Synapse-specific localization of NMDA and GABAA receptor subunits revealed by antigen-retrieval immunohistochemistry

Dual Role of NMDAR Containing NR2A and NR2B Subunits in Alzheimer's Disease

Alzheimer's disease (AD) is the main cause of dementia worldwide. Given that learning and memory are impaired in this pathology, NMDA receptors (NMDARs) appear as key players in the onset and progression of the disease. NMDARs are glutamate receptors, mainly located at the post-synapse, which regulate voltage-dependent influx of calcium into the neurons. They are heterotetramers, and there are different subunits that can be part of the receptors, which are usually composed of two obligatory GluN1 subunits plus either two NR2A or two NR2B subunits. NR2A are mostly located at the synapse, and their activation is involved in the expression of pro-survival genes. Conversely, NR2B are mainly extrasynaptic, and their activation has been related to cell death and neurodegeneration. Thus, activation of NR2A and/or inactivation of NR2B-containing NMDARS has been proposed as a therapeutic strategy to treat AD. Here, we wanted to investigate the main differences between both subunits signalling in neuronal primary cultures of the cortex and hippocampus. It has been observed that Aβ induces a significant increase in calcium release and also in MAPK phosphorylation signalling in NR2B-containing NMDAR in cortical and hippocampal neurons. However, while NR2A-containing NMDAR decreases neuronal death and favours cell viability after Aβ treatment, NR2B-containing NMDAR shows higher levels of cytotoxicity and low levels of neuronal survival. Finally, it has been detected that NMDAR has no effect on pTau axonal transport. The present results demonstrate a different role between GluNA and GluNB subunits in neurodegenerative diseases such as Alzheimer's.

Behavioral as well as hippocampal transcriptomic and microglial responses differ across sexes in adult mouse offspring exposed to a dual genetic and environmental challenge

INTRODUCTION

A wide range of positive, negative, and cognitive symptoms compose the clinical presentation of schizophrenia. Schizophrenia is a multifactorial disorder in which genetic and environmental risk factors interact for a full emergence of the disorder. Infectious challenges during pregnancy are a well-known environmental risk factor for schizophrenia. Also, genetic variants affecting the function of fractalkine signaling between neurons and microglia were linked to schizophrenia. Translational animal models recapitulating these complex gene-environment associations have a great potential to untangle schizophrenia neurobiology and propose new therapeutic strategies.

METHODS

Given that genetic variants affecting the function of fractalkine signaling between neurons and microglia were linked to schizophrenia, we compared the outcomes of a well-characterized model of maternal immune activation induced using the viral mimetic polyinosinic:polycytidylic acid (Poly I:C) in wild-type versus fractalkine receptor knockout mice. Possible behavioral and immune alterations were assessed in male and female offspring during adulthood. Considering the role of the hippocampus in schizophrenia, microglial analyses and bulk RNA sequencing were performed within this region to assess the neuroimmune dynamics at play. Males and females were examined separately.

RESULTS

Offspring exposed to the dual challenge paradigm exhibited symptoms relevant to schizophrenia and unpredictably to mood disorders. Males displayed social and cognitive deficits related to schizophrenia, while females mainly presented anxiety-like behaviors related to mood disorders. Hippocampal microglia in females exposed to the dual challenge were hypertrophic, indicative of an increased surveillance, whereas those in males showed on the other end of the spectrum blunted morphologies with a reduced phagocytosis. Hippocampal bulk-RNA sequencing further revealed a downregulation in females of genes related to GABAergic transmission, which represents one of the main proposed causes of mood disorders.

CONCLUSIONS

Building on previous results, we identified in the current study distinctive behavioral phenotypes in female mice exposed to a dual genetic and environmental challenge, thus proposing a new model of neurodevelopmentally-associated mood and affective symptoms. This paves the way to future sex-specific investigations into the susceptibility to developmental challenges using animal models based on genetic and immune vulnerability as presented here.

Evolutionary conservation of hippocampal mossy fiber synapse properties

Various specialized structural/functional properties are considered essential for contextual memory encoding by hippocampal mossy fiber (MF) synapses. Although investigated to exquisite detail in model organisms, synapses, including MFs, have undergone minimal functional interrogation in humans. To determine the translational relevance of rodent findings, we evaluated MF properties within human tissue resected to treat epilepsy. Human MFs exhibit remarkably similar hallmark features to rodents, including AMPA receptor-dominated synapses with small contributions from NMDA and kainate receptors, large dynamic range with strong frequency facilitation, NMDA receptor-independent presynaptic long-term potentiation, and strong cyclic AMP (cAMP) sensitivity of release. Array tomography confirmed the evolutionary conservation of MF ultrastructure. The astonishing congruence of rodent and human MF core features argues that the basic MF properties delineated in animal models remain critical to human MF function. Finally, a selective deficit in GABAergic inhibitory tone onto human MF postsynaptic targets suggests that unrestrained detonator excitatory drive contributes to epileptic circuit hyperexcitability.

Reclusive chandeliers: Functional isolation of dentate axo-axonic cells after experimental status epilepticus

Axo-axonic cells (AACs) provide specialized inhibition to the axon initial segment (AIS) of excitatory neurons and can regulate network output and synchrony. Although hippocampal dentate AACs are structurally altered in epilepsy, physiological analyses of dentate AACs are lacking. We demonstrate that parvalbumin neurons in the dentate molecular layer express PTHLH, an AAC marker, and exhibit morphology characteristic of AACs. Dentate AACs show high-frequency, non-adapting firing but lack persistent firing in the absence of input and have higher rheobase than basket cells suggesting that AACs can respond reliably to network activity. Early after pilocarpine-induced status epilepticus (SE), dentate AACs receive fewer spontaneous excitatory and inhibitory synaptic inputs and have significantly lower maximum firing frequency. Paired recordings and spatially localized optogenetic stimulation revealed that SE reduced the amplitude of unitary synaptic inputs from AACs to granule cells without altering reliability, short-term plasticity, or AIS GABA reversal potential. These changes compromised AAC-dependent shunting of granule cell firing in a multicompartmental model. These early post-SE changes in AAC physiology would limit their ability to receive and respond to input, undermining a critical brake on the dentate throughput during epileptogenesis.

