How to count synapses unbiasedly and efficiently at the ultrastructural level: proposal for a standard sampling and counting protocol

Unambiguous identification of asymmetric and symmetric synapses using volume electron microscopy

The brain contains thousands of millions of synapses, exhibiting diverse structural, molecular, and functional characteristics. However, synapses can be classified into two primary morphological types: Gray's type I and type II, corresponding to Colonnier's asymmetric (AS) and symmetric (SS) synapses, respectively. AS and SS have a thick and thin postsynaptic density, respectively. In the cerebral cortex, since most AS are excitatory (glutamatergic), and SS are inhibitory (GABAergic), determining the distribution, size, density, and proportion of the two major cortical types of synapses is critical, not only to better understand synaptic organization in terms of connectivity, but also from a functional perspective. However, several technical challenges complicate the study of synapses. Potassium ferrocyanide has been utilized in recent volume electron microscope studies to enhance electron density in cellular membranes. However, identifying synaptic junctions, especially SS, becomes more challenging as the postsynaptic densities become thinner with increasing concentrations of potassium ferrocyanide. Here we describe a protocol employing Focused Ion Beam Milling and Scanning Electron Microscopy for studying brain tissue. The focus is on the unequivocal identification of AS and SS types. To validate SS observed using this protocol as GABAergic, experiments with immunocytochemistry for the vesicular GABA transporter were conducted on fixed mouse brain tissue sections. This material was processed with different concentrations of potassium ferrocyanide, aiming to determine its optimal concentration. We demonstrate that using a low concentration of potassium ferrocyanide (0.1%) improves membrane visualization while allowing unequivocal identification of synapses as AS or SS.

Sublamina-Specific Dynamics and Ultrastructural Heterogeneity of Layer 6 Excitatory Synaptic Boutons in the Adult Human Temporal Lobe Neocortex

Synapses “govern” the computational properties of any given network in the brain. However, their detailed quantitative morphology is still rather unknown, particularly in humans. Quantitative 3D-models of synaptic boutons (SBs) in layer (L)6a and L6b of the temporal lobe neocortex (TLN) were generated from biopsy samples after epilepsy surgery using fine-scale transmission electron microscopy, 3D-volume reconstructions and electron microscopic tomography. Beside the overall geometry of SBs, the size of active zones (AZs) and that of the three pools of synaptic vesicles (SVs) were quantified. SBs in L6 of the TLN were middle-sized (~5 μm²), the majority contained only a single but comparatively large AZ (~0.20 μm²). SBs had a total pool of ~1100 SVs with comparatively large readily releasable (RRP, ~10 SVs L6a), (RRP, ~15 SVs L6b), recycling (RP, ~150 SVs), and resting (~900 SVs) pools. All pools showed a remarkably large variability suggesting a strong modulation of short-term synaptic plasticity. In conclusion, L6 SBs are highly reliable in synaptic transmission within the L6 network in the TLN and may act as “amplifiers,” “integrators” but also as “discriminators” for columnar specific, long-range extracortical and cortico-thalamic signals from the sensory periphery.

Basic quantitative morphological methods applied to the central nervous system

Generating numbers has become an almost inevitable task associated with studies of the morphology of the nervous system. Numbers serve a desire for clarity and objectivity in the presentation of results and are a prerequisite for the statistical evaluation of experimental outcomes. Clarity, objectivity, and statistics make demands on the quality of the numbers that are not met by many methods. This review provides a refresher of problems associated with generating numbers that describe the nervous system in terms of the volumes, surfaces, lengths, and numbers of its components. An important aim is to provide comprehensible descriptions of the methods that address these problems. Collectively known as design‐based stereology, these methods share two features critical to their application. First, they are firmly based in mathematics and its proofs. Second and critically underemphasized, an understanding of their mathematical background is not necessary for their informed and productive application. Understanding and applying estimators of volume, surface, length or number does not require more of an organizational mastermind than an immunohistochemical protocol. And when it comes to calculations, square roots are the gravest challenges to overcome. Sampling strategies that are combined with stereological probes are efficient and allow a rational assessment if the numbers that have been generated are “good enough.” Much may be unfamiliar, but very little is difficult. These methods can no longer be scapegoats for discrepant results but faithfully produce numbers on the material that is assessed. They also faithfully reflect problems that associated with the histological material and the anatomically informed decisions needed to generate numbers that are not only valid in theory. It is within reach to generate practically useful numbers that must integrate with qualitative knowledge to understand the function of neural systems.

Ultrastructural heterogeneity of layer 4 excitatory synaptic boutons in the adult human temporal lobe neocortex

Synapses are fundamental building blocks controlling and modulating the ‘behavior’ of brain networks. How their structural composition, most notably their quantitative morphology underlie their computational properties remains rather unclear, particularly in humans. Here, excitatory synaptic boutons (SBs) in layer 4 (L4) of the temporal lobe neocortex (TLN) were quantitatively investigated. Biopsies from epilepsy surgery were used for fine-scale and tomographic electron microscopy (EM) to generate 3D-reconstructions of SBs. Particularly, the size of active zones (AZs) and that of the three functionally defined pools of synaptic vesicles (SVs) were quantified. SBs were comparatively small (~2.50 μm²), with a single AZ (~0.13 µm²); preferentially established on spines. SBs had a total pool of ~1800 SVs with strikingly large readily releasable (~20), recycling (~80) and resting pools (~850). Thus, human L4 SBs may act as ‘amplifiers’ of signals from the sensory periphery, integrate, synchronize and modulate intra- and extracortical synaptic activity.

