Funding was provided by the Austrian Science Fund FWF grant No. P-22969-B11 to F. Ferraguti, and by the Charitable Hertie Foundation and the Werner Reichardt Centre for Integrative Neuroscience and by the DFG (CIN-Exc. 307) to I. Ehrlich.

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The authors declare that they have no competing financial interests.

Freeze-fracture electron microscopy has been a major technique in ultrastructural research for over 40 years. However, the lack of effective means to study the molecular composition of membranes produced a significant decline in its use. Recently, there has been a major revival in freeze-fracture electron microscopy due to the development of effective ways to reveal integral membrane proteins by immunogold labeling 1, 2, namely the FRIL technique.
The FRIL technique possesses several advantages over other immunogold ultrastructural methods. First, proteins are readily accessible to antibodies increasing the sensitivity. Second, exposure of large portion of plasma membrane specializations, such as the postsynaptic membrane, on the two-dimensional surface of the replica allows the inspection of the spatial distribution and physical contiguity of molecules of interest without laborious and time-consuming reconstruction of serial ultrathin sections. Third, the availability of both halves of the plasma membrane increases the number of proteins that can be labeled for each individual structure, provided suitable antibodies are available. Upon fracturing, the hydrophobic face of the split membrane is coated with carbon-platinum that entrenches protein domains remaining on the fractured surface. This prevents access of antibodies to antigens in these domains. For instance on the P-face of a replica only epitopes facing the protoplasmic space can be detected by antibodies, whereas on the E-face only epitopes facing the exoplasmic space can be bound by antibodies (see Figure 2).
On the other hand, the FRIL technique also suffers from certain limitations 2. As fractures occur randomly, it might be difficult to target specific cells or structures. This can also lead to a sampling bias, e.g., in synapse collection, given the different probability of fracturing along the membrane of structures with different curvature (e.g., spines versus shafts). Moreover, the allocation of membrane proteins to one of the two faces is unpredictable. Therefore, the distribution of a protein to the P-face or E-face, particularly for quantitative studies, should be carefully examined using antibodies reactive to intracellular and extracellular domains. Finally, the identification in the replica of certain structures, such as presynaptic axon terminals, can be difficult when based only on morphological features. However, the use of specific antibodies for marker proteins or the transduction of tagged integral membrane proteins or channels using viral vectors offers additional tools to facilitate identification of the fractured membranes. For example, this study took advantage of the transduction of ChR2-YFP in thalamic neurons to identify their axonal efferents in the amygdala or the labeling for &#181;-opioid receptors to reveal postsynaptic membranes of ITC neurons.
In order to perform the FRIL technique successfully, particular care should be taken concerning tissue fixation. Strong tissue fixation (&#62; 2% paraformaldehyde) can result in a high rate of cross-fractures and a decrease in labeling sensitivity. On the other hand, weak fixations make the tissue handling and preparation (e.g., cutting of sections) difficult. It is also important to control that the thickness of the trimmed blocks matches the thickness of the double-sided tape. If the thickness of the specimen is lower than that of the tape, the surfaces of the tissue might not attach to the surface of the two metal carriers, consequently the frozen specimen is not fractured. If the tissue is thicker, it will be compressed with inevitable structural distortions when the sandwich of the two copper carriers is made. The temperature at which the specimen is fractured (in this protocol, -115 &#176;C) plays also an important role on the structure of the replica. Higher temperatures may produce a higher rate of artifacts such as condensation of water vapor on the surface of the tissue prior or during evaporation. Lower temperatures (&#60; -125 &#176;C) may increase the risk of splitting off of material during fracturing. This material may fall onto the surface of the specimen or stay connected to it. These flakes of material are also coated and contrasted producing dark spots on the image. Fracturing at lower temperatures can also affect the frequency of cross-fractures particularly for small fine structures such as dendritic spines. A further critical step in the preparation of replicas is the detergent-digestion. If the digestion is incomplete, the undigested tissue appears as dark patches on the replica, confounding the analysis of the structure at the TEM. Moreover, the undigested tissue can non-specifically trap or bind antibodies, increasing the background labeling. On the other hand, the use of detergents for tissue digestion can denature the molecules associated to the replica altering their secondary and tertiary structures. Therefore, for certain antigens it might be necessary to gradually dilute the concentration of SDS with additional washing steps.
For immunolabeling, the availability of different sizes of gold particle conjugated to secondary antibodies allows to detect at the same time, but only qualitatively, multiple proteins, even in specific microdomains of the plasma membrane, such as the postsynaptic specialization. However, due to steric hindrance, quantitative studies are generally limited to the detection of just one molecule. The size of the gold particle can also affect the labeling efficacy.
For the interpretation of the labeling in FRIL, it should be kept in mind that the immunogold particle can be located anywhere within a hemisphere with a radius of 20-25 nm from the antigen due to the flexible complex formed by the primary and secondary antibody 26. For further information on the theory and practice of FRIL and related techniques, we refer the reader also to other methodological articles 27, 28.
The FRIL technique has been recently used for high-resolution quantitative analyses of glutamate receptor localization in diverse synapse populations 29, 30. Moreover, the detection sensitivity of the FRIL technique for AMPA-Rs was estimated as high as one immunogold particle per one functional AMPA-R channel 29. Thus, this approach is overall very useful to quantify and analyze the pattern of postsynaptic expression of AMPA-Rs and NMDA-Rs at central synapses. Here, we demonstrated its applicability at PIN/MGn-ITC synapses, a site most likely important for relaying US information during fear conditioning. Using an antibody raised against the highly conserved extracellular amino acid residues of the AMPA receptor subunits GluA1-GluA4, we found an even distribution of gold particles within IMP clusters corresponding to postsynaptic membrane specializations. The density of AMPA-Rs in ITC spines was significantly higher compared to shaft synapses targeted by PIN/MGn thalamic afferents. At both spine and shaft synapses, a positive correlation between labeling for AMPA-Rs and postsynaptic area was detected, a feature common to other glutamatergic synapses 25. The low variance in density of AMPA-Rs in PIN/MGn-ITC synapses indicates a homogeneous distribution similar to other synapses formed by thalamic efferents 7, but different from cortical synapses 25. Conversely, the density of NMDA-Rs was more variable and did not differ between spine and shaft synapses suggesting a different regulation than AMPA-Rs. In the future, the high reproducibility of the FRIL technique will not only allow to assess the basal molecular composition of central synapses but may facilitate detection of changes in ionotropic glutamate receptor numbers and subsynaptic distribution after fear learning, complementing ex-vivo recordings of pre- and postsynaptic properties of these inputs.
In conclusion, this approach could be used by other investigators to gain insights into structure-function relationships of input-specific excitatory synapses in many other neural circuits in which disentangling the origin of the inputs and the nature and composition of postsynaptic elements is crucial but problematic.

