Postnatal development of the vesicular GABA transporter in rat cerebral cortex

GABAergic neuronal lineage development determines clinically actionable targets in diffuse hemispheric glioma, H3G34-mutant

Diffuse hemispheric gliomas, H3G34R/V-mutant (DHG-H3G34), are lethal brain tumors lacking targeted therapies. They originate from interneuronal precursors; however, leveraging this origin for therapeutic insights remains unexplored. Here, we delineate a cellular hierarchy along the interneuron lineage development continuum, revealing that DHG-H3G34 mirror spatial patterns of progenitor streams surrounding interneuron nests, as seen during human brain development. Integrating these findings with genome-wide CRISPR-Cas9 screens identifies genes upregulated in interneuron lineage progenitors as major dependencies. Among these, CDK6 emerges as a targetable vulnerability: DHG-H3G34 tumor cells show enhanced sensitivity to CDK4/6 inhibitors and a CDK6-specific degrader, promoting a shift toward more mature interneuron-like states, reducing tumor growth, and prolonging xenograft survival. Notably, a patient with progressive DHG-H3G34 treated with a CDK4/6 inhibitor achieved 17 months of stable disease. This study underscores interneuronal progenitor-like states, organized in characteristic niches, as a distinct vulnerability in DHG-H3G34, highlighting CDK6 as a promising clinically actionable target.

Ca2+-regulated expression of high affinity methylaminoisobutryic acid transport in hippocampal neurons inhibited by riluzole and novel neuroprotective aminothiazoles

High affinity methylaminoisobutyric acid(MeAIB)/glutamine(Gln) transport activity regulated by neuronal firing occurs at the plasma membrane in mature rat hippocampal neuron-enriched cultures. Spontaneous Ca2+-regulated transport activity was similarly inhibited by riluzole, a benzothiazole anticonvulsant agent, and by novel naphthalenyl substituted aminothiazole derivatives such as SKA-378. Here, we report that spontaneous transport activity is stimulated by 4-aminopyridine (4-AP) and that phorbol-myristate acetate (PMA) increases high K+ stimulated transport activity that is inhibited by staurosporine. 4-AP-stimulated spontaneous and PMA-stimulated high K+-induced transport is not present at 7 days in vitro (DIV) and is maximal by DIV∼21. The relative affinity for MeAIB is similar for spontaneous and high K+-stimulated transport (Km ∼ 50 μM) suggesting that a single transporter is involved. While riluzole and SKA-378 inhibit spontaneous transport with equal potency (IC50 ∼ 1 μM), they exhibit decreased (∼3-5 X) potency for 4-AP-stimulated spontaneous transport. Interestingly, high K+-stimulated MeAIB transport displays lower and differential sensitivity to the two compounds. SKA-378-related halogenated derivatives of SKA-75 (SKA-219, SKA-377 and SKA-375) preferentially inhibit high K+-induced expression of MeAIB transport activity at the plasma membrane (IC50 < 25 μM), compared to SKA-75 and riluzole (IC50 > 100 μM). Ca2+-dependent spontaneous and high K+-stimulated MeAIB transport activity is blocked by ω-conotoxin MVIIC, ω-agatoxin IVA, ω-agatoxin TK (IC50 ∼ 500 nM) or cadmium ion (IC50 ∼ 20 μM) demonstrating that P/Q-type CaV channels that are required for activity-regulated presynaptic vesicular glutamate (Glu) release are also required for high-affinity MeAIB transport expression at the plasma membrane. We suggest that neural activity driven and Ca2+ dependent trafficking of the high affinity MeAIB transporter to the plasma membrane is a unique target to understand mechanisms of Glu/Gln recycling in synapses and acute neuroprotection against excitotoxic presynaptic Glu induced neural injury.

Acute alcohol induces greater dose-dependent increase in the lateral cortical network functional connectivity in adult than adolescent rats

Alcohol misuse and, particularly adolescent drinking, is a major public health concern. While evidence suggests that adolescent alcohol use affects frontal brain regions that are important for cognitive control over behavior little is known about how acute alcohol exposure alters large-scale brain networks and how sex and age may moderate such effects. Here, we employ a recently developed functional magnetic resonance imaging (fMRI) protocol to acquire rat brain functional connectivity data and use an established analytical pipeline to examine the effect of sex, age, and alcohol dose on connectivity within and between three major rodent brain networks: defaul mode, salience, and lateral cortical network. We identify the intra- and inter-network connectivity differences and establish moderation models to reveal significant influences of age on acute alcohol-induced lateral cortical network connectivity. Through this work, we make brain-wide isotropic fMRI data with acute alcohol challenge publicly available, with the hope to facilitate future discovery of brain regions/circuits that are causally relevant to the impact of acute alcohol use.

Quantitative analysis of the GABAergic innervation of the soma and axon initial segment of pyramidal cells in the human and mouse neocortex

Perisomatic GABAergic innervation in the cerebral cortex is carried out mostly by basket and chandelier cells, which differentially participate in the control of pyramidal cell action potential output and synchronization. These cells establish multiple synapses with the cell body (and proximal dendrites) and the axon initial segment (AIS) of pyramidal neurons, respectively. Using multiple immunofluorescence, confocal microscopy and 3D quantification techniques, we have estimated the number and density of GABAergic boutons on the cell body and AIS of pyramidal neurons located through cortical layers of the human and mouse neocortex. The results revealed, in both species, that there is clear variability across layers regarding the density and number of perisomatic GABAergic boutons. We found a positive linear correlation between the surface area of the soma, or the AIS, and the number of GABAergic terminals in apposition to these 2 neuronal domains. Furthermore, the density of perisomatic GABAergic boutons was higher in the human cortex than in the mouse. These results suggest a selectivity for the GABAergic innervation of the cell body and AIS that might be related to the different functional attributes of the microcircuits in which neurons from different layers are involved in both human and mouse.

Gabaergic Interneurons in Early Brain Development: Conducting and Orchestrated by Cortical Network Activity

Throughout early phases of brain development, the two main neural signaling mechanisms—excitation and inhibition—are dynamically sculpted in the neocortex to establish primary functions. Despite its relatively late formation and persistent developmental changes, the GABAergic system promotes the ordered shaping of neuronal circuits at the structural and functional levels. Within this frame, interneurons participate first in spontaneous and later in sensory-evoked activity patterns that precede cortical functions of the mature brain. Upon their subcortical generation, interneurons in the embryonic brain must first orderly migrate to and settle in respective target layers before they can actively engage in cortical network activity. During this process, changes at the molecular and synaptic level of interneurons allow not only their coordinated formation but also the pruning of connections as well as excitatory and inhibitory synapses. At the postsynaptic site, the shift of GABAergic signaling from an excitatory towards an inhibitory response is required to enable synchronization within cortical networks. Concomitantly, the progressive specification of different interneuron subtypes endows the neocortex with distinct local cortical circuits and region-specific modulation of neuronal firing. Finally, the apoptotic process further refines neuronal populations by constantly maintaining a controlled ratio of inhibitory and excitatory neurons. Interestingly, many of these fundamental and complex processes are influenced—if not directly controlled—by electrical activity. Interneurons on the subcellular, cellular, and network level are affected by high frequency patterns, such as spindle burst and gamma oscillations in rodents and delta brushes in humans. Conversely, the maturation of interneuron structure and function on each of these scales feeds back and contributes to the generation of cortical activity patterns that are essential for the proper peri- and postnatal development. Overall, a more precise description of the conducting role of interneurons in terms of how they contribute to specific activity patterns—as well as how specific activity patterns impinge on their maturation as orchestra members—will lead to a better understanding of the physiological and pathophysiological development and function of the nervous system.