Spaceflight-induced synaptic modifications within hair cells of the mammalian utricle

Exposure to the microgravity conditions of spaceflight alleviates the load normally imposed by the Earth's gravitational field on the inner ear utricular epithelia. Previous ultrastructural investigations have shown that spaceflight induces an increase in synapse density within hair cells of the rat utricle. However, the utricle exhibits broad physiological heterogeneity across different epithelial regions, and it is unknown whether capabilities for synaptic plasticity generalize to hair cells across its topography. To achieve systematic and broader sampling of the epithelium than was previously conducted, we used immunohistochemistry and volumetric image analyses to quantify synapse distributions across representative utricular regions in specimens from mice exposed to spaceflight (a 15-day mission of the space shuttle Discovery). These measures were compared with similarly sampled Earth-bound controls. Following paraformaldehyde fixation and microdissection, immunohistochemistry was performed on intact specimens to label presynaptic ribbons (anti-CtBP2) and postsynaptic receptor complexes (anti-Shank1A). Synapses were identified as closely apposed pre- and postsynaptic puncta. Epithelia from horizontal semicircular canal cristae served as "within-specimen" controls, whereas utricles and cristae from Earth-bound cohorts served as experimental controls. We found that synapse densities decreased in the medial extrastriolae of microgravity specimens compared with experimental controls, whereas they were unchanged in the striolae and horizontal cristae from the two conditions. These data demonstrate that structural plasticity was topographically localized to the utricular region that encodes very low frequency and static changes in linear acceleration, and illuminates the remarkable capabilities of utricular hair cells for synaptic plasticity in adapting to novel gravitational environments.NEW & NOTEWORTHY Spaceflight imposes a radically different sensory environment from that in which the inner ear utricle normally operates. We investigated synaptic modifications in utricles from mice flown aboard a space shuttle mission. Structural synaptic plasticity was detected in the medial extrastriola, a region associated with encoding static head position, as decreased synapse density. These results are remarkably congruent with a recent report of decreased utricular function in astronauts immediately after returning from the International Space Station.

Microglomerular Synaptic Complexes in the Sky-Compass Network of the Honeybee Connect Parallel Pathways from the Anterior Optic Tubercle to the Central Complex

While the ability of honeybees to navigate relying on sky-compass information has been investigated in a large number of behavioral studies, the underlying neuronal system has so far received less attention. The sky-compass pathway has recently been described from its input region, the dorsal rim area (DRA) of the compound eye, to the anterior optic tubercle (AOTU). The aim of this study is to reveal the connection from the AOTU to the central complex (CX). For this purpose, we investigated the anatomy of large microglomerular synaptic complexes in the medial and lateral bulbs (MBUs/LBUs) of the lateral complex (LX). The synaptic complexes are formed by tubercle-lateral accessory lobe neuron 1 (TuLAL1) neurons of the AOTU and GABAergic tangential neurons of the central body’s (CB) lower division (TL neurons). Both TuLAL1 and TL neurons strongly resemble neurons forming these complexes in other insect species. We further investigated the ultrastructure of these synaptic complexes using transmission electron microscopy. We found that single large presynaptic terminals of TuLAL1 neurons enclose many small profiles (SPs) of TL neurons. The synaptic connections between these neurons are established by two types of synapses: divergent dyads and divergent tetrads. Our data support the assumption that these complexes are a highly conserved feature in the insect brain and play an important role in reliable signal transmission within the sky-compass pathway.

EphB3 signaling propagates synaptic dysfunction in the traumatic injured brain

Traumatic brain injury (TBI), ranging from mild concussion to severe penetrating wounds, can involve brain regions that contain damaged or lost synapses in the absence of neuronal death. These affected regions significantly contribute to sensory, motor and/or cognitive deficits. Thus, studying the mechanisms responsible for synaptic instability and dysfunction is important for protecting the nervous system from the consequences of progressive TBI. Our controlled cortical impact (CCI) injury produces ~20% loss of synapses and mild changes in synaptic protein levels in the CA3-CA1 hippocampus without neuronal losses. These synaptic changes are associated with functional deficits, indicated by >50% loss in synaptic plasticity and impaired learning behavior. We show that the receptor tyrosine kinase EphB3 participates in CCI injury-induced synaptic damage, where EphB3(-/-) mice show preserved long-term potentiation and hippocampal-dependent learning behavior as compared with wild type (WT) injured mice. Improved synaptic function in the absence of EphB3 results from attenuation in CCI injury-induced synaptic losses and reduced d-serine levels compared with WT injured mice. Together, these findings suggest that EphB3 signaling plays a deleterious role in synaptic stability and plasticity after TBI.

A high-throughput framework to detect synapses in electron microscopy images

Motivation: Synaptic connections underlie learning and memory in the brain and are dynamically formed and eliminated during development and in response to stimuli. Quantifying changes in overall density and strength of synapses is an important pre-requisite for studying connectivity and plasticity in these cases or in diseased conditions. Unfortunately, most techniques to detect such changes are either low-throughput (e.g. electrophysiology), prone to error and difficult to automate (e.g. standard electron microscopy) or too coarse (e.g. magnetic resonance imaging) to provide accurate and large-scale measurements.

Results: To facilitate high-throughput analyses, we used a 50-year-old experimental technique to selectively stain for synapses in electron microscopy images, and we developed a machine-learning framework to automatically detect synapses in these images. To validate our method, we experimentally imaged brain tissue of the somatosensory cortex in six mice. We detected thousands of synapses in these images and demonstrate the accuracy of our approach using cross-validation with manually labeled data and by comparing against existing algorithms and against tools that process standard electron microscopy images. We also used a semi-supervised algorithm that leverages unlabeled data to overcome sample heterogeneity and improve performance. Our algorithms are highly efficient and scalable and are freely available for others to use.