The definition of the functional architecture of biomembranes at nanometer scale has been challenged in recent years by the development of a number of immunolabeling techniques suitable for transmission electron microscopy. However, these techniques, e.g., pre- and post-embedding immunogold, have a number of important limitations, which include poor detection of antigens and/or limited quantitative assessment of membrane-bound proteins. These limitations become particularly critical in the investigation of the fine structure of the nervous system, which is characterized by a high degree of cell diversity and synapse heterogeneity. This heterogeneity results from both structural and functional diversity dictated by the pre- and postsynaptic elements and by the differential expression, enrichment, or interaction of signalling proteins, such as receptors, transporters, and effector molecules.
A new approach for direct immunolabeling of integral or cross-linked membrane proteins in detergent-solubilized freeze-fracture replicas (FRIL) was originally introduced by Fujimoto two decades ago 1. This original method had, however, several limitations, i.e., severe fragmentation of replicas, which hampered meaningful correlations of labeled molecules with individually mapped cells in complex tissues such as the brain. Approximately 10 years ago, Shigemoto and Fukazawa progressively improved the technique 2. This was paralleled by efforts from another group of scientists at the Boulder laboratories of the Colorado State University, who also significantly improved the technique, in particular for the study of gap junctions 3.
The improvement in freeze-fracturing protocols and machines, as well as the introduction of quick freezing (under high pressure), now allows investigators to produce unbroken replicas of specimens of relatively large size and high quality images of most cellular components without the limitations and artifacts produced by strong chemical fixations.
The FRIL technique offers the great advantage of a highly quantitative in situ identification of one or more proteins (simultaneously) in histologically mapped and cytologically identified cells within complex tissues such as the brain, with the additional advantage of a planar view of pre- and postsynaptic elements in a single replica. Therefore, the FRIL technique, despite its many technical hurdles, holds the promise for a number of very significant scientific breakthroughs, particularly for the correlation of structural and functional properties of individual synapses. During the last several decades, a great deal of information has been obtained on the structure, molecular make up, and physiological function of synapses; yet synapses are morphologically and molecularly highly diverse depending on the pre and postsynaptic parent neurons 4. Only for a handful of synapse types were structure-function studies accomplished so far 5-7. This was mostly due to technical constrains that prevented a precise identification of the nature of the pre- and postsynaptic elements.
Ultrastructural analysis has provided critical insights into the variability of postsynaptic membrane specializations across distinct synaptic contacts both in terms of synaptic size and content in neurotransmitter receptors 6, which has a large impact on the strength and plasticity of synaptic transmission. Furthermore, a large body of research indicates that the number of ionotropic glutamate receptors expressed at different types of synapse is regulated in an afferent- and target-dependent fashion 7-10.
Here, a method is outlined that allows analysis of the structure and receptor composition of postsynaptic membrane specializations with defined presynaptic elements and function. This approach takes advantage of presynaptic expression of recently developed light sensitive algal proteins, such as channelrhodopsin2 (ChR2), and of the FRIL technique to analyze the pattern of postsynaptic expression of &#945;-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA-Rs) and N-methyl-D-aspartate (NMDA-Rs) glutamate receptors. This is demonstrated at synapses formed by axons originating from the posterior thalamic-medial geniculate nuclei (PIN/MGn) onto neurons of the intercalated cell masses of the amygdala (ITC). ITC neurons are small spiny GABAergic cells organized in clusters surrounding the basolateral amygdaloid complex (BLA) 11, 12. ITC neurons are known to receive excitatory inputs from BLA principal neurons and to target the central nucleus (CeA), thus functioning as an inhibitory gate for the information flow between the BLA and CeA 12-15.
Recently, we demonstrated that ITC neurons located in the medio-dorsal cluster between BLA and CeA also receive direct and convergent excitatory inputs from sensory thalamic and temporal cortical regions, which are modified upon fear learning during Pavlovian auditory fear conditioning 16. Fear conditioning is one of the best understood forms of associative learning in terms of brain mechanisms. In fear conditioning, an initially neutral conditioned stimulus (CS, e.g., a tone) is paired with an aversive unconditioned stimulus (US, e.g., a mild foot shock) resulting in a CS-US association and conditioned fear response 17, 18. Excitatory inputs from both thalamic and neocortical areas, which carry information representing the CS and US, respectively, were known to converge onto pyramidal neurons of the lateral nucleus of the amygdala (LA) and to undergo plasticity 19. Our previous work revealed that sensory input information is also in parallel relayed to ITC neurons 16.
As a first step towards a mechanistic molecular analysis of individual sensory input synapses onto ITC neurons, we used an adeno-associated virus (AAV) to express ChR2 tagged with the yellow fluorescent protein (YFP). The AAV was injected into the PIN/MGn and the axon terminals were identified by their expression of ChR2-YFP. We used both faces generated by the FRIL technique to assess the density of postsynaptic AMPA-Rs and NMDA-Rs at synapses formed with ITC neurons by PIN/MGn axon terminals.