GABA System in Depression: Impact on Pathophysiology and Psychopharmacology

BACKGROUND

The pathophysiology of major depressive disorder (MDD), one of the major causes of worldwide disability, is still largely unclear, despite the increasing data reporting evidence of multiple alterations of different systems. Recently, there was a renewed interest in the signalling of gamma aminobutyric acid (GABA) - the main inhibitory neurotransmitter.

OBJECTIVE

The aim of this study was to review and comment on the available literature about the involvement of GABA in MDD, as well as on novel GABAergic compounds possibly useful as antidepressants.

METHODS

We carried out a narrative review through Pubmed, Google Scholar and Scopus, by using specific keywords.

RESULTS

The results, derived from various research tools, strongly support the presence of a deficiency of the GABA system in MDD, which appears to be restored by common antidepressant treatments. More recent publications would indicate the complex interactions between GABA and all the other processes involved in MDD, such as monoamine neurotransmission, hypothalamus-pituitary adrenal axis functioning, neurotrophism, and immune response. Taken together, all these findings seem to further support the complexity of the pathophysiology of MDD, possibly reflecting the heterogeneity of the clinical pictures.

CONCLUSION

Although further data are necessary to support the specificity of GABA deficiency in MDD, the available findings would suggest that novel GABAergic compounds might constitute innovative therapeutic strategies in MDD.

A two-hit adversity model in developing rats reveals sex-specific impacts on prefrontal cortex structure and behavior

Adversity early in life substantially impacts prefrontal cortex (PFC) development and vulnerability to later-life psychopathology. Importantly, repeated adverse experiences throughout childhood increase the risk for PFC-mediated behavioral deficits more commonly in women. Evidence from animal models points to effects of adversity on later-life neural and behavioral dysfunction; however, few studies have investigated the neurobiological underpinnings of sex-specific, long-term consequences of multiple developmental stressors. We modeled early life adversity in rats via maternal separation (postnatal day (P)2-20) and juvenile social isolation (P21-35). In adulthood, anxiety-like behavior was assessed in the elevated zero maze and the presence and structural integrity of PFC perineuronal nets (PNNs) enwrapping parvalbumin (PV)-expressing interneurons was quantified. PNNs are extracellular matrix structures formed during critical periods in postnatal development that play a key role in the plasticity of PV cells. We observed a female-specific effect of adversity on hyperactivity and risk-assessment behavior. Moreover, females – but not males – exposed to multiple hits of adversity demonstrated a reduction in PFC PV cells in adulthood. We also observed a sex-specific, potentiated reduction in PV + PNN structural integrity. These findings suggest a sex-specific impact of repeated adversity on neurostructural development and implicate PNNs as a contributor to associated behavioral dysfunction.

Vesicular GABA Uptake Can Be Rate Limiting for Recovery of IPSCs from Synaptic Depression

Synaptic efficacy plays crucial roles in neuronal circuit operation and synaptic plasticity. Presynaptic determinants of synaptic efficacy are neurotransmitter content in synaptic vesicles and the number of vesicles undergoing exocytosis at a time. Bursts of presynaptic firings depress synaptic efficacy, mainly due to depletion of releasable vesicles, whereas recovery from strong depression is initiated by endocytic vesicle retrieval followed by refilling of vesicles with neurotransmitter. We washed out presynaptic cytosolic GABA to induce a rundown of IPSCs at cerebellar inhibitory cell pairs in slices from rats and then allowed fast recovery by elevating GABA concentration using photo-uncaging. The time course of this recovery coincided with that of IPSCs from activity-dependent depression induced by a train of high-frequency stimulation. We conclude that vesicular GABA uptake can be a limiting step for the recovery of inhibitory neurotransmission from synaptic depression.

Distribution of GABAergic Neurons and VGluT1 and VGAT Immunoreactive Boutons in the Ferret (Mustela putorius) Piriform Cortex and Endopiriform Nucleus. Comparison With Visual Areas 17, 18 and 19

We studied the cellular organization of the piriform network [comprising the piriform cortex (PC) and endopiriform nucleus (EP)] of the ferret (Mustela putorius)-a highly excitable region prone to seizures-and, more specifically, the distribution and morphology of different types of gamma-aminobutyric acid (GABA)ergic neurons, and the distribution and ratio of glutamatergic and GABAergic boutons, and we compared our findings to those in primary visual area 17, and secondary areas 18 and 19. We accomplished this by using cytochrome oxidase and immunohistochemistry for mature neuronal nuclei (NeuN), GABAergic neurons [glutamic acid decarboxylase-67 (GAD67), calretinin (CR) and parvalbumin (PV)], and for excitatory (vesicular glutamate transporter 1; VGluT1) and inhibitory (vesicular GABA transporter; VGAT) boutons. In the ferret, the cellular organization of the piriform network is similar to that described in other species such as cats, rats and opossums although some differences also exist. GABAergic immunolabeling showed similarities between cortical layers I-III of the PC and visual areas, such as the relative distribution of GABAergic neurons and the density and area of VGluT1- and VGAT-immunoreactive boutons. However, multiple differences between the piriform network and visual areas (layers I-VI) were found, such as the percentage of GABAergic neurons with respect to the total number of neurons and the ratio of VGluT1- and VGAT-immunoreactive boutons. These findings are relevant to better understand the high excitability of the piriform network.

Differential expression of SV2A in hippocampal glutamatergic and GABAergic terminals during postnatal development

The function of synaptic vesicle protein 2A (SV2A) has not been clearly identified, although it has an essential role in normal neurotransmission. Changes in SV2A expression have been linked to several diseases that could implicate an imbalance between excitation and inhibition, such as epilepsy. Although it is known that SV2A expression is necessary for survival, SV2A expression and its relationship with γ-aminobutyric acid (GABA) and glutamate neurotransmitter systems along development has not been addressed. This report follows SV2A expression levels in the rat hippocampus and their association with glutamatergic and GABAergic terminals along postnatal development. Total SV2A expression was assessed by real time PCR and western blot, while immunofluorescence was used to identify SV2A protein in the different hippocampal layers and its co-localization with GABA or glutamate vesicular transporters. SV2A was dynamically regulated along development and its association with GABA or glutamate transporters varied in the different hippocampal layers. In the principal cells layers (granular and pyramidal), SV2A protein was preferentially localized to GABAergic terminals, while in the hilus and stratum lucidum SV2A was associated mainly to glutamatergic terminals. Although SV2A was ubiquitously expressed in the entire hippocampus, it established a differential association with excitatory or inhibitory terminals, which could contribute to the maturation of excitatory/inhibitory balance.