Availability: Code is available at http://www.cs.cmu.edu/∼saketn/detect_synapses/

Contact:zivbj@cs.cmu.edu

Aberrant restoration of spines and their synapses in L-DOPA-induced dyskinesia: involvement of corticostriatal but not thalamostriatal synapses

We examined the structural plasticity of excitatory synapses from corticostriatal and thalamostriatal pathways and their postsynaptic targets in adult Sprague-Dawley rats to understand how these striatal circuits change in l-DOPA-induced dyskinesias (LIDs). We present here detailed electron and light microscopic analyses that provide new insight into the nature of the structural and synaptic remodeling of medium spiny neurons in response to LIDs. Numerous studies have implicated enhanced glutamate signaling and persistent long-term potentiation as central to the behavioral sensitization phenomenon of LIDs. Moreover, experience-dependent alterations in behavior are thought to involve structural modifications, specifically alterations in patterns of synaptic connectivity. Thus, we hypothesized that in the striatum of rats with LIDs, one of two major glutamatergic pathways would form new or altered contacts, especially onto the spines of medium spiny neuron (MSNs). Our data provide compelling evidence for a dramatic rewiring of the striatum of dyskinetic rats and that this rewiring involves corticostriatal but not thalamostriatal contacts onto MSNs. There is a dramatic increase in corticostriatal contacts onto spines and dendrites that appear to be directly linked to dyskinetic behaviors, since they were not seen in the striatum of animals that did not develop dyskinesia. There is also an aberrant increase in spines receiving more than one excitatory contact(i.e., multisynaptic spines) in the dyskinetic animals compared with the 6-hydroxydopamine-treated and control rats. Such alterations could substantially impair the ability of striatal neurons to gate cortically driven signals and contribute to the loss of bidirectional synaptic plasticity.

Studying synapses in human brain with array tomography and electron microscopy

Postmortem studies of synapses in human brain are problematic because of the axial resolution limit of light microscopy and the difficulty in preserving and analyzing ultrastructure with electron microscopy (EM). Array tomography (AT) overcomes these problems by embedding autopsy tissue in resin and cutting ribbons of ultrathin serial sections. Ribbons are imaged with immunofluorescence, allowing high-throughput imaging of tens of thousands of synapses to assess synapse density and protein composition. The protocol takes ~3 d per case, excluding image analysis, which is done at the end of the study. Parallel processing for transmission electron microscopy (TEM) using a protocol modified to preserve the structure in human samples allows complementary ultrastructural studies. Incorporation of AT and TEM into brain banking is a potent way of phenotyping synapses in well-characterized clinical cohorts in order to develop clinicopathological correlations at the synapse level. This will be important for research in neurodegenerative disease, developmental disease and psychiatric illness.

Defective synaptic transmission and structure in the dentate gyrus and selective fear memory impairment in the Rsk2 mutant mouse model of Coffin-Lowry syndrome

The Coffin-Lowry syndrome (CLS) is a syndromic form of intellectual disability caused by loss-of-function of the RSK2 serine/threonine kinase encoded by the rsk2 gene. Rsk2 knockout mice, a murine model of CLS, exhibit spatial learning and memory impairments, yet the underlying neural mechanisms are unknown. In the current study, we examined the performance of Rsk2 knockout mice in cued, trace and contextual fear memory paradigms and identified selective deficits in the consolidation and reconsolidation of hippocampal-dependent fear memories as task difficulty and hippocampal demand increase. Electrophysiological, biochemical and electron microscopy analyses were carried out in the dentate gyrus of the hippocampus to explore potential alterations in neuronal functions and structure. In vivo and in vitro electrophysiology revealed impaired synaptic transmission, decreased network excitability and reduced AMPA and NMDA conductance in Rsk2 knockout mice. In the absence of RSK2, standard measures of short-term and long-term potentiation (LTP) were normal, however LTP-induced CREB phosphorylation and expression of the transcription factors EGR1/ZIF268 were reduced and that of the scaffolding protein SHANK3 was blocked, indicating impaired activity-dependent gene regulation. At the structural level, the density of perforated and non-perforated synapses and of multiple spine boutons was not altered, however, a clear enlargement of spine neck width and post-synaptic densities indicates altered synapse ultrastructure. These findings show that RSK2 loss-of-function is associated in the dentate gyrus with multi-level alterations that encompass modifications of glutamate receptor channel properties, synaptic transmission, plasticity-associated gene expression and spine morphology, providing novel insights into the mechanisms contributing to cognitive impairments in CLS.

Chronic alcoholism-mediated impairment in the medulla oblongata: a mechanism of alcohol-related mortality in traumatic brain injury?

Alcohol-related traumatic brain injury (TBI) is a common condition in medical and forensic practice, and results in high prehospital mortality. We investigated the mechanism of chronic alcoholism-related mortality by examining the effects of alcohol on the synapses of the medulla oblongata in a rat model of TBI. Seventy adult male Sprague-Dawley rats were randomly assigned to either ethanol (EtOH) group, EtOH-TBI group, or control groups (water group, water-TBI group). To establish chronic alcoholism model, rats in the EtOH group were given EtOH twice daily (4 g/kg for 2 weeks and 6 g/kg for another 2 weeks). The rats also received a minor strike on the occipital tuberosity with an iron pendulum. Histopathologic and ultrastructure changes and the numerical density of the synapses in the medulla oblongata were examined. Expression of postsynaptic density-95 (PSD-95) in the medulla oblongata was measured by ELISA. Compared with rats in the control group, rats in the chronic alcoholism group showed: (1) minor axonal degeneration; (2) a significant decrease in the numerical density of synapses (p < 0.01); and (3) compensatory increase in PSD-95 expression (p < 0.01). Rats in the EtOH-TBI group showed: (1) high mortality (50%, p < 0.01); (2) inhibited respiration before death; (3) severe axonal injury; and (4) decrease in PSD-95 expression (p < 0.05). Chronic alcoholism induces significant synapse loss and axonal impairment in the medulla oblongata and renders the brain more susceptible to TBI. The combined effects of chronic alcoholism and TBI induce significant synapse and axon impairment and result in high mortality.

FAST INTERACTIVE QUANTIFICATION OF SYNAPSES IN THE CEREBRAL CORTEX

Synapses are key elements in the organization of neuronal circuits. To analyse these structures, it is now possible to obtain serial images from large samples of nervous tissue in an automated manner through combined focused ion beam milling and scanning electron microscopy. However, the identification, 3D reconstruction and quantification of synapses within these samples are labor intensive procedures that require continuous user intervention. We have developed a software tool to achieve the segmentation of synapses in a reconstructed 3D volume of the cerebral cortex, thereby greatly facilitating and accelerating these processes. The tool is interactive, allowing the user to supervise the segmentation process, to modify the appropriate parameters and to validate the results. Some experimental results obtained with our tool in the rat cerebral cortex are also presented.