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combined optogenetic and freeze-fracture replica immunolabeling to examine input-specific arrangement of glutamate receptors in the mouse amygdala

Freeze-fracture electron microscopy has been a major technique in ultrastructural research for over 40 years. However, the lack of effective means to study the molecular composition of membranes produced a significant decline in its use. Recently, there has been a major revival in freeze-fracture electron microscopy thanks to the development of effective ways to reveal integral membrane proteins by immunogold labeling. One of these methods is known as detergent-solubilized Freeze-fracture Replica Immunolabeling (FRIL).
The combination of the FRIL technique with optogenetics allows a correlated analysis of the structural and functional properties of central synapses. Using this approach it is possible to identify and characterize both pre- and postsynaptic neurons by their respective expression of a tagged channelrhodopsin and specific molecular markers. The distinctive appearance of the postsynaptic membrane specialization of glutamatergic synapses further allows, upon labeling of ionotropic glutamate receptors, to quantify and analyze the intrasynaptic distribution of these receptors. Here, we give a step-by-step description of the procedures required to prepare paired replicas and how to immunolabel them. We will also discuss the caveats and limitations of the FRIL technique, in particular those associated with potential sampling biases. The high reproducibility and versatility of the FRIL technique, when combined with optogenetics, offers a very powerful approach for the characterization of different aspects of synaptic transmission at identified neuronal microcircuits in the brain.
Here, we provide an example how this approach was used to gain insights into structure-function relationships of excitatory synapses at neurons of the intercalated cell masses of the mouse amygdala. In particular, we have investigated the expression of ionotropic glutamate receptors at identified inputs originated from the thalamic posterior intralaminar and medial geniculate nuclei. These synapses were shown to relay sensory information relevant for fear learning and to undergo plastic changes upon fear conditioning.