RECLUSIVE CHANDELIERS: FUNCTIONAL ISOLATION OF DENTATE AXO-AXONIC CELLS AFTER EXPERIMENTAL STATUS EPILEPTICUS

Axo-axonic cells (AACs) provide specialized inhibition to the axon initial segment (AIS) of excitatory neurons and can regulate network output and synchrony. Although hippocampal dentate AACs are structurally altered in epilepsy, physiological analyses of dentate AACs are lacking. We demonstrate that parvalbumin neurons in the dentate molecular layer express PTHLH, an AAC marker, and exhibit morphology characteristic of AACs. Dentate AACs show high-frequency, non-adapting firing but lack persistent firing in the absence of input and have higher rheobase than basket cells suggesting that AACs can respond reliably to network activity. Early after pilocarpine-induced status epilepticus (SE), dentate AACs receive fewer spontaneous excitatory and inhibitory synaptic inputs and have significantly lower maximum firing frequency. Paired recordings and spatially localized optogenetic stimulation revealed that SE reduced the amplitude of unitary synaptic inputs from AACs to granule cells without altering reliability, short-term plasticity, or AIS GABA reversal potential. These changes compromised AAC-dependent shunting of granule cell firing in a multicompartmental model. These early post-SE changes in AAC physiology would limit their ability to receive and respond to input, undermining a critical brake on the dentate throughput during epileptogenesis.

Glyoxal fixation: An approach to solve immunohistochemical problem in neuroscience research

The gold-standard fixative for immunohistochemistry is 4% formaldehyde; however, it limits antibody access to target molecules that are buried within specialized neuronal components, such as ionotropic receptors at the postsynapse and voltage-gated ion channels at the axon initial segment, often requiring additional antigen-exposing techniques to detect their authentic signals. To solve this problem, we used glyoxal, a two-carbon atom di-aldehyde. We found that glyoxal fixation greatly improved antibody penetration and immunoreactivity, uncovering signals for buried molecules by conventional immunohistochemical procedures at light and electron microscopic levels. It also enhanced immunosignals of most other molecules, which are known to be detectable in formaldehyde-fixed sections. Furthermore, we unearthed several specific primary antibodies that were once judged to be unusable in formaldehyde-fixed tissues, allowing us to successfully localize so far controversial synaptic adhesion molecule Neuroligin 1. Thus, glyoxal is a highly effective fixative for immunostaining, and a side-by-side comparison of glyoxal and formaldehyde fixation is recommended for routine immunostaining in neuroscience research.

Different modes of synaptic and extrasynaptic NMDA receptor alteration in the hippocampus of P301S tau transgenic mice

N-methyl-d-aspartate receptors (NMDARs) are pivotal players in the synaptic transmission and synaptic plasticity underlying learning and memory. Accordingly, dysfunction of NMDARs has been implicated in the pathophysiology of Alzheimer disease (AD). Here, we used histoblot and sodium dodecylsulphate-digested freeze-fracture replica labelling (SDS-FRL) techniques to investigate the expression and subcellular localisation of GluN1, the obligatory subunit of NMDARs, in the hippocampus of P301S mice. Histoblots showed that GluN1 expression was significantly reduced in the hippocampus of P301S mice in a laminar-specific manner at 10 months of age but was unaltered at 3 months. Using the SDS-FRL technique, excitatory synapses and extrasynaptic sites on spines of pyramidal cells and interneuron dendrites were analysed throughout all dendritic layers in the CA1 field. Our ultrastructural approach revealed a high density of GluN1 in synaptic sites and a substantially lower density at extrasynaptic sites. Labelling density for GluN1 in excitatory synapses established on spines was significantly reduced in P301S mice, compared with age-matched wild-type mice, in the stratum oriens (so), stratum radiatum (sr) and stratum lacunosum-moleculare (slm). Density for synaptic GluN1 on interneuron dendrites was significantly reduced in P301S mice in the so and sr but unaltered in the slm. Labelling density for GluN1 at extrasynaptic sites showed no significant differences in pyramidal cells, and only increased density in the interneuron dendrites of the sr. This differential alteration of synaptic versus extrasynaptic NMDARs supports the notion that the progressive accumulation of phospho-tau is associated with changes in NMDARs, in the absence of amyloid-β pathology, and may be involved in the mechanisms causing abnormal network activity of the hippocampal circuit.

p21-activated kinase 4 controls the aggregation of α-synuclein by reducing the monomeric and aggregated forms of α-synuclein: involvement of the E3 ubiquitin ligase NEDD4-1

Aggregation of misfolded alpha-synuclein (α-synuclein) is a central player in the pathogenesis of neurodegenerative diseases. Therefore, the regulatory mechanism underlying α-synuclein aggregation has been intensively studied in Parkinson's disease (PD) but remains poorly understood. Here, we report p21-activated kinase 4 (PAK4) as a key regulator of α-synuclein aggregation. Immunohistochemical analysis of human PD brain tissues revealed an inverse correlation between PAK4 activity and α-synuclein aggregation. To investigate their causal relationship, we performed loss-of-function and gain-of-function studies using conditional PAK4 depletion in nigral dopaminergic neurons and the introduction of lentivirus expressing a constitutively active form of PAK4 (caPAK4; PAK4^S445N/S474E), respectively. For therapeutic relevance in the latter setup, we injected lentivirus into the striatum following the development of motor impairment and analyzed the effects 6 weeks later. In the loss-of-function study, Cre-driven PAK4 depletion in dopaminergic neurons enhanced α-synuclein aggregation, intracytoplasmic Lewy body-like inclusions and Lewy-like neurites, and reduced dopamine levels in PAK4^DAT-CreER mice compared to controls. Conversely, caPAK4 reduced α-synuclein aggregation, as assessed by a marked decrease in both proteinase K-resistant and Triton X100-insoluble forms of α-synuclein in the AAV-α-synuclein-induced PD model. Mechanistically, PAK4 specifically interacted with the NEDD4-1 E3 ligase, whose pharmacological inhibition and knockdown suppressed the PAK4-mediated downregulation of α-synuclein. Collectively, these results provide new insights into the pathogenesis of PD and suggest PAK4-based gene therapy as a potential disease-modifying therapy in PD.