Automated detection and segmentation of synaptic contacts in nearly isotropic serial electron microscopy images

We describe a protocol for fully automated detection and segmentation of asymmetric, presumed excitatory, synapses in serial electron microscopy images of the adult mammalian cerebral cortex, taken with the focused ion beam, scanning electron microscope (FIB/SEM). The procedure is based on interactive machine learning and only requires a few labeled synapses for training. The statistical learning is performed on geometrical features of 3D neighborhoods of each voxel and can fully exploit the high z-resolution of the data. On a quantitative validation dataset of 111 synapses in 409 images of 1948×1342 pixels with manual annotations by three independent experts the error rate of the algorithm was found to be comparable to that of the experts (0.92 recall at 0.89 precision). Our software offers a convenient interface for labeling the training data and the possibility to visualize and proofread the results in 3D. The source code, the test dataset and the ground truth annotation are freely available on the website http://www.ilastik.org/synapse-detection.

Advanced microscopy techniques for quantitative analysis in neuromorphology and neuropathology research: current status and requirements for the future

Visualizing neuromorphology and in particular neuropathology has been the focus of many researchers in the quest to solve the numerous questions that are still remaining related to several neurological and neuropsychiatric diseases. Over the last years, intense research into microscopy techniques has resulted in the development of various new types of microscopes, software imaging systems, and analysis programs. This review briefly discusses some key techniques, such as confocal stereology and automated neuron tracing and reconstruction, and their applications in neuroscience research. Special emphasis is placed on needs for further developments, such as the demand for higher-throughput analyses in quantitative neuromorphology. These developments will advance basic neuroscience research as well as pharmaceutical and biotechnology research and development.

Circadian plasticity in photoreceptor cells controls visual coding efficiency in Drosophila melanogaster

In the fly Drosophila melanogaster, neuronal plasticity of synaptic terminals in the first optic neuropil, or lamina, depends on early visual experience within a critical period after eclosion [1]. The current study revealed two additional and parallel mechanisms involved in this type of synaptic terminal plasticity. First, an endogenous circadian rhythm causes daily oscillations in the volume of photoreceptor cell terminals. Second, daily visual experience precisely modulates the circadian time course and amplitude of the volume oscillations that the photoreceptor-cell terminals undergo. Both mechanisms are separable in their molecular basis. We suggest that the described neuronal plasticity in Drosophila ensures continuous optimal performance of the visual system over the course of a 24 h-day. Moreover, the sensory system of Drosophila cannot only account for predictable, but also for acute, environmental changes. The volumetric changes in the synaptic terminals of photoreceptor cells are accompanied by circadian and light-induced changes of presynaptic ribbons as well as extensions of epithelial glial cells into the photoreceptor terminals, suggesting that the architecture of the lamina is altered by both visual exposure and the circadian clock. Clock-mutant analysis and the rescue of PER protein rhythmicity exclusively in all R1-6 cells revealed that photoreceptor-cell plasticity is autonomous and sufficient to control visual behavior. The strength of a visually guided behavior, the optomotor turning response, co-varies with synaptic-terminal volume oscillations of photoreceptor cells when elicited at low light levels. Our results show that behaviorally relevant adaptive processing of visual information is performed, in part, at the level of visual input level.

The proportion of synapses formed by the axons of the lateral geniculate nucleus in layer 4 of area 17 of the cat

The connection between the dorsal lateral geniculate nucleus (dLGN) and area 17 of the cat is a classical model for studying thalamocortical relations. We investigated the proportion of asymmetric synapses in layer 4 of area 17 of cats formed by axons of the dLGN, because this is an important morphological parameter in understanding the impact of dLGN axons on their target neurons. Although the present consensus is that this proportion is small, the exact percentage remains in doubt. Most previous work estimated that the thalamus contributes less than 10% of excitatory synapses in layer 4, but one estimate was as high as 28%. Two issues contribute to these widely different estimates, one being the tracers used, the other being the use of biased stereological approaches. We have addressed both of these issues. Thalamic axons were labeled in vivo by injections of biotinylated dextran amine into the A lamina of the dLGN of anesthetized cats. After processing, the brain was cut serially and prepared for light and electron microscopy. The density of asymmetric synapses in the neuropil and the density of synapses formed by labeled dLGN boutons were measured by using an unbiased sampling method called the physical disector. Our counts indicate that, in the fixed cat brain, there are 5.9 x 10(8) +/- 0.9 x 10(8) asymmetric synapses per cubic millimeter of layer 4 in area 17, and the dLGN input provides only 6% of all asymmetric synapses in layer 4. The vast majority of synapses of layer 4 probably originate from other neurons in area 17.

Counting Synapses Using FIB/SEM Microscopy: A True Revolution for Ultrastructural Volume Reconstruction

The advent of transmission electron microscopy (TEM) in the 1950s represented a fundamental step in the study of neuronal circuits. The application of this technique soon led to the realization that the number of synapses changes during the course of normal life, as well as under certain pathological or experimental circumstances. Since then, one of the main goals in neurosciences has been to define simple and accurate methods to estimate the magnitude of these changes. Contrary to analysing single sections, TEM reconstructions are extremely time-consuming and difficult. Therefore, most quantitative studies use stereological methods to define the three-dimensional characteristics of synaptic junctions that are studied in two dimensions. Here, to count the exact number of synapses per unit of volume we have applied a new three-dimensional reconstruction method that involves the combination of focused ion beam milling and scanning electron microscopy (FIB/SEM). We show that the images obtained with FIB/SEM are similar to those obtained with TEM, but with the advantage that FIB/SEM permits serial reconstructions of large volumes of tissue to be generated rapidly and automatically. Furthermore, we compared the estimates of the number of synapses obtained with stereological methods with the values obtained by FIB/SEM reconstructions. We concluded that FIB/SEM not only provides the actual number of synapses per volume but it is also much easier and faster to use than other currently available TEM methods. More importantly, it also avoids most of the errors introduced by stereological methods and overcomes the difficulties associated with these techniques.