Expression of Neurofilament Subunits at Neocortical Glutamatergic and GABAergic Synapses

Neurofilaments (NFs) are neuron-specific heteropolymers that have long been considered as structural proteins. However, it has recently been documented that they may play a functional role at synapses. Indeed, the four NF subunits—NFL, NFM, NFH and α-internexin—are integral components of synapses in the striatum and hippocampus, since their elimination disrupts synaptic plasticity and impairs social memory, an observation that might have important implications for some neuropsychiatric diseases. Here, we studied NFs localization in VGLUT1-, VGLUT2-, VGAT-, PSD-95- and gephyrin-positive (+) puncta, and in glutamatergic and GABAergic synapses in the cerebral cortex of adult rats. Synapses were identified by pre- and postsynaptic markers: glutamatergic synapses by VGLUT1+ or VGLUT2+ puncta contacting PSD-95+ puncta; and GABAergic synapses by VGAT+ puncta contacting gephyrin+ puncta. In VGLUT1 glutamatergic synapses NF showed a greater expression in the compartment labeled by postsynaptic markers (20%–30%) than in those labeled by presynaptic markers (10%–20%), whereas in GABAergic synapses a similar expression was detected in both compartments (20%–30%). Moreover, NF expression was higher in the GABAergic (20%–30%) than in the glutamatergic (10%–15%) compartments labeled by presynaptic markers. Finally, a higher colocalization of VGLUT1+, VGLUT2+ and VGAT+ puncta with NFs was seen when presynaptic puncta contacted elements labeled by postsynaptic markers. These findings show that the four NF subunits are expressed at some neocortical synapses, and contribute to glutamatergic and GABAergic synapse heterogeneity.

Enhancing Oligodendrocyte Myelination Rescues Synaptic Loss and Improves Functional Recovery after Chronic Hypoxia

To address the significance of enhancing myelination for functional recovery after white matter injury (WMI) in preterm infants, we characterized hypomyelination associated with chronic hypoxia and identified structural and functional deficits of excitatory cortical synapses with a prolonged motor deficit. We demonstrate that genetically delaying myelination phenocopies the synaptic and functional deficits observed in mice after hypoxia, suggesting that myelination may possibly facilitate excitatory presynaptic innervation. As a gain-of-function experiment, we specifically ablated the muscarinic receptor 1 (M1R), a negative regulator of oligodendrocyte differentiation in oligodendrocyte precursor cells. Genetically enhancing oligodendrocyte differentiation and myelination rescued the synaptic loss after chronic hypoxia and promoted functional recovery. As a proof of concept, drug-based myelination therapies also resulted in accelerated differentiation and myelination with functional recovery after chronic hypoxia. Together, our data indicate that myelination-enhancing strategies in preterm infants may represent a promising therapeutic approach for structural/functional recovery after hypoxic WMI.

MultiMap: A Tool to Automatically Extract and Analyse Spatial Microscopic Data From Large Stacks of Confocal Microscopy Images

The development of 3D visualization and reconstruction methods to analyse microscopic structures at different levels of resolutions is of great importance to define brain microorganization and connectivity. MultiMap is a new tool that allows the visualization, 3D segmentation and quantification of fluorescent structures selectively in the neuropil from large stacks of confocal microscopy images. The major contribution of this tool is the posibility to easily navigate and create regions of interest of any shape and size within a large brain area that will be automatically 3D segmented and quantified to determine the density of puncta in the neuropil. As a proof of concept, we focused on the analysis of glutamatergic and GABAergic presynaptic axon terminals in the mouse hippocampal region to demonstrate its use as a tool to provide putative excitatory and inhibitory synaptic maps. The segmentation and quantification method has been validated over expert labeled images of the mouse hippocampus and over two benchmark datasets, obtaining comparable results to the expert detections.

Astrocytes, neurons, synapses: a tripartite view on cortical circuit development

In the mammalian cerebral cortex neurons are arranged in specific layers and form connections both within the cortex and with other brain regions, thus forming a complex mesh of specialized synaptic connections comprising distinct circuits. The correct establishment of these connections during development is crucial for the proper function of the brain. Astrocytes, a major type of glial cell, are important regulators of synapse formation and function during development. While neurogenesis precedes astrogenesis in the cortex, neuronal synapses only begin to form after astrocytes have been generated, concurrent with neuronal branching and process elaboration. Here we provide a combined overview of the developmental processes of synapse and circuit formation in the rodent cortex, emphasizing the timeline of both neuronal and astrocytic development and maturation. We further discuss the role of astrocytes at the synapse, focusing on astrocyte-synapse contact and the role of synapse-related proteins in promoting formation of distinct cortical circuits.

Regulation of Synapse Development by Vgat Deletion from ErbB4-Positive Interneurons

GABA signaling has been implicated in neural development; however, in vivo genetic evidence is missing because mutant mice lacking GABA activity die prematurely. Here, we studied synapse development by ablating vesicular GABA transporter (Vgat) in ErbB4⁺ interneurons. We show that inhibitory axo-somatic synapses onto pyramidal neurons vary from one cortical layer to another; however, inhibitory synapses on axon initial segments (AISs) were similar across layers. Conversely, parvalbumin-positive (PV⁺)/ErbB4⁺ interneurons and PV-only interneurons receive a higher number of inhibitory synapses from PV⁺ErbB4⁺ interneurons compared with ErbB4-only interneurons. Vgat deletion from ErbB4⁺ interneurons reduced axo-somatic or axo-axonic synapses from PV⁺ErbB4⁺ interneurons onto excitatory neurons. This effect was associated with corresponding changes in neurotransmission. However, the Vgat mutation seemed to have little effect on inhibitory synapses onto PV⁺ and/or ErbB4⁺ interneurons. Interestingly, perineuronal nets, extracellular matrix structures implicated in maturation, survival, protection, and plasticity of PV⁺ interneurons, were increased in the cortex of ErbB4-Vgat^-/- mice. No apparent difference was observed between males and females. These results demonstrate that Vgat of ErbB4⁺ interneurons is essential for the development of inhibitory synapses onto excitatory neurons and suggest a role of GABA in circuit assembly.SIGNIFICANCE STATEMENT GABA has been implicated in neural development, but in vivo genetic evidence is missing because mutant mice lacking GABA die prematurely. Here, we ablated Vgat in ErbB4⁺ interneurons in an inducible manner. We provide evidence that the formation of inhibitory and excitatory synapses onto excitatory neurons requires Vgat in interneurons. In particular, inhibitory axo-somatic and axo-axonic synapses are more vulnerable. Our results suggest a role of GABA in circuit assembly.