Identifying Neural Progenitor Cells in the Adult Human Brain

The discovery, in 1998, that the adult human brain contains at least two populations of progenitor cells and that progenitor cells are upregulated in response to a range of degenerative brain diseases has raised hopes for their use in replacing dying brain cells. Since these early findings, the race has been on to understand the biology of progenitor cells in the human brain, and they have now been isolated and studied in many major neurodegenerative diseases. Before these cells can be exploited for cell replacement purposes, it is important to understand how to (1) locate them, (2) label them, (3) determine what receptors they express, (4) isolate them, and (5) examine their electrophysiological properties when differentiated. In this chapter we have described the methods we use for studying progenitor cells in the adult human brain and in particular the tissue processing, immunohistochemistry, autoradiography, progenitor cell culture, and electrophysiology on brain cells. The Neurological Foundation of New Zealand Human Brain Bank has been receiving human tissue for approximately 25 years during which time we have developed a number of unique ways to examine and isolate progenitor cells from resected surgical specimens as well as from postmortem brain tissue. There are ethical and technical considerations that are unique to working with human brain tissue, and these, as well as the processing of this tissue and the culturing of it for the purpose of studying progenitor cells, are the topic of this chapter.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Biophysical and synaptic properties of NMDA receptors in the lateral habenula

Excitatory synaptic transmission in the lateral habenula (LHb), an evolutionarily ancient subcortical structure, encodes aversive stimuli and affective states. Habenular glutamatergic synapses contribute to these processes partly through the activation of AMPA receptors. Yet, N-methyl-d-aspartate receptors (NMDARs) are also expressed in the LHb and support the emergence of depressive symptoms. Indeed, local NMDAR blockade in the LHb rescues anhedonia and behavioral despair in rodent models of depression. However, the subunit composition and biophysical properties of habenular NMDARs remain unknown, thereby hindering their study in the context of mental health. Here, we performed electrophysiological recordings and optogenetic-assisted circuit mapping in mice, to study pharmacologically-isolated NMDAR currents in LHb neurons that receive innervation from different brain regions (entopeduncular nucleus, lateral hypothalamic area, bed nucleus of the stria terminalis, or ventral tegmental area). This systematic approach revealed that habenular NMDAR currents are sensitive to TCN and ifenprodil - drugs that specifically inhibit GluN2A- and GluN2B-containing NMDARs, respectively. Whilst these pharmacological effects were consistently observed across inputs, we detected region-specific differences in the current-voltage relationship and decay time of NMDAR currents. Finally, inspired by the firing of LHb neurons in vivo, we designed a burst protocol capable of eliciting calcium-dependent long-term potentiation of habenular NMDAR transmission ex vivo. Altogether, we define basic biophysical and synaptic properties of NMDARs in LHb neurons, opening new avenues for studying their plasticity processes in physiological as well as pathological contexts.

A High-Resolution Method for Quantitative Molecular Analysis of Functionally Characterized Individual Synapses

Elucidating the molecular mechanisms underlying the functional diversity of synapses requires a high-resolution, sensitive, diffusion-free, quantitative localization method that allows the determination of many proteins in functionally characterized individual synapses. Array tomography permits the quantitative analysis of single synapses but has limited sensitivity, and its application to functionally characterized synapses is challenging. Here, we aim to overcome these limitations by searching the parameter space of different fixation, resin, embedding, etching, retrieval, and elution conditions. Our optimizations reveal that etching epoxy-resin-embedded ultrathin sections with Na-ethanolate and treating them with SDS dramatically increase the labeling efficiency of synaptic proteins. We also demonstrate that this method is ideal for the molecular characterization of individual synapses following paired recordings, two-photon [Ca²⁺] or glutamate-sensor (iGluSnFR) imaging. This method fills a missing gap in the toolbox of molecular and cellular neuroscience, helping us to reveal how molecular heterogeneity leads to diversity in function.

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Etching and antigen retrieval enhance immunoreactions in epoxy-resin-embedded tissue

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Biocytin-filled nerve cells can be visualized in epoxy-resin-embedded tissue

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Molecular composition of functionally characterized individual synapses is revealed

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Multiplexed, postembedding reactions are compatible with STED imaging

Immunohistochemical detection of GluA1 subunit of AMPA receptor in the rat nucleus accumbens following cocaine exposure

α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors are differentially regulated in the nucleus accumbens (NAcc) of the brain after cocaine exposure. However, these results are supported only by biochemical and electrophysiological methods, but have not been validated with immunohistochemistry. To overcome the restriction of antigen loss on the postsynaptic target molecules that occurs during perfusion-fixation, we adopted an immersion-fixation method that enabled us to immunohistochemically quantify the expression levels of the AMPA receptor GluA1 subunit in the NAcc. Interestingly, compared to saline exposure, cocaine significantly increased the immunofluorescence intensity of GluA1 in two sub-regions, the core and the shell, of the NAcc on withdrawal day 21 following cocaine exposure, which led to locomotor sensitization. Increases in GluA1 intensity were observed in both the extra-post synaptic density (PSD) and PSD areas in the two sub-regions of the NAcc. These results clearly indicate that AMPA receptor plasticity, as exemplified by GluA1, in the NAcc can be visually detected by immunohistochemistry and confocal imaging. These results expand our understanding of the molecular changes occurring in neuronal synapses by adding a new form of analysis to conventional biochemical and electrophysiological methods.

A Viral Toolbox of Genetically Encoded Fluorescent Synaptic Tags

Fibronectin intrabodies generated with mRNA display (FingRs) are a recently developed tool for labeling excitatory or inhibitory synapses, with the benefit of not altering endogenous synaptic protein expression levels or synaptic transmission. Here, we generated a viral vector FingR toolbox that allows for multi-color, neuron-type-specific labeling of excitatory or inhibitory synapses in multiple brain regions. We screened various fluorophores, FingR fusion configurations, and transcriptional control regulations in adeno-associated virus (AAV) and retrovirus vector designs. We report the development of a red FingR variant and demonstrated dual labeling of excitatory and inhibitory synapses in the same cells. Furthermore, we developed cre-inducible FingR AAV variants and demonstrated their utility, finding that the density of inhibitory synapses in aspiny striatal cholinergic interneurons remained unchanged in response to dopamine depletion. Finally, we generated FingR retroviral vectors, which enabled us to track the development of excitatory and inhibitory synapses in hippocampal adult-born granule cells.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca²⁺ ) into the post-synaptic compartment. Ca²⁺ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

Potentiating α2 subunit containing perisomatic GABAA receptors protects against seizures in a mouse model of Dravet syndrome

KEY POINTS

Dravet syndrome mice (Scn1a^+/- ) demonstrate a marked strain dependence for the severity of seizures which is correlated with GABA_A receptor α₂ subunit expression. The α₂ /α₃ subunit selective positive allosteric modulator (PAM) AZD7325 potentiates inhibitory postsynaptic currents (IPSCs) specifically in perisomatic synapses. AZD7325 demonstrates stronger effects on IPSCs in the seizure resistant mouse strain, consistent with higher α₂ subunit expression. AZD7325 demonstrates seizure protective effects in Scn1a^+/- mice without apparent sedative effects in vivo.