The FRIL technique, when combined with expression of optogenetic actuators of microbial origin 21, i.e., channels integrated in the plasma membrane and effectively transported anterogradely along axons, allows to examine quantitatively the postsynaptic expression of AMPA-Rs and NMDA-Rs at a defined subgroup of synapses. This is shown here for axons originating from distinct thalamic nuclei (e.g., PIN/MGn) onto ITC neurons in the amygdala. This approach enables a molecular analysis of individual sensory input synapses onto ITC neurons, a group of cells that have been refractory to a detailed anatomical and molecular characterization so far.
Four weeks after stereotactic injection of the rAAV-ChR2-YFP into the PIN/MGn, ChR2-YFP-positive axons densely innervated the LA, the amygdalostriatal transition area (Astria) and the medial paracapsular ITC cluster in the amygdala, a pattern fully consistent with previous tracing studies 16, 22. We also detected intense gold immunolabeling for the ChR2-YFP on the P-face of axons and terminals in freeze-fracture replica from rAAV-ChR2-YFP-injected mice (Figure 5A), but not in replicas from non-injected mice. The postsynaptic membrane specialization of glutamatergic synapses in a replica can be observed as a cluster of intramembrane particles (IMPs) on the E-face of the plasma membrane 2, 23, and is often accompanied by the P-face of its presynaptic plasma membrane 7 (Figure 5B-C). These features allowed to identify the postsynaptic specialization of glutamatergic synapses formed by PIN/MGn axon terminals (Figure 5 and 6). We labeled AMPA-Rs with an antibody that recognizes all four subunits (GluA1-4), whereas NMDA-Rs were detected using an antibody against the essential NR1 subunit.
Because of the lack of tools to detect on the same replica whether these synapses were made with dendritic spines or shafts of ITC neurons, we labeled the corresponding replica face for &#181;-opioid receptors, as ITC neurons express postsynaptically high levels of these receptors 24. This required the identification of the same postsynaptic profiles in the two replicas (Figure 5C-F and Figure 6A-D) using a strategy that employs landmarks (Figure 4E).
<img alt="Figure 5" src="/files/ftp_upload/53853/53853fig5.jpg" /> 
Figure 5. Detection of ChR2-YFP and Ionotropic Glutamate Receptors on Replica by Immunogold Particles. (A) A cross-fractured axon terminal (light blue) and small portions of its P-face labeled with 15 nm gold particles detecting ChR2-YFP. Within the terminal, the membrane of numerous synaptic vesicles can be observed (sv). Note the specificity of the immunolabeling in large part restricted to the plasma membrane. Labeling for ChR2 identifies the terminal as originating from the PIN/MGn. The terminal forms an asymmetric synapse with a spine. The postsynaptic membrane specialization (PSD) on the E-face shows a characteristic cluster of intramembrane particles and is labeled with 5 nm gold particles revealing AMPA-Rs. (B) The P-face of a ChR2-expressing axon (labeled with 15 nm gold particles) is shown bordering two dendrites, one of them possessing two PSDs labeled with 5 nm gold particles revealing NMDA-Rs. (C-D) Opposite faces of the pre- and postsynaptic membranes of a PIN/MGn-ITC synapse. (C) The P-face of the terminal expresses ChR2 (labeled with 15 nm gold particles) and extends over the E-face of two dendritic shafts, one of them containing two PSDs labeled for AMPA-Rs (5 nm gold particles). (D) The corresponding P-face of the two dendrites is labeled for &#181;-opioid receptors (10 nm gold particles). (E-F) Enlarged views of the areas outlined by the dashed lines. Scale bars: 500 nm. <a href="https://www.jove.com/files/ftp_upload/53853/53853fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Immunoparticles for AMPA-Rs in the PIN/MGn-ITC synapses were found all over the IMP cluster, suggesting a homogeneous distribution within the postsynaptic specialization (Figure 5E). A significantly higher (unpaired t-test p&#60;0.018) density of AMPA-R labeling was observed in PIN/MGn synapses onto ITC spines (715&#177;38 gold particles/&#181;m2, n = 32) compared to synapses onto ITC dendrites (590&#177;44 gold particles/&#181;m2, n = 32). Overall, the density of AMPA-Rs in PIN/MGn-ITC synapses showed a relatively low variance (Coefficient of variance, CV = 0.37) consistent with a homogeneous distribution.
Immunoparticles for NMDA-Rs in the PIN/MGn-ITC synapses were often observed unevenly distributed within postsynaptic IMP clusters (Figure 5B). The density of NMDA-R labeling was similar (unpaired t-test p=0.39) between PIN/MGn synapses onto ITC spines (1070&#177;153 gold particles/&#181;m2, n=8) and ITC dendrites (812&#177;183 gold particles/&#181;m2, n=9). Unlike what was observed for AMPA-Rs, the density of NMDA-Rs in PIN/MGn-ITC synapses was highly variable (CV = 0.54).
<img alt="Figure 6" src="/files/ftp_upload/53853/53853fig6.jpg" /> 
Figure 6. AMPA-Rs and NMDA-Rs Immunolabeling at Identified PIN/MGn-ITC Synapses. 
(A-B) Opposite faces of the pre- and postsynaptic membranes of a PIN/MGn-ITC synapse made onto a dendritic spine in which the P-face of the terminal expresses ChR2 (labeled with 15 nm gold particles) and the PSD onto a dendritic spine is labeled for AMPA-Rs (5 nm gold particles). (C-D) Enlarged views of the areas outlined by the dashed line. These areas have been rotated approximately 45&#176; anticlockwise to allow a better view of the PSD. (E) Scatterplots of the number of AMPA-R particles versus synaptic area in ITC spines and dendrites. In both structures, a positive correlation has been observed. (F) Scatterplots of the number of NMDA-R particles against synaptic area in ITC spines and dendrites. A significant positive correlation was detected only in dendritic spines. Scale bars: 500 nm. <a href="https://www.jove.com/files/ftp_upload/53853/53853fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Because the P-face of the presynaptic plasma membrane often overlaid in part the postsynaptic IMP cluster, we could estimate the synaptic area of only 30% of the synapses (spines: mean area 0.032 &#181;m2, range: 0.007 to 0.063 &#181;m2, n = 8; dendrites: 0.047 &#181;m2, range: 0.024 to 0.166 &#181;m2, n = 11). These were in a similar range as previously analyzed telencephalic glutamatergic synapses 25.
In both spines and dendrites, the number of gold immunoparticles for AMPA-Rs in individual synapses was positively correlated with the synaptic area (Spearman, spines: r = 0.88, dendrites: r = 0.60, p&#60;0.0001) (Figure 6E). Conversely, the number of gold immunoparticles for NMDA-Rs was found to correlate with the synaptic area in spines (Spearman, spines: r = 0.90, p&#60;0.002) but not in dendrites (r = 0.21, p = 0.29) (Figure 6F).