Levetiracetam Affects Differentially Presynaptic Proteins in Rat Cerebral Cortex

Presynaptic proteins are potential therapeutic targets for epilepsy and other neurological diseases. We tested the hypothesis that chronic treatment with the SV2A ligand levetiracetam affects the expression of other presynaptic proteins. Results showed that in rat neocortex no significant difference was detected in SV2A protein levels in levetiracetam treated animals compared to controls, whereas levetiracetam post-transcriptionally decreased several vesicular proteins and increased LRRK2, without any change in mRNA levels. Analysis of SV2A interactome indicates that the presynaptic proteins regulation induced by levetiracetam reported here is mediated by this interactome, and suggests that LRRK2 plays a role in forging the pattern of effects.

Metabolomics and neuroanatomical evaluation of post-mortem changes in the hippocampus

Understanding the human brain is the ultimate goal in neuroscience, but this is extremely challenging in part due to the fact that brain tissue obtained from autopsy is practically the only source of normal brain tissue and also since changes at different levels of biological organization (genetic, molecular, biochemical, anatomical) occur after death due to multiple mechanisms. Here we used metabolomic and anatomical techniques to study the possible relationship between post-mortem time (PT)-induced changes that may occur at both the metabolomics and anatomical levels in the same brains. Our experiments have mainly focused on the hippocampus of the mouse. We found significant metabolomic changes at 2 h PT, whereas the integrity of neurons and glia, at the anatomical/ neurochemical level, was not significantly altered during the first 5 h PT for the majority of histological markers.

H3 and H4 Lysine Acetylation Correlates with Developmental and Experimentally Induced Adult Experience-Dependent Plasticity in the Mouse Visual Cortex

Histone posttranslational modifications play a fundamental role in orchestrating gene expression. In this work, we analyzed the acetylation of H3 and H4 histones (AcH3-AcH4) and its modulation by visual experience in the mouse visual cortex (VC) during normal development and in two experimental conditions that restore juvenile-like plasticity levels in adults (fluoxetine treatment and enriched environment). We found that AcH3-AcH4 declines with age and is upregulated by treatments restoring plasticity in the adult. We also found that visual experience modulates AcH3-AcH4 in young and adult plasticity-restored mice but not in untreated ones. Finally, we showed that the transporter vGAT is downregulated in adult plasticity-restored models. In summary, we identified a dynamic regulation of AcH3-AcH4, which is associated with high plasticity levels and enhanced by visual experience. These data, along with recent ones, indicate H3-H4 acetylation as a central hub in the control of experience-dependent plasticity in the VC.

GABAergic Function as a Limiting Factor for Prefrontal Maturation during Adolescence

Adolescence is a vulnerable period for the onset of mental illnesses including schizophrenia and affective disorders, yet the neurodevelopmental processes underlying this vulnerability remain poorly understood. The prefrontal cortex (PFC) and its local GABAergic system are thought to contribute to the core of cognitive deficits associated with such disorders. However, clinical and preclinical end-point analyses performed in adults are likely to give limited insight into the cellular mechanisms that are altered during adolescence but are only manifested in adulthood. This perspective summarizes work regarding the developmental trajectories of the GABAergic system in the PFC during adolescence to provide an insight into the increased susceptibility to psychiatric disorders during this critical developmental period.

Developmental and degenerative modulation of GABAergic transmission in the mouse hippocampus

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter involved in synaptic plasticity. GABAergic transmission is also implicated in developmental and degenerative processes in the brain. The goal of the present study was to understand the developmental and degenerative regulation of GABAergic transmission in the mouse hippocampus by examining changes in GABA receptor subunit mRNA levels and GABA-related protein expression during postnatal development of the hippocampus and trimethyltin (TMT)-induced neurodegeneration in the juvenile (postnatal day [PD] 24) and adult hippocampus (PD 56). During postnatal development, the mRNA levels of GABA A receptor (GABAAR) subunits, including α1, α4, β1, β2, and δ; GABA B receptor (GABABR) subunit 2; and the expression of GABA-related proteins, including glutamic acid decarboxylase, vesicular GABA transporter (VGAT), and potassium chloride cotransporter 2 increased gradually in the mouse hippocampus. The results of seizure scoring and histopathological findings in the hippocampus revealed a more pronounced response to the same administered TMT dose in juvenile mice, compared with that in adult mice. The mRNA levels of most GABA receptor subunits in the juvenile hippocampus, excluding GABAAR subunit β3, were dynamically altered after TMT treatment. The mRNA levels of GABAAR subunits γ2 and δ decreased significantly in the adult hippocampus following TMT treatment, whereas the level of GABABR subunit 1 mRNA increased significantly. Among the GABA-related proteins, only VGAT decreased significantly in the juvenile and adult mouse hippocampus after TMT treatment. In conclusion, regulation of GABAergic signaling in the mouse hippocampus may be related to maturation of the central nervous system and the degree of neurodegeneration during postnatal development and TMT-induced neurodegeneration in the experimental animals.

Differential expression of metabotropic glutamate and GABA receptors at neocortical glutamatergic and GABAergic axon terminals

Metabotropic glutamate (Glu) receptors (mGluRs) and GABAB receptors are highly expressed at presynaptic sites. To verify the possibility that the two classes of metabotropic receptors contribute to axon terminals heterogeneity, we studied the localization of mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 in VGLUT1-, VGLUT2-, and VGAT- positive terminals in the cerebral cortex of adult rats. VGLUT1-positive puncta expressed mGluR1α (∼5%), mGluR5 (∼6%), mGluR2/3 (∼22%), mGluR7 (∼17%), and GABAB1 (∼40%); VGLUT2-positive terminals expressed mGluR1α (∼10%), mGluR5 (∼11%), mGluR2/3 (∼20%), mGluR7 (∼28%), and GABAB1 (∼25%); whereas VGAT-positive puncta expressed mGluR1α (∼27%), mGluR5 (∼24%), mGluR2/3 (∼38%), mGluR7 (∼31%), and GABAB1 (∼19%). Control experiments ruled out the possibility that postsynaptic mGluRs and GABAB1 might have significantly biased our results. We also performed functional assays in synaptosomal preparations, and showed that all agonists modify Glu and GABA levels, which return to baseline upon exposure to antagonists. Overall, these findings indicate that mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 expression differ significantly between glutamatergic and GABAergic axon terminals, and that the robust expression of heteroreceptors may contribute to the homeostatic regulation of the balance between excitation and inhibition.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Gestational and early postnatal hypothyroidism alters VGluT1 and VGAT bouton distribution in the neocortex and hippocampus, and behavior in rats