ABSTRACT

GABA_A receptor potentiators are commonly used for the treatment of epilepsy, but it is not clear whether targeting distinct GABA_A receptor subtypes will have disproportionate benefits over adverse effects. Here we demonstrate that the α₂ /α₃ selective positive allosteric modulator (PAM) AZD7325 preferentially potentiates hippocampal inhibitory responses at synapses proximal to the soma of CA1 neurons. The effect of AZD7325 on synaptic responses was more prominent in mice on the 129S6/SvEvTac background strain, which have been demonstrated to be seizure resistant in the model of Dravet syndrome (Scn1a^+/- ), and in which the α₂ GABA_A receptor subunits are expressed at higher levels relative to in the seizure prone C57BL/6J background strain. Consistent with this, treatment of Scn1a^+/- mice with AZD7325 elevated the temperature threshold for hyperthermia-induced seizures without apparent sedative effects. Our results in a model system indicate that selectively targeting α₂ is a potential therapeutic option for Dravet syndrome.

A Computational Synaptic Antibody Characterization Tool for Array Tomography

Application-specific validation of antibodies is a critical prerequisite for their successful use. Here we introduce an automated framework for characterization and screening of antibodies against synaptic molecules for high-resolution immunofluorescence array tomography (AT). The proposed Synaptic Antibody Characterization Tool (SACT) is designed to provide an automatic, robust, flexible, and efficient tool for antibody characterization at scale. SACT automatically detects puncta of immunofluorescence labeling from candidate antibodies and determines whether a punctum belongs to a synapse. The molecular composition and size of the target synapses expected to contain the antigen is determined by the user, based on biological knowledge. Operationally, the presence of a synapse is defined by the colocalization or adjacency of the candidate antibody punctum to one or more reference antibody puncta. The outputs of SACT are automatically computed measurements such as target synapse density and target specificity ratio that reflect the sensitivity and specificity of immunolabeling with a given candidate antibody. These measurements provide an objective way to characterize and compare the performance of different antibodies against the same target, and can be used to objectively select the antibodies best suited for AT and potentially for other immunolabeling applications.

Activity-dependent control of NMDA receptor subunit composition at hippocampal mossy fibre synapses

KEY POINTS

CA3 pyramidal cells display input-specific differences in the subunit composition of synaptic NMDA receptors (NMDARs). Although at low density, GluN2B contributes significantly to NMDAR-mediated EPSCs at mossy fibre synapses. Long-term potentiation (LTP) of NMDARs triggers a modification in the subunit composition of synaptic NMDARs by insertion of GluN2B. GluN2B subunits are essential for the expression of LTP of NMDARs at mossy fibre synapses.

ABSTRACT

Single neurons express NMDA receptors (NMDARs) with distinct subunit composition and biophysical properties that can be segregated in an input-specific manner. The dynamic control of the heterogeneous distribution of synaptic NMDARs is crucial to control input-dependent synaptic integration and plasticity. In hippocampal CA3 pyramidal cells from mice of both sexes, we found that mossy fibre (MF) synapses display a markedly lower proportion of GluN2B-containing NMDARs than associative/commissural synapses. The mechanism involved in such heterogeneous distribution of GluN2B subunits is not known. Here we show that long-term potentiation (LTP) of NMDARs, which is selectively expressed at MF-CA3 pyramidal cell synapses, triggers a modification in the subunit composition of synaptic NMDARs by insertion of GluN2B. This activity-dependent recruitment of GluN2B at mature MF-CA3 pyramidal cell synapses contrasts with the removal of GluN2B subunits at other glutamatergic synapses during development and in response to activity. Furthermore, although expressed at low levels, GluN2B is necessary for the expression of LTP of NMDARs at MF-CA3 pyramidal cell synapses. Altogether, we reveal a previously unknown activity-dependent regulation and function of GluN2B subunits that may contribute to the heterogeneous plasticity induction rules in CA3 pyramidal cells.

Nigral dopaminergic PAK4 prevents neurodegeneration in rat models of Parkinson's disease

Parkinson's disease (PD) is characterized by progressive loss of dopaminergic (DA) neurons in the substantia nigra. No neuroprotective treatments have successfully prevented the progression of this disease. We report that p21-activated kinase 4 (PAK4) is a key survival factor for DA neurons. We observed PAK4 immunoreactivity in rat and human DA neurons in brain tissue, but not in microglia or astrocytes. PAK4 activity was markedly decreased in postmortem brain tissue from PD patients and in rodent models of PD. Expression of constitutively active PAK4^S445N/S474E (caPAK4) protected DA neurons in both the 6-hydroxydopamine and α-synuclein rat models of PD and preserved motor function. This neuroprotective effect of caPAK4 was mediated by phosphorylation of CRTC1 [CREB (adenosine 3',5'-monophosphate response element-binding protein)-regulated transcription coactivator] at S215. The nonphosphorylated form of CRTC1^S215A compromised the ability of caPAK4 to induce the expression of the CREB target proteins Bcl-2, BDNF, and PGC-1α. Our results support a neuroprotective role for the PAK4-CRTC1^S215-CREB signaling pathway and suggest that this pathway may be a useful therapeutic target in PD.

Developing high-quality mouse monoclonal antibodies for neuroscience research - approaches, perspectives and opportunities

High-quality antibodies (Abs) are critical to neuroscience research, as they remain the primary affinity proteomics reagent used to label and capture endogenously expressed protein targets in the nervous system. As in other fields, neuroscientists are frequently confronted with inaccurate and irreproducible Ab-based results and/or reporting. The UC Davis/NIH NeuroMab Facility was created with the mission of addressing the unmet need for high-quality Abs in neuroscience research by applying a unique approach to generate and validate mouse monoclonal antibodies (mAbs) optimized for use against mammalian brain (i.e., NeuroMabs). Here we describe our methodology of multi-step mAb screening focused on identifying mAbs exhibiting efficacy and specificity in labeling mammalian brain samples. We provide examples from NeuroMab screens, and from the subsequent specialized validation of those selected as NeuroMabs. We highlight the particular challenges and considerations of determining specificity for brain immunolabeling. We also describe why our emphasis on extensive validation of large numbers of candidates by immunoblotting and immunohistochemistry against brain samples is essential for identifying those that exhibit efficacy and specificity in those applications to become NeuroMabs. We describe the special attention given to candidates with less common non-IgG1 IgG subclasses that can facilitate simultaneous multiplex labeling with subclass-specific secondary antibodies. We detail our recent use of recombinant cloning of NeuroMabs as a method to archive all NeuroMabs, to unambiguously define NeuroMabs at the DNA sequence level, and to re-engineer IgG1 NeuroMabs to less common IgG subclasses to facilitate their use in multiplex labeling. Finally, we provide suggestions to facilitate Ab development and use, as to design, execution and interpretation of Ab-based neuroscience experiments. Reproducibility in neuroscience research will improve with enhanced Ab validation, unambiguous identification of Abs used in published experiments, and end user proficiency in Ab-based assays.

