possible limitations that preclude the extrapolation of the results to the whole population of epileptic patients with dysplastic neocortex.

Vesicular glutamate transporter 1 immunostaining in the normal and epileptic human cerebral cortex.

Glutamate is the main excitatory neurotransmitter in the brain where, due to the activity of specific vesicular glutamate transporters, it accumulates in synaptic vesicles. The vesicular glutamate transporter 1 is found in the majority of axon terminals that form asymmetrical (excitatory) synapses in the rat neocortex. However, since there is no information available regarding the distribution of vesicular glutamate transporter 1 in the human neocortex, we have used correlative light and electron microscopy to define its expression in this tissue. We found that the distribution of vesicular glutamate transporter 1-immunoreactivity is virtually identical to that found in the rat neocortex, both at the light and electron microscope levels. Therefore, we assessed whether vesicular glutamate transporter 1 immunostaining might be a useful tool to study the pathological alterations of glutamatergic transmission in the epileptic cerebral cortex. We analyzed the distribution of vesicular glutamate transporter 1 in the peritumoral neocortex of patients with epilepsy secondary to low-grade tumors. In these regions, we found alterations in the pattern of vesicular glutamate transporter 1-immunoreactivity that perfectly matched the neuronal loss and gliosis, as well as the decrease in the number of asymmetrical synapses identified by electron microscopy in this tissue. Thus, vesicular glutamate transporter 1 immunostaining appears to be a reliable and simple tool to study glutamatergic synapses in the normal and epileptic human cerebral cortex.

Perisomatic glutamatergic axon terminals: a novel feature of cortical synaptology revealed by vesicular glutamate transporter 1 immunostaining.

The cellular localization of the vesicular glutamate transporter 1, VGLUT1, was studied in the rat cerebral cortex with immunocytochemical techniques. VGLUT1 immunoreactivity (ir) was localized to punctate structures dispersed in the neuropil of all cortical layers as well as around the profile of somata and proximal dendritic segments of virtually all pyramidal neurons. Using a correlative light and electron microscopic method, we found that VGLUT1 ir is expressed in axon terminals forming synapses exclusively with dendritic shafts and spines. Perisomatic VGLUT1-positive terminals never formed synapses with the pyramidal cell bodies to which they were in apposition, but formed asymmetric synapses with adjacent neuropilar dendritic elements. The high probability of a close spatial relationship between glutamatergic and GABAergic terminals in perisomatic regions suggests that spilled-out glutamate may act on inhibitory axon terminals innervating the soma of cortical pyramidal neurons.

Postnatal development of the vesicular GABA transporter in rat cerebral cortex.

Light and electron microscopic immunocytochemical techniques and Western blotting were used to investigate the postnatal development of the vesicular GABA transporter (VGAT) in the rat somatic sensory cortex. VGAT immunoreactivity was low at birth, it increased gradually through the first and second weeks of life and achieved the adult pattern during the third week. At postnatal day (P)0-P5, VGAT immunoreactivity was associated exclusively to fibers and puncta. Electron microscopic studies performed at P5 showed that all identified synaptic contacts formed by VGAT-positive axonal swellings were of the symmetric type and that a substantial proportion of the boutons appeared not to have formed synapses. From P10 onward, labeled puncta were both scattered in the neuropil and in apposition to unstained cellular profiles; VGAT was also expressed in few GABAergic cell bodies. Western blottings at the same postnatal ages revealed a 55-kDa band whose intensity was weak at P0 (17% of adult), it increased constantly until P15 (P2: 35%; P5: 44%; P10: 68%; P15: 97%), and then leveled off. Overall, the present results show that during neocortical development the expression of VGAT slightly precedes the complete maturation of inhibitory synaptogenesis and suggest that it may contribute to the formation of neocortical GABAergic circuitry.

Synaptology of the proximal segment of pyramidal cell basal dendrites.

Pyramidal neurons are covered with dendritic spines, the main postsynaptic targets of excitatory (asymmetrical) synapses. However, the proximal portion of both the apical and basal dendrites is devoid of spines, suggesting a lack of excitatory inputs to this region. In the present study we used electron microscopy to analyse the proximal region of the basal dendrites of supra- and infragranular pyramidal cells to determine if this is the case. The proximal region of 80 basal dendrites sampled from the rat hindlimb representation in the primary somatosensory cortex was studied by electron microscopy. A total of 317 synapses were found within this region of the dendrites, all of which were of the symmetrical type. These results suggest that glutamate receptors, although present in the cytoplasm, are not involved in synaptic junctions in the proximal portion of the dendrites. These data further support the idea that inhibitory terminals exclusively innervate the proximal region of basal dendrites.