Thyroid hormones are fundamental for the expression of genes involved in the development of the CNS and their deficiency is associated with a wide spectrum of neurological diseases including mental retardation, attention deficit-hyperactivity disorder and autism spectrum disorders. We examined in rat whether developmental and early postnatal hypothyroidism affects the distribution of vesicular glutamate transporter-1 (VGluT1; glutamatergic) and vesicular inhibitory amino acid transporter (VGAT; GABAergic) immunoreactive (ir) boutons in the hippocampus and somatosensory cortex, and the behavior of the pups. Hypothyroidism was induced by adding 0.02% methimazole (MMI) and 1% KClO4 to the drinking water starting at embryonic day 10 (E10; developmental hypothyroidism) and E21 (early postnatal hypothyroidism) until day of sacrifice at postnatal day 50. Behavior was studied using the acoustic prepulse inhibition (somatosensory attention) and the elevated plus-maze (anxiety-like assessment) tests. The distribution, density and size of VGluT1-ir and VGAT-ir boutons in the hippocampus and somatosensory cortex was abnormal in MMI pups and these changes correlate with behavioral changes, as prepulse inhibition of the startle response amplitude was reduced, and the percentage of time spent in open arms increased. In conclusion, both developmental and early postnatal hypothyroidism significantly decreases the ratio of GABAergic to glutamatergic boutons in dentate gyrus leading to an abnormal flow of information to the hippocampus and infragranular layers of the somatosensory cortex, and alter behavior in rats. Our data show cytoarchitectonic alterations in the basic excitatory hippocampal loop, and in local inhibitory circuits of the somatosensory cortex and hippocampus that might contribute to the delayed neurocognitive outcome observed in thyroid hormone deficient children born in iodine deficient areas, or suffering from congenital hypothyroidism.

Plasma membrane transporters GAT-1 and GAT-3 contribute to heterogeneity of GABAergic synapses in neocortex

Cortical GABAergic synapses exhibit a high degree of molecular, anatomical and functional heterogeneity of their neurons of origins, presynaptic mechanisms, receptors, and scaffolding proteins. GABA transporters (GATs) have an important role in regulating GABA levels; among them, GAT-1 and GAT-3 play a prominent role in modulating tonic and phasic GABA_AR-mediated inhibition. We asked whether GAT-1 and GAT-3 contribute to generating heterogeneity by studying their ultrastructural localization at cortical symmetric synapses using pre- and post-embedding electron microcopy. GAT-1 and GAT-3 staining at symmetric synapses showed that in some cases the transporters were localized exclusively over axon terminals; in others they were in both axon terminals and perisynaptic astrocytic processes; and in some others GAT-1 and GAT-3 were in perisynaptic astrocytic processes only. Moreover, we showed that the organizational pattern of GAT-1, but not of GAT-3, exhibits a certain degree of specificity related to the post-synaptic target of GABAergic synapses. These findings show that symmetric synapses expressing GAT-1 or GAT-3 are heterogeneous, and indicate that plasma membrane transporters can contribute to synaptic heterogeneity.

Role of tonic GABAergic currents during pre- and early postnatal rodent development

In the last three decades it became evident that the GABAergic system plays an essential role for the development of the central nervous system, by influencing the proliferation of neuronal precursors, neuronal migration and differentiation, as well as by controlling early activity patterns and thus formation of neuronal networks. GABA controls neuronal development via depolarizing membrane responses upon activation of ionotropic GABA receptors. However, many of these effects occur before the onset of synaptic GABAergic activity and thus require the presence of extrasynaptic tonic currents in neuronal precursors and immature neurons. This review summarizes our current knowledge about the role of tonic GABAergic currents during early brain development. In this review we compare the temporal sequence of the expression and functional relevance of different GABA receptor subunits, GABA synthesizing enzymes and GABA transporters. We also refer to other possible endogenous agonists of GABAA receptors. In addition, we describe functional consequences mediated by the GABAergic system during early developmental periods and discuss current models about the origin of extrasynaptic GABA and/or other endogenous GABAergic agonists during early developmental states. Finally, we present evidence that tonic GABAergic activity is also critically involved in the generation of physiological as well as pathophysiological activity patterns before and after the establishment of functional GABAergic synaptic connections.

GABAergic interneurons shape the functional maturation of the cortex

From early embryonic development to adulthood, GABA release participates in the construction of the mammalian cerebral cortex. The maturation of GABAergic neurotransmission is a protracted process which takes place in discrete steps and results from the dynamic interaction between developmentally directed gene expression and brain activity. During the course of development, GABAergic interneurons contribute to key aspects of the functional maturation of the cortex in different ways, from exerting a trophic role to pacing immature neural networks. In this review, we provide an overview of the maturation of GABAergic neurotransmission and discuss the role of GABAergic interneurons in cortical wiring, plasticity, and network activity during pre- and postnatal development. We also discuss psychiatric diseases that may be considered at least in part developmental disorders of the GABAergic system.

Shift from phasic to tonic GABAergic transmission following laser-lesions in the rat visual cortex

Reduction in the strength of GABAergic neurotransmission has often been reported following brain lesions. This weakened inhibition is believed to influence neurological deficits, neuronal hyperexcitability and functional recovery after brain injuries. Uncovering the mechanisms underlying the altered inhibition is therefore crucial. In the present study we used an ex vivo-in vitro model of laser lesions in the rat visual cortex to characterize the cellular correlates of changes in GABAergic transmission in the tissue adjacent to the injury. In the first week post-injury the number of VGAT positive GABAergic terminals as well as the expression level of the GABA synthesizing enzymes GAD67 and GAD65 remained unaltered. However, a reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs) together with an increased paired-pulse ratio (PPR) of evoked IPSCs suggested a functional reduction of phasic GABA release. In parallel, we found an enhancement in the GABAA receptor-mediated tonic inhibition. On the basis of these findings, we propose that cortical lesions provoke a shift in GABAergic transmission, decreasing the phasic and reinforcing the tonic component. We therefore suggest that it is not, as traditionally assumed, the overall inhibitory strength to be primarily compromised by a cortical lesion but rather the temporal accuracy of the GABAergic synaptic signaling.

Synaptotagmin-2 is a reliable marker for parvalbumin positive inhibitory boutons in the mouse visual cortex

BACKGROUND

Inhibitory innervation by parvalbumin (PV) expressing interneurons has been implicated in the onset of the sensitive period of visual plasticity. Immunohistochemical analysis of the development and plasticity of these inhibitory inputs is difficult because PV expression is low in young animals and strongly influenced by neuronal activity. Moreover, the synaptic boutons that PV neurons form onto each other cannot be distinguished from the innervated cell bodies by immunostaining for this protein because it is present throughout the cells. These problems call for the availability of a synaptic, activity-independent marker for PV+ inhibitory boutons that is expressed before sensitive period onset. We investigated whether synaptotagmin-2 (Syt2) fulfills these properties in the visual cortex. Syt2 is a synaptic vesicle protein involved in fast Ca(2+) dependent neurotransmitter release. Its mRNA expression follows a pattern similar to that of PV throughout the brain and is present in 30-40% of hippocampal PV expressing basket cells. Up to now, no quantitative analyses of Syt2 expression in the visual cortex have been carried out.