Similar GABAA receptor subunit composition in somatic and axon initial segment synapses of hippocampal pyramidal cells

Hippocampal pyramidal cells (PCs) express many GABA_AR subunit types and receive GABAergic inputs from distinct interneurons. Previous experiments revealed input-specific differences in α1 and α2 subunit densities in perisomatic synapses, suggesting distinct IPSC decay kinetics. However, IPSC decays evoked by axo-axonic, parvalbumin- or cholecystokinin-expressing basket cells were found to be similar. Using replica immunogold labeling, here we show that all CA1 PC somatic and AIS synapses contain the α1, α2, β1, β2, β3 and γ2 subunits. In CA3 PCs, 90% of the perisomatic synapses are immunopositive for the α1 subunit and all synapses are positive for the remaining five subunits. Somatic synapses form unimodal distributions based on their immunoreactivity for these subunits. The α2 subunit densities in somatic synapses facing Cav2.1 (i.e. parvalbumin) or Cav2.2 (cholecystokinin) positive presynaptic active zones are comparable. We conclude that perisomatic synapses made by three distinct interneuron types have similar GABA_A receptor subunit content.

DOI:http://dx.doi.org/10.7554/eLife.18426.001

Immunostaining for Homer reveals the majority of excitatory synapses in laminae I-III of the mouse spinal dorsal horn

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Identifying glutamatergic synapses is important for tracing synaptic circuits.

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Most proteins at glutamatergic synapses are masked by tissue fixation.

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Homer can reveal glutamatergic synapses without the need for antigen retrieval.

The spinal dorsal horn processes somatosensory information before conveying it to the brain. The neuronal organization of the dorsal horn is still poorly understood, although recent studies have defined several distinct populations among the interneurons, which account for most of its constituent neurons. All primary afferents, and the great majority of neurons in laminae I–III are glutamatergic, and a major factor limiting our understanding of the synaptic circuitry has been the difficulty in identifying glutamatergic synapses with light microscopy. Although there are numerous potential targets for antibodies, these are difficult to visualize with immunocytochemistry, because of protein cross-linking following tissue fixation. Although this can be overcome by antigen retrieval methods, these lead to difficulty in detecting other antigens. The aim of this study was to test whether the postsynaptic protein Homer can be used to reveal glutamatergic synapses in the dorsal horn. Immunostaining for Homer gave punctate labeling when viewed by confocal microscopy, and this was restricted to synapses at the ultrastructural level. We found that Homer puncta were colocalized with the AMPA receptor GluR2 subunit, but not with the inhibitory synapse-associated protein gephyrin. We also examined several populations of glutamatergic axons and found that most boutons were in contact with at least one Homer punctum. These results suggest that Homer antibodies can be used to reveal the great majority of glutamatergic synapses without antigen retrieval. This will be of considerable value in tracing synaptic circuits, and also in investigating plasticity of glutamatergic synapses in pain states.

α2-containing GABA(A) receptors: a requirement for midazolam-escalated aggression and social approach in mice

RATIONALE

Benzodiazepines (BZDs) are prescribed to reduce anxiety, agitation, and muscle spasms and for their sedative-hypnotic and anticonvulsant effects. Under specific conditions, BZDs escalate aggression in some individuals. Specific effects of BZDs have been linked to the α-subunit subtype composition of GABAA receptors.

OBJECTIVES

Point-mutated mice rendered selectively insensitive to BZDs at α1-, α2-, or α3-containing GABAA receptors were used to determine which α-subunit subtypes are necessary for BZDs to escalate aggression and social approach and to reduce fear-motivated behavior.

METHODS

During resident-intruder confrontations, male wild-type (WT) and point-mutated α1(H101R), α2(H101R), and α3(H126R) mice were treated with midazolam (0-1.7 mg/kg, i.p.) and evaluated for aggression in an unfamiliar environment. Separate midazolam-treated WT and point-mutated mice were assessed for social approach toward a female or investigated in a 6-day fear-potentiated startle procedure.

RESULTS

Moderate doses of midazolam (0.3-0.56 mg/kg, i.p.) escalated aggression in WT and α3(H126R) mutants and increased social approach in WT and α1(H101R) mice. The highest dose of midazolam (1.0 mg/kg) reduced fear-potentiated startle responding. All mice were sensitive to the sedative effect of midazolam (1.7 mg/kg) except α1(H101R) mutants.

CONCLUSIONS

Midazolam requires BZD-sensitive α1- and α2-containing GABAA receptors in order to escalate aggression and α2- and α3-containing receptors to reduce social anxiety-like behavior. GABAA receptors containing the α1-subunit are crucial for BZD-induced sedation, while α2-containing GABAA receptors may be a shared site of action for the pro-aggressive and anxiolytic effects of BZDs.

Hunting increases phosphorylation of calcium/calmodulin-dependent protein kinase type II in adult barn owls

Juvenile barn owls readily adapt to prismatic spectacles, whereas adult owls living under standard aviary conditions do not. We previously demonstrated that phosphorylation of the cyclic-AMP response element-binding protein (CREB) provides a readout of the instructive signals that guide plasticity in juveniles. Here we investigated phosphorylation of calcium/calmodulin-dependent protein kinase II (pCaMKII) in both juveniles and adults. In contrast to CREB, we found no differences in pCaMKII expression between prism-wearing and control juveniles within the external nucleus of the inferior colliculus (ICX), the major site of plasticity. For prism-wearing adults that hunted live mice and are capable of adaptation, expression of pCaMKII was increased relative to prism-wearing adults that fed passively on dead mice and are not capable of adaptation. This effect did not bear the hallmarks of instructive information: it was not localized to rostral ICX and did not exhibit a patchy distribution reflecting discrete bimodal stimuli. These data are consistent with a role for CaMKII as a permissive rather than an instructive factor. In addition, the paucity of pCaMKII expression in passively fed adults suggests that the permissive default setting is "off" in adults.