A systematic random sampling scheme optimized to detect the proportion of rare synapses in the neuropil

Synapses can only be morphologically identified by electron microscopy and this is often a very labor-intensive and time-consuming task. When quantitative estimates are required for pathways that contribute a small proportion of synapses to the neuropil, the problems of accurate sampling are particularly severe and the total time required may become prohibitive. Here we present a sampling method devised to count the percentage of rarely occurring synapses in the neuropil using a large sample (approximately 1000 sampling sites), with the strong constraint of doing it in reasonable time. The strategy, which uses the unbiased physical disector technique, resembles that used in particle physics to detect rare events. We validated our method in the primary visual cortex of the cat, where we used biotinylated dextran amine to label thalamic afferents and measured the density of their synapses using the physical disector method. Our results show that we could obtain accurate counts of the labeled synapses, even when they represented only 0.2% of all the synapses in the neuropil.

Role of mental retardation-associated dystrophin-gene product Dp71 in excitatory synapse organization, synaptic plasticity and behavioral functions

Background

Duchenne muscular dystrophy (DMD) is caused by deficient expression of the cytoskeletal protein, dystrophin. One third of DMD patients also have mental retardation (MR), likely due to mutations preventing expression of dystrophin and other brain products of the DMD gene expressed from distinct internal promoters. Loss of Dp71, the major DMD-gene product in brain, is thought to contribute to the severity of MR; however, the specific function of Dp71 is poorly understood.

Methodology/Principal Findings

Complementary approaches were used to explore the role of Dp71 in neuronal function and identify mechanisms by which Dp71 loss may impair neuronal and cognitive functions. Besides the normal expression of Dp71 in a subpopulation of astrocytes, we found that a pool of Dp71 colocalizes with synaptic proteins in cultured neurons and is expressed in synaptic subcellular fractions in adult brains. We report that Dp71-associated protein complexes interact with specialized modular scaffolds of proteins that cluster glutamate receptors and organize signaling in postsynaptic densities. We then undertook the first functional examination of the brain and cognitive alterations in the Dp71-null mice. We found that these mice display abnormal synapse organization and maturation in vitro, altered synapse density in the adult brain, enhanced glutamatergic transmission and reduced synaptic plasticity in CA1 hippocampus. Dp71-null mice show selective behavioral disturbances characterized by reduced exploratory and novelty-seeking behavior, mild retention deficits in inhibitory avoidance, and impairments in spatial learning and memory.

Conclusions/Significance

Results suggest that Dp71 expression in neurons play a regulatory role in glutamatergic synapse organization and function, which provides a new mechanism by which inactivation of Dp71 in association with that of other DMD-gene products may lead to increased severity of MR.

Age related optic nerve axonal loss in adult Brown Norway rats

The effect of age on the number and morphology of optic nerve axons in adult Brown Norway rats (5-31 months old) (n=29) was examined using transmission electron microscopy (TEM). By manually counting every axon in areas representing 60% of the optic nerve cross-section, we found a significant negative correlation between age and axon count (R(2)=0.18, P<0.05). However, when the oldest animals were omitted, the relationship was no longer statistically significant. Simultaneously, the proportion of spontaneously degenerating axons increased at an exponential rate (R(2)=0.79, P<0.05), with significantly more degeneration in the 31-month group than in 5-month-old animals (ANOVA, P<0.05). This study demonstrates, using quantitative TEM methods, that optic nerve axonal numbers are relatively constant throughout the majority of the adult life of the Brown Norway rat, an increasingly popular strain for glaucoma research. Total axonal loss with aging is substantially less than that reported for other strains. The reduction in axonal numbers and the rate of axonal degeneration do not appear significantly altered until the last few months of life, failing to support some studies that have concluded that optic nerve axon loss in adult rats is linear. However, they do agree with other studies in the rat, and a similar study performed in non-human primate eyes, that concluded that aging changes in the optic nerve and retina follow a complex pattern. Therefore, the impact of animal age must be considered when modeling the course and pathophysiology of experimental glaucomatous optic nerve damage in rats.

The morphology of bi-directional experience-dependent cortical plasticity: a meta-analysis

Describing the neural mechanisms underlying learning and memory continues to be an intensive area of study within neuroscience. Of specific interest are changes in synaptic number and structure in the neocortex, which may play a distinct role in learning and memory. As such, characterizing the structural correlates of neocortical learning and memory may be critical to understanding the link between synaptic structure and function. Towards this understanding, a meta-analysis was conducted on several well-researched paradigms of behavioral plasticity, categorized by those which enhance or deprive plasticity-inducing experience (PIE). Results revealed several distinct groups. Several variables (spine size, density of multisynaptic terminals, vesicular content) showed distinct dynamics under enhanced vs. deprived PIE, but changed consistently within these categories, regardless of the manipulation. A second set of variables (i.e., density of excitatory, inhibitory, excitatory spinuous, and inhibitory spinuous synapses) showed the same qualitative changes following both enhanced and impoverished PIE. A third group (total synapse density, total basilar branches, apical spine density, total postsynaptic density size, and total bouton size) showed significant heterogeneity that could not be accounted for by partitioning enhancement and deprivation of PIE. However, this variance was accounted for by the modality and duration of the manipulation, the delay between this manipulation and sacrifice, and the stereological/methodological rigor of the study. These data, along with suggestions for future investigation based on gaps in the literature may go far towards the goal of relating neural structure and function.