Watch this Scientific Journal Video about Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate Receptors in the Mouse Amygdala at JoVE.com

Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate Receptors in the Mouse Amygdala

This article illustrates how the expression of neurotransmitter receptors can be quantified and the pattern analyzed at synapses with identified pre and postsynaptic elements using a combination of viral transduction of optogenetic tools and the freeze-fracture replica immunolabeling technique.

Freeze-fracture electron microscopy has been a major technique in ultrastructural research for over 40 years. However, the lack of effective means to ...

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JoVE Journal

Neuroscience

11.0K Views.  Medical University of Innsbruck. This article illustrates how the expression of neurotransmitter receptors can be quantified and the pattern analyzed at synapses with identified pre and postsynaptic elements using a combination of viral transduction of optogenetic tools and the freeze-fracture replica immunolabeling technique.

Video: Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate Receptors in the Mouse Amygdala

Post-embedding Immunogold Labeling of Synaptic Proteins in Hippocampal Slice Cultures

Immunoelectron microscopy is a powerful tool to study biological molecules at the subcellular level. Antibodies coupled to electron-dense markers such as colloidal gold can reveal the localization and distribution of specific antigens in various tissues1. The two most widely used techniques are pre-embedding and post-embedding techniques. In pre-embedding immunogold-electron microscopy (EM) techniques, the tissue must be permeabilized to allow antibody penetration before it is embedded. These techniques are ideal for preserving structures but poor penetration of the antibody (often only the first few micrometers) is a considerable drawback2. The post-embedding labeling methods can avoid this problem because labeling takes place on sections of fixed tissues where antigens are more easily accessible. Over the years, a number of modifications have improved the post-embedding methods to enhance immunoreactivity and to preserve ultrastructure3-5.
Tissue fixation is a crucial part of EM studies. Fixatives chemically crosslink the macromolecules to lock the tissue structures in place. The choice of fixative affects not only structural preservation but also antigenicity and contrast. Osmium tetroxide (OsO4), formaldehyde, and glutaraldehyde have been the standard fixatives for decades, including for central nervous system (CNS) tissues that are especially prone to structural damage during chemical and physical processing. Unfortunately, OsO4 is highly reactive and has been shown to mask antigens6, resulting in poor and insufficient labeling. Alternative approaches to avoid chemical fixation include freezing the tissues. But these techniques are difficult to perform and require expensive instrumentation. To address some of these problems and to improve CNS tissue labeling, Phend et al. replaced OsO4 with uranyl acetate (UA) and tannic acid (TA), and successfully introduced additional modifications to improve the sensitivity of antigen detection and structural preservation in brain and spinal cord tissues7. We have adopted this osmium-free post-embedding method to rat brain tissue and optimized the immunogold labeling technique to detect and study synaptic proteins.
We present here a method to determine the ultrastructural localization of synaptic proteins in rat hippocampal CA1 pyramidal neurons. We use organotypic hippocampal cultured slices. These slices maintain the trisynaptic circuitry of the hippocampus, and thus are especially useful for studying synaptic plasticity, a mechanism widely thought to underlie learning and memory. Organotypic hippocampal slices from postnatal day 5 and 6 mouse/rat pups can be prepared as described previously8, and are especially useful to acutely knockdown or overexpress exogenous proteins. We have previously used this protocol to characterize neurogranin (Ng), a neuron-specific protein with a critical role in regulating synaptic function8,9 . We have also used it to characterize the ultrastructural localization of calmodulin (CaM) and Ca2+/CaM-dependent protein kinase II (CaMKII)10. As illustrated in the results, this protocol allows good ultrastructural preservation of dendritic spines and efficient labeling of Ng to help characterize its distribution in the spine8. Furthermore, the procedure described here can have wide applicability in studying many other proteins involved in neuronal functions.