Optical imaging. Expansion microscopy

In optical microscopy, fine structural details are resolved by using refraction to magnify images of a specimen. We discovered that by synthesizing a swellable polymer network within a specimen, it can be physically expanded, resulting in physical magnification. By covalently anchoring specific labels located within the specimen directly to the polymer network, labels spaced closer than the optical diffraction limit can be isotropically separated and optically resolved, a process we call expansion microscopy (ExM). Thus, this process can be used to perform scalable superresolution microscopy with diffraction-limited microscopes. We demonstrate ExM with apparent ~70-nanometer lateral resolution in both cultured cells and brain tissue, performing three-color superresolution imaging of ~10(7) cubic micrometers of the mouse hippocampus with a conventional confocal microscope.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Diurnal inhibition of NMDA-EPSCs at rat hippocampal mossy fibre synapses through orexin-2 receptors

Diurnal release of the orexin neuropeptides orexin-A (Ox-A, hypocretin-1) and orexin-B (Ox-B, hypocretin-2) stabilises arousal, regulates energy homeostasis and contributes to cognition and learning. However, whether cellular correlates of brain plasticity are regulated through orexins, and whether they do so in a time-of-day-dependent manner, has never been assessed. Immunohistochemically we found sparse but widespread innervation of hippocampal subfields through Ox-A- and Ox-B-containing fibres in young adult rats. The actions of Ox-A were studied on NMDA receptor (NMDAR)-mediated excitatory synaptic transmission in acute hippocampal slices prepared around the trough (Zeitgeber time (ZT) 4-8, corresponding to 4-8 h into the resting phase) and peak (ZT 23) of intracerebroventricular orexin levels. At ZT 4-8, exogenous Ox-A (100 nm in bath) inhibited NMDA receptor-mediated excitatory postsynaptic currents (NMDA-EPSCs) at mossy fibre (MF)-CA3 (to 55.6 ± 6.8% of control, P = 0.0003) and at Schaffer collateral-CA1 synapses (70.8 ± 6.3%, P = 0.013), whereas it remained ineffective at non-MF excitatory synapses in CA3. Ox-A actions were mediated postsynaptically and blocked by the orexin-2 receptor (OX2R) antagonist JNJ10397049 (1 μm), but not by orexin-1 receptor inhibition (SB334867, 1 μm) or by adrenergic and cholinergic antagonists. At ZT 23, inhibitory effects of exogenous Ox-A were absent (97.6 ± 2.9%, P = 0.42), but reinstated (87.2 ± 3.3%, P = 0.002) when endogenous orexin signalling was attenuated for 5 h through i.p. injections of almorexant (100 mg kg(-1)), a dual orexin receptor antagonist. In conclusion, endogenous orexins modulate hippocampal NMDAR function in a time-of-day-dependent manner, suggesting that they may influence cellular plasticity and consequent variations in memory performance across the sleep-wake cycle.

Localisation and stress-induced plasticity of GABAA receptor subunits within the cellular networks of the mouse dorsal raphe nucleus

The dorsal raphe nucleus (DRN) provides the major source of serotonin to the central nervous system (CNS) and modulates diverse neural functions including mood. Furthermore, DRN cellular networks are engaged in the stress-response at the CNS level allowing for adaptive behavioural responses, whilst stress-induced dysregulation of DRN and serotonin release is implicated in psychiatric disorders. Therefore, identifying the molecules regulating DRN activity is fundamental to understand DRN function in health and disease. GABAA receptors (GABAARs) allow for brain region, cell type and subcellular domain-specific GABA-mediated inhibitory currents and are thus key regulators of neuronal activity. Yet, the GABAAR subtypes expressed within the neurochemically diverse cell types of the mouse DRN are poorly described. In this study, immunohistochemistry and confocal microscopy revealed that all serotonergic neurons expressed immunoreactivity for the GABAAR alpha2 and 3 subunits, although the respective signals were co-localised to varying degrees with inhibitory synaptic marker proteins. Only a topographically located sub-population of serotonergic neurons exhibited GABAAR alpha1 subunit immunoreactivity. However, all GABAergic as well as non-GABAergic, non-serotonergic neurons within the DRN expressed GABAAR alpha1 subunit immunoreactivity. Intriguingly, immunoreactivity for the GABAAR gamma2 subunit was enriched on GABAergic rather than serotonergic neurons. Finally, repeated restraint stress increased the expression of the GABAAR alpha3 subunit at the mRNA and protein level. The study demonstrates the identity and location of distinct GABAAR subunits within the cellular networks of the mouse DRN and that stress impacts on the expression levels of particular subunits at the gene and protein level.

N-cadherin regulates molecular organization of excitatory and inhibitory synaptic circuits in adult hippocampus in vivo

N-Cadherin and β-catenin form a transsynaptic adhesion complex required for spine and synapse development. In adulthood, N-cadherin mediates persistent synaptic plasticity, but whether the role of N-cadherin at mature synapses is similar to that at developing synapses is unclear. To address this, we conditionally ablated N-cadherin from excitatory forebrain synapses in mice starting in late postnatal life and examined hippocampal structure and function in adulthood. In the absence of N-cadherin, β-catenin levels were reduced, but numbers of excitatory synapses were unchanged, and there was no impact on number or shape of dendrites or spines. However, the composition of synaptic molecules was altered. Levels of GluA1 and its scaffolding protein PSD95 were diminished and the density of immunolabeled puncta was decreased, without effects on other glutamate receptors and their scaffolding proteins. Additionally, loss of N-cadherin at excitatory synapses triggered increases in the density of markers for inhibitory synapses and decreased severity of hippocampal seizures. Finally, adult mutant mice were profoundly impaired in hippocampal-dependent memory for spatial episodes. These results demonstrate a novel function for the N-cadherin/β-catenin complex in regulating ionotropic receptor composition of excitatory synapses, an appropriate balance of excitatory and inhibitory synaptic proteins and the maintenance of neural circuitry necessary to generate flexible yet persistent cognitive and synaptic function.

Neuronal Ig/Caspr recognition promotes the formation of axoaxonic synapses in mouse spinal cord

Inhibitory microcircuits are wired with a precision that underlies their complex regulatory roles in neural information processing. In the spinal cord, one specialized class of GABAergic interneurons (GABApre) mediates presynaptic inhibitory control of sensory-motor synapses. The synaptic targeting of these GABAergic neurons exhibits an absolute dependence on proprioceptive sensory terminals, yet the molecular underpinnings of this specialized axoaxonic organization remain unclear. Here, we show that sensory expression of an NB2 (Contactin5)/Caspr4 coreceptor complex, together with spinal interneuron expression of NrCAM/CHL1, directs the high-density accumulation of GABAergic boutons on sensory terminals. Moreover, genetic elimination of NB2 results in a disproportionate stripping of inhibitory boutons from high-density GABApre-sensory synapses, suggesting that the preterminal axons of GABApre neurons compete for access to individual sensory terminals. Our findings define a recognition complex that contributes to the assembly and organization of a specialized GABAergic microcircuit.