METHODOLOGY/PRINCIPAL FINDINGS

We used immunohistochemistry to analyze colocalization of Syt2 with multiple interneuron markers including vesicular GABA transporter VGAT, calbindin, calretinin, somatostatin and PV in the primary visual cortex of mice during development and after dark-rearing.

CONCLUSIONS/SIGNIFICANCE

We show that in the adult visual cortex Syt2 is only found in inhibitory, VGAT positive boutons. Practically all Syt2 positive boutons also contain PV and vice versa. During development, Syt2 expression can be detected in synaptic boutons prior to PV and in contrast to PV expression, Syt2 is not down-regulated by dark-rearing. These properties of Syt2 make it an excellent marker for analyzing the development and plasticity of perisomatic inhibitory innervations onto both excitatory and inhibitory neurons in the visual cortex.

Ontogenetic changes in the distribution of the vesicular GABA transporter VGAT correlate with the excitation/inhibition shift of GABA action

GABA is the major inhibitory neurotransmitter in the adult CNS and is among others involved in the synchronization of large neuronal networks. During development, GABA acts as a morphogenetic factor and has transient excitatory actions in many brain regions. One distinct protein, the vesicular GABA transporter (VGAT), has been identified accumulating GABA into presynaptic vesicles prior to its exocytotic release. The function of VGAT and its distribution is well defined in the adult, but its contribution to the transient excitatory action at putative GABAergic nerve terminals in the immature brain and its potential roles in putative glutamatergic nerve terminals remain elusive. We have studied VGAT expression in the brain from late embryonic stages through several postnatal stages until adulthood. Quantitative immunoblotting and immunolabeling of tissue sections at the light microscope and the electron microscope levels show an abrupt augmentation in VGAT staining in the cerebral cortex during the first three postnatal weeks, resembling the increase in other proteins involved in GABA synthesis and recycling in the same time frame - such as GAD65, GAD67, GAT1 (Slc6a1) and SN1 (Slc38a3) - and coincides with the synaptogenetic spurt. Dynamic changes in the expression of VGAT are seen in many cellular populations and in several layers in different brain regions. However, mossy fiber terminals (MFT) elude staining for VGAT. We also demonstrate that VGAT(+) nerve terminals undergo a developmental reorganization so that from targeting primarily the dendrites of the principal neurons in several brain regions in the immature brain, they target the soma of the same cells in the adult. This shift in the targeted subcellular compartment coincides with the conversion of the chloride gradient across neuronal membranes and suggests that it may be important for the shift of GABA action from excitation to inhibition and for the establishment of the potent synchronization of neuronal networks.

Analysis of Synaptotagmin, SV2, and Rab3 Expression in Cortical Glutamatergic and GABAergic Axon Terminals

We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the nature and amount of proteins they express, by performing a quantitative analysis of the degree of co-localization of synaptotagmin (SYT) 1 and 2, synaptic vesicle protein 2 (SV2) A and B, and Rab3a and c in VGLUT1+, VGLUT2+, and VGAT+ terminals and synaptic vesicles (SVs) in rat cerebral cortex. Co-localization studies showed that VGLUT1 puncta had high levels of SV2A and B and of Rab3c, intermediate levels of SYT1, and low levels of SYT2 and Rab3c; VGLUT2 puncta exhibited intermediate levels of all presynaptic proteins studied; whereas vesicular GABA transporter (VGAT) puncta had high levels of SV2A and SYT2, intermediate levels of SYT1, Rab3a, and Rab3c, and low levels of SV2B. Since SV2B is reportedly expressed by glutamatergic neurons and we observed SV2B expression in VGAT puncta, we performed electron microscopic studies and found SV2B positive axon terminals forming symmetric synapses. Immunoisolation studies showed that the expression levels of the protein isoforms varied in the three populations of SVs. Expression of SYT1 was highest in VGLUT1–SVs, while SYT2 expression was similar in the three SV groups. Expression of SV2A was similarly high in all three SV populations, except for SV2B levels that were very low in VGAT SVs. Finally, Rab3a levels were similar in the three SV groups, while Rab3c levels were highest in VGLUT1–SVs. These quantitative results extend our previous studies on the differential expression of presynaptic proteins involved in neurotransmitter release in GABAergic and glutamatergic terminals and indicate that heterogeneity of the respective release machineries can be generated by the differential complement of SV proteins involved in distinct stages of the release process.

Developmental programming: reproductive endocrinopathies in the adult female sheep after prenatal testosterone treatment are reflected in altered ontogeny of GnRH afferents

The GnRH system represents a useful model of long-term neural plasticity. An unexplored facet of this plasticity relates to the ontogeny of GnRH neural afferents during critical periods when the hypothalamic-pituitary-gonadal axis is highly susceptible to perturbation by sex steroids. Sheep treated with testosterone (T) in utero exhibit profound reproductive neuroendocrine dysfunctions during their lifespan. The current study tested the hypothesis that these changes are associated with alterations in the normal ontogeny of GnRH afferents and glial associations. Adult pregnant sheep (n=50) were treated with vehicle [control (CONT)] or T daily from gestational day (GD)30 to GD90. CONT and T fetuses (n=4-6/treatment per age group) were removed by cesarean section on GD90 and GD140 and the brains frozen at -80°C. Brains were also collected from CONT and T females at 20-23 wk (prepubertal), 10 months (normal onset of puberty and oligo-anovulation), and 21 months (oligo-anovulation in T females). Tissue was analyzed for GnRH immunoreactivity (ir), total GnRH afferents (Synapsin-I ir), glutamate [vesicular glutamate transporter-2 (VGLUT2)-ir], and γ-aminobutyric acid [GABA, vesicular GABA transporter (VGAT)-ir] afferents and glial associations (glial fibrillary acidic protein-ir) with GnRH neurons using optical sectioning techniques. The results revealed that: 1) GnRH soma size was slightly reduced by T, 2) the total (Synapsin-I) GnRH afferents onto both somas and dendrites increased significantly with age and was reduced by T, 3) numbers of both VGAT and VGLUT inputs increased significantly with age and were also reduced by T, and 4) glial associations with GnRH neurons were reduced (<10%) by T. Together, these findings reveal a previously unknown developmental plasticity in the GnRH system of the sheep. The altered developmental trajectory of GnRH afferents after T reinforces the notion that prenatal programming plays an important role in the normal development of the reproductive neuroendocrine axis.