The localization and distribution of proteins provide important information for understanding their cellular functions. The superior spatial resolution of electron microscopy (EM) can be used to determine the subcellular localization of a given antigen following immunohistochemistry. For tissues of the central nervous system (CNS), preserving structural integrity while maintaining antigenicity has been especially difficult in EM studies. Here, we adopt a procedure that has been used to preserve structures and antigens in the CNS to study and characterize synaptic proteins in rat hippocampal CA1 pyramidal neurons.

Immunoelectron microscopy is a powerful tool to study biological molecules at the subcellular level. Antibodies coupled to electron-dense markers such as ...

High-Resolution Quantitative Immunogold Analysis of Membrane Receptors at Retinal Ribbon Synapses

Retinal ganglion cells (RGCs) receive excitatory glutamatergic input from bipolar cells. Synaptic excitation of RGCs is mediated postsynaptically by NMDA receptors (NMDARs) and AMPA receptors (AMPARs). Physiological data have indicated that glutamate receptors at RGCs are expressed not only in postsynaptic but also in perisynaptic or extrasynaptic membrane compartments. However, precise anatomical locations for glutamate receptors at RGC synapses have not been determined. Although a high-resolution quantitative analysis of glutamate receptors at central synapses is widely employed, this approach has had only limited success in the retina. We developed a postembedding immunogold method for analysis of membrane receptors, making it possible to estimate the number, density and variability of these receptors at retinal ribbon synapses. Here we describe the tools, reagents, and the practical steps that are needed for: 1) successful preparation of retinal fixation, 2) freeze-substitution, 3) postembedding immunogold electron microscope (EM) immunocytochemistry and, 4) quantitative visualization of glutamate receptors at ribbon synapses.

The postembedding immunogold method is one of the most effective ways to provide high-resolution analyses of the subcellular localization of specific molecules. Here we describe a protocol to quantitatively analyze glutamate receptors at retinal ribbon synapses.

Retinal ganglion cells (RGCs) receive excitatory glutamatergic input from bipolar cells. Synaptic excitation of RGCs is mediated postsynaptically by NMDA ...

Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices

Optogenetic approaches are now widely used to study the function of neural populations and circuits by combining targeted expression of light-activated proteins and subsequent manipulation of neural activity by light. Channelrhodopsins (ChRs) are light-gated cation-channels and when fused to a fluorescent protein their expression allows for visualization and concurrent activation of specific cell types and their axonal projections in defined areas of the brain. Via stereotactic injection of viral vectors, ChR fusion proteins can be constitutively or conditionally expressed in specific cells of a defined brain region, and their axonal projections can subsequently be studied anatomically and functionally via ex vivo optogenetic activation in brain slices. This is of particular importance when aiming to understand synaptic properties of connections that could not be addressed with conventional electrical stimulation approaches, or in identifying novel afferent and efferent connectivity that was previously poorly understood. Here, a few examples illustrate how this technique can be applied to investigate these questions to elucidating fear-related circuits in the amygdala. The amygdala is a key region for acquisition and expression of fear, and storage of fear and emotional memories. Many lines of evidence suggest that the medial prefrontal cortex (mPFC) participates in different aspects of fear acquisition and extinction, but its precise connectivity with the amygdala is just starting to be understood. First, it is shown how ex vivo optogenetic activation can be used to study aspects of synaptic communication between mPFC afferents and target cells in the basolateral amygdala (BLA). Furthermore, it is illustrated how this ex vivo optogenetic approach can be applied to assess novel connectivity patterns using a group of GABAergic neurons in the amygdala, the paracapsular intercalated cell cluster (mpITC), as an example.

Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices

Optogenetic approaches are widely used to manipulate neural activity and assess the consequences for brain function. Here, a technique is outlined that&#160;upon in vivo expression of the optical activator Channelrhodopsin, allows for ex vivo analysis of synaptic properties of specific long range and local neural connections in fear-related circuits.