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Sensory Ig/Caspr4 complex directs inhibitory synapse formation in mouse spinal cord

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Eliminating NB2 results in a reduced number of GABApre-sensory synapses

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Quantitative modeling suggests competition for formation of axoaxonic synapses

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Role for a contactin/Caspr complex in central synapse formation

Nuclear phospholipase C-β1 and diacylglycerol LIPASE-α in brain cortical neurons

Phosphoinositide (PtdIns) signaling involves the generation of lipid second messengers in response to stimuli in a receptor-mediated manner at the plasma membrane. In neuronal cells of adult brain, the standard model proposes that activation of metabotropic receptors coupled to Phospholipase C-β1 (PLC-β1) is linked to endocannabinoid signaling through the production of diacylglycerol (DAG), which could be systematically metabolized by 1,2-diacylglycerol Lipases (DAGL) to produce an increase of 2-arachidonoyl-glycerol (2-AG), the most abundant endocannabinoid in the brain. However, the existence of a nuclear PtdIns metabolism independent from that occurring elsewhere in the cell is now widely accepted, suggesting that the nucleus constitutes both a functional and a distinct compartment for PtdIns metabolism. In this review, we shall highlight the main achievements in the field of neuronal nuclear inositol lipid metabolism with particular attention to progress made linked to the 2-AG biosynthesis. Our aim has been to identify potential sites of 2-AG synthesis other than the neuronal cytoplasmic compartment by determining the subcellular localization of PLC-β1 and DAGL-α, which is much more abundant than DAGL-β in brain. Our data show that PLC-β1 and DAGL-α are detected in discrete brain regions, with a marked predominance of pyramidal morphologies of positive cortical cells, consistent with their role in the biosynthesis and release of 2-AG by pyramidal neurons to control their synaptic inputs. However, as novelty, we showed here an integrated description of the localization of PLC-β1 and DAGL-α in the neuronal nuclear compartment. We discuss our comparative analysis of the expression patterns of PLC-β1 and DAGL-α, providing some insight into the potential autocrine role of 2-AG production in the neuronal nuclear compartment that probably subserve additional roles to the recognized activation of the CB1 cannabinoid receptor.

The effects of caffeine on sleep and maturational markers in the rat

Adolescence is a critical period for brain maturation during which a massive reorganization of cortical connectivity takes place. In humans, slow wave activity (<4.5 Hz) during NREM sleep was proposed to reflect cortical maturation which relies on use-dependent processes. A stimulant like caffeine, whose consumption has recently increased especially in adolescents, is known to affect sleep wake regulation.

The goal of this study was to establish a rat model allowing to assess the relationship between cortical maturation and sleep and to further investigate how these parameters are affected by caffeine consumption. To do so, we assessed sleep and markers of maturation by electrophysiological recordings, behavioral and structural readouts in the juvenile rat. Our results show that sleep slow wave activity follows a similar inverted U-shape trajectory as already known in humans. Caffeine treatment exerted short-term stimulating effects and altered the trajectory of slow wave activity. Moreover, caffeine affected behavioral and structural markers of maturation. Thus, caffeine consumption during a critical developmental period shows long lasting effects on sleep and brain maturation.

GABA(A) receptor characterization and subunit localization in the human sub-ventricular zone

It is now well established that the human brain continuously produces new stem cells until well into old age. One of these stem-cell rich areas in the human brain is the sub-ventricular zone (SVZ). The human SVZ is organized in four distinctive layers containing type A, B and C cells. To date, no studies have investigated the distribution of inhibitory neurotransmitters such as γ-aminobutyric acid (GABA) and their respective receptors on the different cell types in the human SVZ. GABA(A) receptors (GABA(A)R) are ubiquitously expressed, inhibitory heteropentameric chloride ion channels comprised of a variety of subunits that are targeted by many prescribed drugs. In this study we present detailed immunohistochemical data on the regional and cellular localization of α₁, α₂, α3, β₂,₃ and γ₂ subunits of GABA(A)R in the human SVZ. The results from our double and triple labeling studies demonstrate that the cell types and subunit composition throughout the SVZ is heterogeneous; the thickness of the SVZ and GABA(A)R α₂ and γ₂ expression is increased especially in the vicinity of large SVZ blood vessels. GABA(A)R γ₂ is the most specific to the SVZ and present on various cells that express, either glial fibrillary acidic protein (GFAPδ) or polysialic acid-neural cell adhesion molecule (PSA-NCAM) separately, or together in a respective ratio of 7:6:2. Proliferating (type C) cells in the SVZ express GAD65/67, GFAPδ and GABA(A)R β₂,₃ receptor subunits. Within the SVZ the majority of cells have an unexpected nuclear GABA(A)R β₂,₃ expression that is inversely proportional to that of PCNA (proliferating cell nuclear antigen marker), which is a very different pattern of expression compared with underlying caudate nucleus cells. Taken together our results provide a detailed description of the chemo-architecture of the adult human SVZ demonstrating the importance of GABA and GABA(A) receptors on the various cell types in the SVZ.

Selective distribution of GABA(A) receptor subtypes in mouse spinal dorsal horn neurons and primary afferents

In the spinal cord dorsal horn, presynaptic GABA(A) receptors (GABA(A)Rs) in the terminals of nociceptors as well as postsynaptic receptors in spinal neurons regulate the transmission of nociceptive and somatosensory signals from the periphery. GABA(A)Rs are heterogeneous and distinguished functionally and pharmacologically by the type of α subunit variant they contain. This heterogeneity raises the possibility that GABA(A)R subtypes differentially regulate specific pain modalities. Here, we characterized the subcellular distribution of GABA(A)R subtypes in nociceptive circuits by using immunohistochemistry with subunit-specific antibodies combined with markers of primary afferents and dorsal horn neurons. Confocal laser scanning microscopy analysis revealed a distinct, partially overlapping laminar distribution of α1-3 and α5 subunit immunoreactivity in laminae I-V. Likewise, a layer-specific pattern was evident for their distribution among glutamatergic, γ-aminobutyric acid (GABA)ergic, and glycinergic neurons (detected in transgenic mice expressing vesicular glutamate transporter 2-enhanced green fluorescent protein [vGluT2-eGFP], glutamic acid decarboxylase [GAD]67-eGFP, and glycine transporter 2 (GlyT2)-eGFP, respectively). Finally, all four subunits could be detected within primary afferent terminals. C-fibers predominantly contained either α2 or α3 subunit immunoreactivity; terminals from myelinated (Aβ/Aδ) fibers were colabeled in roughly equal proportion with each subunit. The presence of axoaxonic GABAergic synapses was determined by costaining with gephyrin and vesicular inhibitory amino acid transporter to label GABAergic postsynaptic densities and terminals, respectively. Colocalization of the α2 or α3 subunit with these markers was observed in a subset of C-fiber synapses. Furthermore, gephyrin mRNA and protein expression was detected in dorsal root ganglia. Collectively, these results show that differential GABA(A)R distribution in primary afferent terminals and dorsal horn neurons allows for multiple, circuit-specific modes of regulation of nociceptive circuits.