An interspecific comparison using immunofluorescence reveals that synapse density in the avian song system is related to sex but not to male song repertoire size

Immunofluorescent labeling of synaptic vesicle protein 2 (SV2) and confocal microscopy were employed to assess the role of synapse density in the functioning of the avian song system. Synapse density in premotor nuclei HVC and RA was measured, in both sexes of two species characterized by male-only singing behavior: the zebra finch Taeniopygia guttata, which sings a single, stereotyped song, and the Carolina wren Thryothorus ludovicianus, which sings a large repertoire of different songs. Multiple levels of analyses demonstrate overall similarity of synapse density between nuclei HVC and RA, suggesting that synapse density is regulated uniformly across these regions within individuals. Male zebra finches and male Carolina wrens have equivalent synapse densities, suggesting a common pattern of masculinized development despite dramatic behavioral differences. Female Carolina wrens have synaptic density similar to that of males of both species, while female zebra finches exhibit greater synaptic densities in both regions than do male zebra finches or both sexes of wrens. Prior reports implicate testosterone as a regulator of synapse density in this system; sex differences in circulating or neural testosterone may contribute to the sexual dimorphism of synapse density observed here. Interspecific comparison of song system synapse density in nonsinging females suggests that synapse density in female songbirds may be a particularly labile trait.

Design-based stereology in neuroscience

Quantitative morphology of the CNS has recently undergone major developments. In particular, several new approaches, known as design-based stereologic methods, have become available and have been successfully applied to neuromorphological research. However, much confusion and uncertainty remains about the meaning, implications, and advantages of these design-based stereologic methods. The objective of this review is to provide some clarification. It does not comprise a full description of all stereologic methods available. Rather, it is written by users for users, provides the reader with a guided tour through the relevant literature. It has been the experience of the authors that most neuroscientists potentially interested in design-based stereology need to analyze volumes of brain regions, numbers of cells (neurons, glial cells) within these brain regions, mean volumes (nuclear, perikaryal) of these cells, length densities of linear biological structures such as vessels and nerve fibers within brain regions, and the cytoarchitecture of brain regions (i.e. the spatial distribution of cells within a region of interest). Therefore, a comprehensive introduction to design-based stereologic methods for estimating these parameters is provided. It is demonstrated that results obtained with design-based stereology are representative for the entire brain region of interest, and are independent of the size, shape, spatial orientation, and spatial distribution of the cells to be investigated. Also, it is shown that bias (i.e. systematic error) in results obtained with design-based stereology can be limited to a minimum, and that it is possible to assess the variability of these results. These characteristics establish the advantages of design-based stereologic methods in quantitative neuromorphology.

Constitutive Ras activity induces hippocampal hypertrophy and remodeling of pyramidal neurons in synRas mice

The small G protein Ras, which is involved critically in neurotrophic signal transduction, has been implicated in neuronal plasticity of both the developing and the adult nervous systems. In the present study, the cumulative effects of constitutive Ras activity from early in postnatal development into the adult upon the morphology of hippocampal pyramidal neurons were investigated in synRas mice overexpressing Val12-Ha-Ras postmitotically under the control of the rat synapsin I promoter. In synRas mice, stereologic investigations revealed hypertrophy of the hippocampus associated with an increase in perikaryal size of pyramidal neurons within the CA2/CA3 region and the gyrus dentatus. Morphometric analyses of Lucifer Yellow-filled CA1 pyramidal neurons, in addition, demonstrated considerable expansion of dendritic arbors. The increase in basal dendritic size was caused primarily by alterations of intermediate and distal segments and was associated with an enlarged dendritic surface. Apical dendrites showed similar but more moderate changes, which were attributed mainly to elongation of terminal segments. Sholl analyses illustrated higher complexity of both basal and apical trees. Despite significant morphologic alterations, dendritic arbors preserve their major design principles. The synaptic density within the stratum radiatum of CA1 remained unchanged; however, increases in the total hippocampal volume and in apical dendritic size imply an increment in the absolute number of synaptic contacts. The data presented here suggest a critical involvement of Ras dependent signaling in morphoregulatory processes during the maturation and in the maintenance of hippocampal pyramidal neurons.

Constitutive expression of p21H-ras(Val12) in pyramidal neurons results in reorganization of mouse neocortical afferents

The effect of constitutive expression of p21H-ras(Val12) in pyramidal neurons upon the establishment of afferent input has been investigated in the primary somatosensory cortex of transgenic mice. In these animals, relevant transgene expression is confined to cortical pyramidal neurons and starts postnatally at a period when neuronal morphogenesis has been largely completed. We have shown recently that overexpression of p21H-ras(Val12) in these cells results in considerable enlargement of their size and consequently in expansion of the cortex. In the present study we demonstrate that the density of terminals representing intra- or interhemispheric afferents within cortical layers II/III, however, is only slightly decreased. The density of thalamocortical boutons within layer IV is even higher and the number of afferent contacts to transgenic pyramidal neurons is significantly increased compared to the wild-type. The number of catecholaminergic and cholinergic terminals is augmented proportionally to cortical size or even overproportionally, respectively. Along intercortical and striatal fibers arising from p21H-ras(Val12)-expressing pyramidal neurons, frequency of varicosities is significantly increased, but remains unchanged on cortical cholinergic and catecholaminergic axons originating from "nontransgenic" neurons. Additionally, a higher number of multiple synaptic bodies are found in transgenic mice, suggesting subtle effects on synaptic plasticity. It is concluded that the enlargement of pyramidal neurons due to transgenic expression of p21H-ras(Val12) is paralleled by significant changes in the quantity and pattern of afferent connections. Moreover, expression of p21H-ras(Val12) in pyramidal cells induces an enhanced establishment of efferent boutons.