Optogenetic approaches are now widely used to study the function of neural populations and circuits by combining targeted expression of light-activated ...

An Antibody Feeding Approach to Study Glutamate Receptor Trafficking in Dissociated Primary Hippocampal Cultures

Cellular responses to external stimuli heavily rely on the set of receptors expressed at the cell surface at a given moment. Accordingly, the population of surface-expressed receptors is constantly adapting and subject to strict mechanisms of regulation. The paradigmatic example and one of the most studied trafficking events in biology is the regulated control of the synaptic expression of glutamate receptors (GluRs). GluRs mediate the vast majority of excitatory neurotransmission in the central nervous system and control physiological activity-dependent functional and structural changes at the synaptic and neuronal levels (e.g., synaptic plasticity). Modifications in the number, location, and subunit composition of surface expressed GluRs deeply affect neuronal function and, in fact, alterations in these factors are associated with different neuropathies. Presented here is a method to study GluR trafficking in dissociated hippocampal primary neurons. An &#34;antibody-feeding&#34; approach is used to differentially visualize GluR populations expressed at the surface and internal membranes. By labeling surface receptors on live cells and fixing them at different times to allow for receptors endocytosis and/or recycling, these trafficking processes can be evaluated and selectively studied. This is a versatile protocol that can be used in combination with pharmacological approaches or overexpression of altered receptors to gain valuable information about stimuli and molecular mechanisms affecting GluR trafficking. Similarly, it can be easily adapted to study other receptors or surface expressed proteins.

This article presents a method to study glutamate receptor (GluR) trafficking in dissociated primary hippocampal cultures. Using an antibody-feeding approach to label endogenous or overexpressed receptors in combination with pharmacological approaches, this method allows for the identification of molecular mechanisms regulating GluR surface expression by modulating internalization or recycling processes.

Cellular responses to external stimuli heavily rely on the set of receptors expressed at the cell surface at a given moment. Accordingly, the population ...

Single Synapse Indicators of Glutamate Release and Uptake in Acute Brain Slices from Normal and Huntington Mice

Synapses are highly compartmentalized functional units that operate independently on each other. In Huntington&#39;s disease (HD) and other neurodegenerative disorders, this independence might be compromised due to insufficient glutamate clearance and the resulting spill-in and spill-out effects. Altered astrocytic coverage of the presynaptic terminals and/or dendritic spines as well as a reduced size of glutamate transporter clusters at glutamate release sites have been implicated in the pathogenesis of diseases resulting in symptoms of dys-/hyperkinesia. However, the mechanisms leading to the dysfunction of glutamatergic synapses in HD are not well understood. Improving and applying synapse imaging we have obtained data shedding new light on the mechanisms impeding the initiation of movements. Here, we describe the principle elements of a relatively inexpensive approach to achieve single synapse resolution by using the new genetically encoded ultrafast glutamate sensor iGluu, wide-field optics, a scientific CMOS (sCMOS) camera, a 473 nm laser and a laser positioning system to evaluate the state of corticostriatal synapses in acute slices from age appropriate healthy or diseased mice. Glutamate transients were constructed from single or multiple pixels to obtain estimates of i) glutamate release based on the maximal elevation of the glutamate concentration [Glu] next to the active zone and ii) glutamate uptake as reflected in the time constant of decay (TauD) of the perisynaptic [Glu]. Differences in the resting bouton size and contrasting patterns of short-term plasticity served as criteria for the identification of corticostriatal terminals as belonging to the intratelencephalic (IT) or the pyramidal tract (PT) pathway. Using these methods, we discovered that in symptomatic HD mice ~40% of PT-type corticostriatal synapses exhibited insufficient glutamate clearance, suggesting that these synapses might be at risk to excitotoxic damage. The results underline the usefulness of TauD as a biomarker of dysfunctional synapses in Huntington mice with a hypokinetic phenotype.

We present a protocol to evaluate the balance between glutamate release and clearance at single corticostriatal glutamatergic synapses in acute slices from adult mice. This protocol uses the fluorescent sensor iGluu for glutamate detection, a sCMOS camera for signal acquisition and a device for focal laser illumination.

Synapses are highly compartmentalized functional units that operate independently on each other. In Huntington&#39;s disease (HD) and other ...

Immunolabeling of Neuronal Membrane Proteins in a Freeze-fractured Specimen of Mouse Brain Tissue

<a href='https://app.jove.com/v/53853/combined-optogenetic-freeze-fracture-replica-immunolabeling-to'>Source: Sch&#246;nherr, S., et. al. Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate Receptors in the Mouse Amygdala. J. Vis. Exp. (2016)</a>This video demonstrates the immunolabelling of freeze-fractured replicas of mouse brain tissue for electron microscopy. The fractured and coated replicas are digested to expose receptors. Primary and gold-tagged secondary antibodies are added to label specific neuronal membrane receptors. The replica is mounted on a grid and visualized under an electron microscope.