Development of the GABAergic system from birth to adolescence

The neurotransmitter GABA (γ-aminobutyric acid), acting via inotropic GABA(A) and metabotropic GABA(B) receptors, plays an essential role in a variety of distinct neuronal processes, including regulation of neuronal excitability, determination of temporal aspects of spike trains, control of the size and propagation of neuronal assemblies, generation of oscillatory activity, and neuronal plasticity. Although the developmental switch between excitatory and inhibitory GABA(A) receptor-mediated responses is widely appreciated, the fact that the postnatal maturation of the GABAergic system lasts until late adolescence is not so persuasively promoted. This review summarizes recent knowledge of the maturation of various aspects of the GABAergic systems, like functional expression of GABA synthesizing/degrading enzymes and transporters, density of GABAergic synapses, GABAergic projection patterns, GABA receptor subunit composition, and properties of GABAergic interneurons, with an emphasis on the late developmental alterations. In addition, some aspects of the development of mental capabilities during adolescence and their relation the delayed maturation of the GABAergic system are presented.

GABAergic complex basket formations in the human neocortex

Certain GABAergic interneurons in the cerebral cortex, basket cells, establish multiple connections with cell bodies that typically outline the somata and proximal dendrites of pyramidal cells. During studies into the distribution of the vesicular GABA transporter (VGAT) in the human cerebral cortex, we were struck by the presence of a very dense, pericellular arrangement of multiple VGAT-immunoreactive (-ir) terminals in certain cortical areas. We called these terminals "Complex basket formations" (Cbk-formations) to distinguish them from the simpler and more typical pericellular GABAergic innervations of most cortical neurons. Here we examined the distribution of these VGAT-ir Cbk-formations in various cortical areas, including the somatosensory (area 3b), visual (areas 17 and 18), motor (area 4), associative frontal (dorsolateral areas 9, 10, 45, 46, and orbital areas 11, 12, 13, 14, 47), associative temporal (areas 20, 21, 22, and 38), and limbic cingulate areas (areas 24, 32). Furthermore, we used dual or triple staining techniques to study the chemical nature of the innervated cells. We found that VGAT-ir Cbk-formations were most frequently found in area 4 followed by areas 3b, 13, and 18. In addition, they were mostly observed in layer III, except in area 17, where they were most dense in layer IV. We also found that 70% of the innervated neurons were pyramidal cells, while the remaining 30% were multipolar cells. Most of these multipolar cells expressed the calcium-binding protein parvalbumin and the lectin Vicia villosa agglutinin.

Differential distribution of neurons in the gyral white matter of the human cerebral cortex

The neurons in the cortical white matter (WM neurons) originate from the first set of postmitotic neurons that migrates from the ventricular zone. In particular, they arise in the subplate that contains the earliest cells generated in the telencephalon, prior to the appearance of neurons in gray matter cortical layers. These cortical WM neurons are very numerous during development, when they are thought to participate in transient synaptic networks, although many of these cells later die, and relatively few cells survive as WM neurons in the adult. We used light and electron microscopy to analyze the distribution and density of WM neurons in various areas of the adult human cerebral cortex. Furthermore, we examined the perisomatic innervation of these neurons and estimated the density of synapses in the white matter. Finally, we examined the distribution and neurochemical nature of interneurons that putatively innervate the somata of WM neurons. From the data obtained, we can draw three main conclusions: first, the density of WM neurons varies depending on the cortical areas; second, calretinin-immunoreactive neurons represent the major subpopulation of GABAergic WM neurons; and, third, the somata of WM neurons are surrounded by both glutamatergic and GABAergic axon terminals, although only symmetric axosomatic synapses were found. By contrast, both symmetric and asymmetric axodendritic synapses were observed in the neuropil. We discuss the possible functional implications of these findings in terms of cortical circuits.

A Role for GAT-1 in Presynaptic GABA Homeostasis?

In monoamine-releasing terminals, neurotransmitter transporters – in addition to terminating synaptic transmission by clearing released transmitters from the extracellular space – are the primary mechanism for replenishing transmitter stores and thus regulate presynaptic homeostasis. Here, we analyze whether GAT-1, the main plasma membrane GABA transporter, plays a similar role in GABAergic terminals. Re-examination of existing literature and recent data gathered in our laboratory show that GABA homeostasis in GABAergic terminals is dominated by the activity of the GABA synthesizing enzyme and that GAT-1-mediated GABA transport contributes to cytosolic GABA levels. However, analysis of GAT-1 KO, besides demonstrating the effects of reduced clearance, reveals the existence of changes compatible with an impaired presynaptic function, as miniature IPSCs frequency is reduced by one-third and glutamic acid decarboxylases and phosphate-activated glutaminase levels are significantly up-regulated. Although the changes observed are less robust than those reported in mice with impaired dopamine, noradrenaline, and serotonin plasma membrane transporters, they suggest that in GABAergic terminals GAT-1 impacts on presynaptic GABA homeostasis, and may contribute to the activity-dependent regulation of inhibitory efficacy.

Expression of VGluT1 and VGAT mRNAs in human dorsolateral prefrontal cortex during development and in schizophrenia

A balance between excitatory and inhibitory neurotransmission is important in normal brain function, and in schizophrenia a deficit in γ-aminobutyric acid (GABA)ergic inhibitory neurotransmission has been indicated by postmortem studies. We examined the ratio of excitatory to inhibitory vesicular neurotransmitter transporter mRNAs (VGluT1 to VGAT) and their ratio in the dorsolateral prefrontal cortex during normal human development and in people with schizophrenia and controls by quantitative RT-PCR. The ratio of VGluT1/VGAT increased gradually in development to reach a peak at school age (5-12 years), after which levels remained fairly constant into adulthood. The VGluT1 mRNA/VGAT mRNA ratio was unchanged in schizophrenia, as was the ratio of complexin 2 mRNA to complexin 1 mRNA (related to synaptic vesicle fusion in excitatory and inhibitory terminals, respectively). This suggests that the excitatory/inhibitory balance is attained prior to adolescence and is maintained across the rest of the life-span and also indicates that in schizophrenia this balance is not greatly disturbed.

Pericellular innervation of neurons expressing abnormally hyperphosphorylated tau in the hippocampal formation of Alzheimer's disease patients

Neurofibrillary tangles (NFT) represent one of the main neuropathological features in the cerebral cortex associated with Alzheimer's disease (AD). This neurofibrillary lesion involves the accumulation of abnormally hyperphosphorylated or abnormally phosphorylated microtubule-associated protein tau into paired helical filaments (PHF-tau) within neurons. We have used immunocytochemical techniques and confocal microscopy reconstructions to examine the distribution of PHF-tau-immunoreactive (ir) cells, and their perisomatic GABAergic and glutamatergic innervations in the hippocampal formation and adjacent cortex of AD patients. Furthermore, correlative light and electron microscopy was employed to examine these neurons and the perisomatic synapses. We observed two patterns of staining in PHF-tau-ir neurons, pattern I (without NFT) and pattern II (with NFT), the distribution of which varies according to the cortical layer and area. Furthermore, the distribution of both GABAergic and glutamatergic terminals around the soma and proximal processes of PHF-tau-ir neurons does not seem to be altered as it is indistinguishable from both control cases and from adjacent neurons that did not contain PHF-tau. At the electron microscope level, a normal looking neuropil with typical symmetric and asymmetric synapses was observed around PHF-tau-ir neurons. These observations suggest that the synaptic connectivity around the perisomatic region of these PHF-tau-ir neurons was apparently unaltered.