Benzodiazepines counteract rostral anterior cingulate cortex activation induced by cholecystokinin-tetrapeptide in humans

BACKGROUND

Benzodiazepines modulate γ-aminobutyric acid type A (GABA(A)) receptors throughout the brain. However, it is not fully understood which brain regions within anxiety-related brain circuits are really responsible for their anxiolytic effects and how these regions interact.

METHODS

We investigated whether the benzodiazepine alprazolam affects activity in distinct brain regions within anxiety-related circuits during an experimental anxiety paradigm by means of functional magnetic resonance imaging (fMRI). Panic symptoms were elicited by a bolus injection of the neuropeptide cholecystokinin-tetrapeptide (CCK-4) in 16 healthy male subjects in a double-blind, placebo-controlled design. Functional brain activation patterns were determined before and during the CCK-4-challenge without pretreatment and after treatment with either placebo or 1 mg alprazolam.

RESULTS

The CCK-4 induced anxiety and elicited widely distributed activation patterns in anxiety-related brain circuits, especially in the rostral anterior cingulate cortex (rACC), which was attenuated after alprazolam treatment. In contrast to placebo, alprazolam abolished the activation of the rACC after challenge with CCK-4 (p<.005, corrected for multiple comparisons) and increased functional connectivity between the rACC and other anxiety-related brain regions such as amygdala and prefrontal cortex. Moreover, the reduction in the CCK-4 induced activation of the rACC correlated with the anxiolytic effect of alprazolam (r(p) = .52; p = .04).

CONCLUSIONS

These findings put forward the rACC as a target for benzodiazepines and suggest that the CCK-4/fMRI paradigm might represent a human translational model for the investigation of anxiolytic drugs.

Identifying neural progenitor cells in the adult human brain

The discovery, in 1998, that the adult human brain contains at least two populations of progenitor cells and that progenitor cells are upregulated in response to a range of degenerative brain diseases has raised hopes for their use in replacing dying brain cells. Since these early findings the race has been on to understand the biology of progenitor cells in the human brain and they have now been isolated and studied in many major neurodegenerative diseases. Before these cells can be exploited for cell replacement purposes it is important to understand how to: (1) find them, (2) label them, (3) determine what receptors they express, (4) isolate them, and (5) examine their electrophysiological properties when differentiated. In this chapter we have described the methods we use for studying progenitor cells in the adult human brain and in particular the tissue processing, immunohistochemistry, autoradiography, progenitor cell culture, and electrophysiology on brain cells. The Neurological Foundation of New Zealand Human Brain Bank has been receiving human tissue for approximately 20 years during which time we have developed a number of unique ways to examine and isolate progenitor cells from resected surgical specimens as well as from postmortem brain tissue. There are ethical and technical considerations that are unique to working with human brain tissue and these, as well as the processing of this tissue and the culturing of it for the purpose of studying progenitor cells, are the topic of this chapter.

α2-containing GABAA receptors expressed in hippocampal region CA3 control fast network oscillations

GABA(A) receptors are critically involved in hippocampal oscillations. GABA(A) receptor α1 and α2 subunits are differentially expressed throughout the hippocampal circuitry and thereby may have distinct contributions to oscillations. It is unknown which GABA(A) receptor α subunit controls hippocampal oscillations and where these receptors are expressed. To address these questions we used transgenic mice expressing GABA(A) receptor α1 and/or α2 subunits with point mutations (H101R) that render these receptors insensitive to allosteric modulation at the benzodiazepine binding site, and tested how increased or decreased function of α subunits affects hippocampal oscillations. Positive allosteric modulation by zolpidem prolonged decay kinetics of hippocampal GABAergic synaptic transmission and reduced the frequency of cholinergically induced oscillations. Allosteric modulation of GABAergic receptors in CA3 altered oscillation frequency in CA1, while modulation of GABA receptors in CA1 did not affect oscillations. In mice having a point mutation (H101R) at the GABA(A) receptor α2 subunit, zolpidem effects on cholinergically induced oscillations were strongly reduced compared to wild-type animals, while zolpidem modulation was still present in mice with the H101R mutation at the α1 subunit. Furthermore, genetic knockout of α2 subunits strongly reduced oscillations, whereas knockout of α1 subunits had no effect. Allosteric modulation of GABAergic receptors was strongly reduced in unitary connections between fast spiking interneurons and pyramidal neurons in CA3 of α2H101R mice, but not of α1H101R mice, suggesting that fast spiking interneuron to pyramidal neuron synapses in CA3 contain α2 subunits. These findings suggest that α2-containing GABA(A) receptors expressed in the CA3 region provide the inhibition that controls hippocampal rhythm during cholinergically induced oscillations.

Dissecting the role of diazepam-sensitive γ-aminobutyric acid type A receptors in defensive behavioral reactivity to mild threat

Moderate reductions in synaptic γ-aminobutyric acid(A) receptors (GABA(A)Rs) have been associated with an enhanced defensive behavioral reactivity to mild threat, sensitive to diazepam. We here tested whether a deficit in α2 subunit-containing GABAergic synapses is sufficient to cause this anxiety-related phenotype and to prevent its attenuation by the benzodiazepine. Wild type (α2+/+), heterozygous (α2+/-) and homozygous (α2-/-) knock-out littermates were tested in the free-choice exploratory (FCE) and the light/dark choice (LDC) paradigms. α2-/- mice, double mutant α1H101Rα2-/- and α3H126Rα2-/- mice, which combine a lack of α2-GABA(A)Rs with point-mutated diazepam-insensitive either α1H101R or α3H126R-GABA(A)Rs, and double point-mutated α1H101Rα2H101R and α1H101Rα3H126R mice were used to uncover the GABA(A)R subtype(s) mediating the drug effects. Data show that in the FCE, α2-/- mice exhibited more retractions (i.e. risk assessment) and longer latencies to first occurrence into the novel compartment and less transitions and time spent inside it in comparison to α2+/- and α2+/+ mice. In the LDC, α2-/- mice visited and spent less time in the lit box and stayed longer in the tunnel than the other two groups. Minor differences were found between α2+/- and α2+/+ mice in the two paradigms. Diazepam (1.5mg/kg per os) normalized retractions and latencies in the FCE in α2-/- and α3H126Rα2-/- mice, but not in α1H101Rα2-/- mice. The same drug treatment failed to attenuate behavioral aversion in both paradigms in all mutants with impaired α2-GABA(A)R function. These results reveal α2-containing GABA(A)Rs as key molecular determinants in the regulation of anxiety-related responses elicited by exposure to relative novelty and mild threat. In the absence of these receptors, diazepam through activation of α1-GABA(A)Rs remains effective in reducing risk assessment, but not behavioral aversion.