Source: Sch&#246;nherr, S., et. al. Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate ...

JoVE Encyclopedia of Experiments

Neuropathology

Epilepsy and Seizure Disorders

Measuring Protein Expression in the Rodent Brain Using Near-Infrared Fluorescence and High-Resolution Scanning 

Source:&#160;<a style='text-decoration:none;' href='https://app.jove.com/v/59685/using-near-infrared-fluorescence-and-high-resolution-scanning-to-measure-protein-expression-in-the-rodent-brain'><span style='-webkit-text-decoration-skip:none;font-family:Arial,sans-serif;font-size:12pt;font-style:normal;font-variant:normal;font-weight:400;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre-wrap;'>Kimmelmann-Shultz, B.,&#160;et. al.<span style='-webkit-text-decoration-skip:none;font-family:Arial,sans-serif;font-size:12pt;font-style:normal;font-variant:normal;font-weight:400;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre-wrap;'>&#160;<span style='-webkit-text-decoration-skip:none;font-family:Arial,sans-serif;font-size:12pt;font-style:normal;font-variant:normal;font-weight:400;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre-wrap;'>Using Near-infrared Fluorescence and High-resolution Scanning to Measure Protein Expression in the Rodent Brain.&#160;J. Vis. Exp.<span style='-webkit-text-decoration-skip:none;font-family:Arial,sans-serif;font-size:12pt;font-style:normal;font-variant:normal;font-weight:400;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre-wrap;'>&#160;(2019)</a> &#160;This video demonstrates a near-infrared fluorescence and high-resolution scanning method for measuring protein expression in the rodent brain.

Source:&#160;Kimmelmann-Shultz, B.,&#160;et. al.&#160;Using Near-infrared Fluorescence and High-resolution Scanning to Measure Protein Expression in the ...

Neuroimaging

Synaptic & Cellular Imaging

Visualization and Mapping of Neuronal Pathways in an Amygdala Brain Slice

Source: Bosch, D., et al. <a href="https://app.jove.com/v/53628">Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices.</a> J. Vis. Exp. (2016).
This video demonstrates a procedure for analyzing synaptic connectivity between medial prefrontal cortex (mPFC) and amygdala neurons using acute amygdala brain slices from mice. The mPFC neurons in these mice express channelrhodopsin fused to a fluorescent protein. The channelrhodopsins facilitate ion influx and action potential generation in the mPFC neurons upon light activation. Consequently, mPFC neurons release neurotransmitters that bind to postsynaptic amygdala neurons, triggering postsynaptic currents that are recorded to visualize mPFC-amygdala connectivity.

All procedures involving animal samples have been reviewed and approved by the appropriate animal ethical review committee.
1. Visualization and ...

Neurophysiology

Imaging and Optical Techniques

An In Vitro Technique to Study Glutamate Receptor Trafficking in Hippocampal Neurons

Source: Chiu, A. M., et al. <a href="https://app.jove.com/v/59982">An Antibody Feeding Approach to Study Glutamate Receptor Trafficking in Dissociated Primary Hippocampal Cultures.</a> J. Vis. Exp. (2019)
This video demonstrates an antibody-feeding approach to study the trafficking of glutamate receptors in primary hippocampal neuronal cultures. This approach allows the observation of receptor populations at both the plasma and internal membranes. Discrimination between these subsets is achieved by labeling the receptors before and after membrane permeabilization, using the same primary antibody but secondary antibodies conjugated to different fluorophores.

1. Preparation Before Labeling

	Preparation and maintenance of primary hippocampal cultures&#8203;
	
		Prepare primary hippocampal cultures at a density ...

Immunology

Antibody-Based Technologies

Characterization and Functional Analysis

Generation of Tilted Amygdala Slices from a Mouse Brain

Source: Bosch, D., et al. <a href="https://app.jove.com/v/53628">Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices.</a> J. Vis. Exp. (2016)
The video demonstrates the generation of tilted amygdala slices from a mouse brain. The brain is positioned at an angle on the vibratome specimen stage to obtain slices containing the amygdala and axons from the medial prefrontal cortex that form synaptic connections with the amygdala. The brain slices are maintained at the physiological temperature to preserve the tissue for further experiments.

All procedures involving sample collection have been performed in accordance with the institute&#39;s IRB guidelines.
1. Preparation of Acute Slices

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Combined Optogenetic and Freeze-fracture Replica Immunolabeling to Examine Input-specific Arrangement of Glutamate Receptors in the Mouse Amygdala

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