Neuronal activation of Ras regulates synaptic connectivity

A synRas mouse model was used expressing constitutively activated Ha-Ras (Val12 mutation) in neurons to investigate the role of Ras-MAPkinase signalling for neuronal connectivity in adult brain. Expression of the transgene in the cortex of these mice starts after neuronal differentiation is completed and allows to directly investigate the effects of enhanced Ras activity in differentiated neurons. Activation of Ha-Ras induced an increase in soma size which was sensitive to MEK inhibitor in postnatal organotypic cultures. Adult cortical pyramidal neurons showed complex structural rearrangements associated with an increased size and ramification of dendritic arborization. Dendritic spine density was elevated and correlated with a twofold increase in number of synapses. In acute brain slices of the somatosensory and of the visual cortex, extracellular field potentials were recorded from layer II/III neurons. The input-output relation of synaptically evoked field potentials revealed a significantly higher basal excitability of the transgenic mice cortex compared to wild-type animals. In whole cell patch clamp preparations, the frequency of AMPA receptor-mediated spontaneous excitatory postsynaptic currents was increased while the ratio between NMDA and AMPA-receptor mediated signal amplitude was unchanged. A pronounced depression of paired pulse facilitation indicated that Ras contributes to changes at the presynaptic site. Furthermore, synRas mice showed an increased synaptic long-term potentiation, which was sensitive to blockers of NMDA-receptors and of MEK. We conclude that neuronal Ras is a common switch of plasticity in adult mammalian brain sculpturing neuronal architecture and synaptic connectivity in concert with tuning synaptic efficacy.

Comparative analyses of synaptic densities during reactive synaptogenesis in the rat dentate gyrus

Advancements in the field of synaptic plasticity have created the need for a reexamination of classic paradigms using new and more precise techniques. One prime candidate for such a reexamination is the process of reactive synaptogenesis (RS). Since the time course of RS was initially outlined in the 1970s and 1980s, advances in stereology have allowed for better characterization of synaptic ultrastructure. Thus, a reexamination was undertaken in the hippocampal dentate gyrus by assessing the densities and proportions of several synaptic subtypes in Long-Evans hooded rats at 3, 6, 10, 15 and 30 days following induction of unilateral lesions of the entorhinal cortex. Although initial synaptic loss in the denervated region was similar to previous reports, recovery during the first 30 days is not as dramatic as previously observed. Following lesioning, concave and perforated synapses retained pre-lesion density despite massive degeneration, underscoring their theoretical importance in plasticity and maintenance of neural function. Convex synapses showed opposite changes, having implications for excitation/inhibition imbalance following lesion induction. These complementary alterations in synaptic structures support ultrastructural changes as a means for compensation following synaptic loss. Nearby areas also seem to participate in this response, with a striking similarity to other models of plasticity, such as long-term potentiation.

Increased expression of insulin-like growth factor I augments the progressive phase of synaptogenesis without preventing synapse elimination in the hypoglossal nucleus

The in vivo actions of insulin-like growth factor I (IGF-I) on synaptogenesis in the hypoglossal nucleus were investigated in transgenic mice that overexpress IGF-I in the brain postnatally and in normal nontransgenic littermate controls. In a previous study using these mice, we found that IGF-I increases the total volume of the hypoglossal nucleus by increasing the volume of neuropil rather than by increasing total neuron number; therefore, the progressive and regressive phases of synaptogenesis could be evaluated without the confounding effects of altered neuron number. The volume of the hypoglossal nucleus was significantly increased by 28% to 59% in transgenic mice after postnatal day (P) 7, whereas the total number of hypoglossal neurons did not differ significantly from controls. The numerical density of neurons was significantly decreased by 21% to 38% after P7, and the density of myelinated axons was significantly increased by 19%. Although the numerical density of synapses did not differ between groups at any age, the total number of synapses in transgenic mice was increased by 42% to 52% after P14. Total synapse number in controls increased from P7 (7.9 million) to peak values at P21 (36.0 million), followed by a significant decrease (33%) at P130 (24.2 million). In transgenic mice, total synapses increased from 8.2 million on P7 to 51.1 million on P21, followed by a significant decrease (28%) to 36.7 million at P130. Our results demonstrated that IGF-I can stimulate a persistent increase in the number of hypoglossal synapses, thereby augmenting the progressive phase of synaptogenesis without preventing synapse elimination during the regressive phase.

Chronic food restriction is associated with subtle dendritic alterations in granule cells of the rat hippocampal formation

The hippocampal formation undergoes significant morphological and functional changes after prolonged feeding with low-protein diets. In this study we tested whether prolonged food restriction causes deleterious alterations in this brain region as well. It was found that the total number of dentate granule cells and hippocampal CA3 and CA1 pyramidal neurons did not differ between controls and rats submitted to food restriction (40%) for 36 weeks. Likewise, no effects of this dietary regimen have been detected on the morphology of the dendritic trees of hippocampal pyramids, and on the total number of the mossy fiber-CA3 synapses. By contrast, the dendritic arborizations of granule cells were found to have a reduced number of segments in food-restricted rats. However, the spine density on the distal segments of their dendritic trees and the total number of axospinous synapses in the outer molecular layer of the dentate gyrus were increased in these animals. In addition, the total dendritic length of the granule cells and the overall surface area of the active zones of the synapses in the outer molecular layer were preserved, indicating that the capacity of dentate granule cells to process afferent stimuli is likely to be unaffected by this dietary treatment. Supporting this view are the results obtained in the water maze experiment which show that food-restricted rats exhibit unimpaired spatial abilities, which are known to be dependent on the entorhinal drive towards the hippocampal formation. These results show that, among hippocampal neurons, dentate granule cells are selectively vulnerable to food restriction. Nonetheless, the reorganization which takes place in their dendrites and synapses is capable of minimizing the functional impairments that were expected to occur following changes in the hippocampal neuronal circuitry induced by this type of dietary restriction.

Morphometry of E-PTA stained synapses at the periphery of pathological lesions

We carried out a novel application of the disector sampling and counting method, in a biopsy material from the pathologic human brain, to estimate the synaptic structural dynamics, quantitatively. Parietal cortex biopsies of adult (mean age: 40.0 years) and old (mean age: 66.2 years) patients having undergone surgical intervention were investigated. The tissue samples were excised at the periphery of meningioma masses. Synaptic contact zones were stained en bloc by the ethanol phosphotungstic acid (E-PTA) preferential technique which selectively enhances both the pre- and post-synaptic paramembranous material separated by a sharp cleft against a very faint background, thus facilitating and objectifying synaptic morphometry. The disector method, associated with currently used morphometric formulas, enabled us to measure the number of synapses/m3 of tissue (numeric density: Nv); the total area of the synaptic contact zones/m3 of tissue (surface density: Sv) and the average synaptic size (S). In old vs. adult patients, Nv decreased by 7.5% (Mean (SEM): Adult 2.0040(0.0452); Old 1.6780(0.0623)), while S increased by 17.5% (Adult 0.0203(0.0026); Old 0.0246(0.0035)). Sv did not show any age-related difference. The same negative correlation between Nv and S has also been reported in physiological aging, and this suggests the active presence of age-related synaptic restructuring mechanisms in the nervous tissue surrounding a tumoral mass.

Age-related decline of presumptive inhibitory synapses in the sensorimotor cortex as revealed by the physical disector

The synapse, as the site of functional neural interaction, has been suggested as a possible substrate for age-related impairment of cognitive ability. Using the physical disector probe with tissue prepared for ultrastructural analysis, we find an age-related decline in the numerical density of presumptive inhibitory synapses in layer 2 of the sensorimotor cortex of the Brown Norway x Fisher 344 rat. This age-related decline in presumptive inhibitory synapses is maintained when the density of synapses is combined with the numerical density of neurons quantified from the same anatomical space to arrive at a ratio of synapses per neuron. The numerical density of these synapses declines between middle-aged (18 months) and old (29 months) animals by 36% whereas numerical density of neurons does not change between these ages, resulting in a decline in the ratio of presumptive inhibitory synapses per neuron in this cortical area. This study demonstrates a deficit in the intrinsic inhibitory circuitry of the aging neocortex, which suggests an anatomical substrate for age-related cognitive impairment.

Insulin-like growth factor-I promotes neurogenesis and synaptogenesis in the hippocampal dentate gyrus during postnatal development

The in vivo actions of insulin-like growth factor-I (IGF-I) on the growth and development of the hippocampal dentate gyrus were investigated in transgenic mice that overexpress IGF-I postnatally in the brain and in normal nontransgenic littermate controls. Stereological analyses of the dentate gyrus were performed by light and electron microscopy on days 7, 14, 21, 28, 35, and 130 to determine postnatal changes in the numerical density and total number of neurons and synapses. The volumes of both the granule cell layer and the molecular layer of the dentate gyrus were significantly increased by 27-69% in transgenic mice after day 7, with the greatest relative increases occurring by day 35. Although the numerical density of neurons in the granule cell layer did not differ significantly between transgenic and control mice at any age studied, the total number of neurons was significantly greater in transgenic mice by 29-61% beginning on day 14. The total number of synapses in the molecular layer was significantly increased by 42-105% in transgenic mice from day 14 to day 130. A transient increase in the synapse-to-neuron ratio was found in transgenic mice at postnatal days 28 and 35 but not at day 130. This finding indicates a disproportionate increase in synaptogenesis, exceeding that expected for the observed increase in neuron number. Our results demonstrate that IGF-I overexpression produces persistent increases in the total number of neurons and synapses in the dentate gyrus, indicating that IGF-I promotes both neurogenesis and synaptogenesis in the developing hippocampus in vivo.

Sexual dimorphism in the subiculum of the rat hippocampal formation

Data accumulated over the last years demonstrate that the hippocampal formation of rodents is sexually dimorphic with respect to its functional attributes. Neuroanatomical substrates that might contribute to explain these gender-related differences have been described in the dentate gyrus, and in the CA3 and CA1 hippocampal fields. However, the subiculum, the source of the major efferent projection of the hippocampal formation, has not been searched for the presence of sex-related differences. To address this issue, we have used stereological methods applied to adult rats of both sexes to estimate the volume of the subiculum, the total number of subicular neurons, and the total number and size of the synapses established by subicular neurons. The apical dendritic trees of Golgi-impregnated subicular neurons were also quantitatively analyzed. We have found that the volume of the subiculum and of its neuronal layer, and the total number of subicular neurons were greater in males than in females. Conversely, the total dendritic length of the apical arborization of the subicular neurons, and the number of dendritic spines and axospinous synapses were higher in females than in males. However, the size of the postsynaptic densities of the individual synapses was smaller in female than in male rats and, as a result, the surface area of the total active synaptic zones did not differ between the sexes. These findings provide an additional morphological clue for the comprehension of the sex dimorphisms within the hippocampal circuitries and, consequently, for a better understanding of the functional sex differences ascribed to the hippocampal formation.

Synaptic density of the prefrontal cortex regulated by dopamine instead of serotonin in rats

Recent findings indicate that monoamine contributes to synaptic plasticity. We examined the synaptic density of the prefrontal cortex and parietal cortex of rats using dopamine (DA) antagonists and agonists, as well as serotonin (5-HT) depleters and found a reduction in synaptic density in the prefrontal cortex lamina V-VI at a maximum of 20% with administration of a D1 antagonist (SCH23390) and at a maximum of 30% with a D2 antagonist (YM09151). Further, with the administration of D1+D2 antagonists there was a 27% decrease in synaptic density, which was a larger reduction than the total of the single dosages of each DA antagonist at equal levels. Increase in synaptic density was seen at a maximum of 8.5% with dosage of a D1 agonist (SKF38390) and 14.5% with dosage of a D2 agonist (PPHT). The dosage of D1+D2 agonists showed a 27.1% increase in synaptic density. There was no change in synaptic density of the parietal cortex with either DA antagonist or agonist administration. Administration of 5-HT depleter pCPA resulted in a 13.8% reduction of synaptic density in the parietal cortex, though there was no change identified in the synaptic density in the prefrontal cortex. Based on these results, it was suggested that the area of the brain with affected synaptic plasticity could differ, depending on the type of monoamine.