Developmental changes in GABAergic mechanisms in human visual cortex across the lifespan

Functional maturation of visual cortex is linked with dynamic changes in synaptic expression of GABAergic mechanisms. These include setting the excitation-inhibition balance required for experience-dependent plasticity, as well as, intracortical inhibition underlying development and aging of receptive field properties. Animal studies have shown that there is developmental regulation of GABAergic mechanisms in visual cortex. In this study, we show for the first time how these mechanisms develop in the human visual cortex across the lifespan. We used Western blot analysis of postmortem tissue from human primary visual cortex (n = 30, range: 20 days to 80 years) to quantify expression of eight pre- and post-synaptic GABAergic markers. We quantified the inhibitory modulating cannabinoid receptor (CB1), GABA vesicular transporter (VGAT), GABA synthesizing enzymes (GAD65/GAD67), GABA(A) receptor anchoring protein (Gephyrin), and GABA(A) receptor subunits (GABA(A)alpha1, GABA(A)alpha2, GABA(A)alpha3). We found a complex pattern of different developmental trajectories, many of which were prolonged and continued well into the teen, young adult, and even older adult years. These included a monotonic increase or decrease (GABA(A)alpha1, GABA(A)alpha2), a biphasic increase then decrease (GAD65, Gephyrin), or multiple increases and decreases (VGAT, CB1) across the lifespan. Comparing the balances between the pre- and post-synaptic markers we found three main transition stages (early childhood, early teen years, aging) when there were rapid switches in the composition of the GABAergic signaling system, indicating that functioning of the GABAergic system must change as the visual cortex develops and ages. Furthermore, these results provide key information for translating therapies developed in animal models into effective treatments for amblyopia in humans.

Densities of glutamatergic and GABAergic presynaptic terminals are altered in experimental cortical dysplasia

PURPOSE

Cortical dysplasia (CD) is a major cause of epilepsy in children and adults, but underlying mechanisms of epileptogenesis in this disorder are poorly understood. We have utilized the irradiated rat model to study an injury-based form of diffuse CD in rats. Prior studies in this model have shown reduced numbers of γ-aminobutyric acid (GABA)ergic interneurons and reduced inhibitory synaptic currents in pyramidal cells in CD. We analyzed the number of excitatory and inhibitory presynaptic terminals in the neocortex of irradiated rats to better characterize altered connectivity in experimental CD.

METHODS

Antibodies to vesicular glutamate transporter 1 (VGLUT1), vesicular glutamate transporter 2 (VGLUT2), vesicular GABA transporter (VGAT), and parvalbumin (PV) were used to quantify glutamatergic and GABAergic presynaptic terminals in control and dysplastic cortex.

RESULTS

We found that the density of VGLUT1 terminals was increased in CD in comparison to layers IV, V, and VI in control cortex. VGLUT2 terminals were increased in CD compared to layers IV and VI. VGAT terminals were reduced in CD compared to layers II/III, IV, and V in controls as were PV-immunoreactive somata and terminals.

DISCUSSION

These findings suggest an overall increase in excitatory synaptic connectivity and decrease in inhibitory synaptic connectivity in CD in irradiated rat. We propose that these changes contribute to hyperexcitability in these animals and may contribute to epileptogenicity in some forms of human CD.

Overexpression of the cell adhesion protein neuroligin-1 induces learning deficits and impairs synaptic plasticity by altering the ratio of excitation to inhibition in the hippocampus

Trans-synaptic cell-adhesion molecules have been implicated in regulating CNS synaptogenesis. Among these, the Neuroligin (NL) family (NLs 1-4) of postsynaptic adhesion proteins has been shown to promote the development and specification of excitatory versus inhibitory synapses. NLs form a heterophilic complex with the presynaptic transmembrane protein Neurexin (NRX). A differential association of NLs with postsynaptic scaffolding proteins and NRX isoforms has been suggested to regulate the ratio of excitatory to inhibitory synapses (E/I ratio). Using transgenic mice, we have tested this hypothesis by overexpressing NL1 in vivo to determine whether the relative levels of these cell adhesion molecules may influence synapse maturation, long-term potentiation (LTP), and/or learning. We found that NL1-overexpressing mice show significant deficits in memory acquisition, but not in memory retrieval. Golgi and electron microscopy analysis revealed changes in synapse morphology indicative of increased maturation of excitatory synapses. In parallel, electrophysiological examination indicated a shift in the synaptic activity toward increased excitation as well as impairment in LTP induction. Our results demonstrate that altered balance in the expression of molecules necessary for synapse specification and development (such as NL1) can lead to defects in memory formation and synaptic plasticity and outline the importance of rigidly controlled synaptic maturation processes.

Excitation-transcription coupling via calcium/calmodulin-dependent protein kinase/ERK1/2 signaling mediates the coordinate induction of VGLUT2 and Narp triggered by a prolonged increase in glutamatergic synaptic activity

Homeostatic scaling of glutamatergic and GABAergic transmission is triggered by prolonged alterations in synaptic neuronal activity. We have previously described a presynaptic mechanism for synaptic homeostasis and plasticity that involves scaling the level of vesicular glutamate (VGLUT1) and gamma-aminobutyric acid (GABA) (VGAT) transporter biosynthesis. These molecular determinants of vesicle filling and quantal size are regulated by neuronal activity in an opposite manner and bi-directionally. Here, we report that a striking induction of VGLUT2 mRNA and synaptic protein is triggered by a prolonged increase in glutamatergic synaptic activity in mature neocortical neuronal networks in vitro together with two determinants of inhibitory synaptic strength, the neuronal activity-regulated pentraxin (Narp), and glutamate decarboxylase (GAD65). Activity-dependent induction of VGLUT2 and Narp exhibits a similar intermediate-early gene response that is blocked by actinomycin D and tetrodotoxin, by inhibitors of ionotropic glutamate receptors and L-type voltage-gated calcium channels, and is dependent on downstream signaling via calmodulin, calcium/calmodulin-dependent protein kinase (CaMK) and extracellular signal-regulated kinase 1/2 (ERK1/2). The co-induction of VGLUT2 and Narp triggered by prolonged gamma-aminobutyric acid type A receptor blockade is independent of brain-derived nerve growth factor and TrkB receptor signaling. VGLUT2 protein induction occurs on a subset of cortically derived synaptic vesicles in excitatory synapses on somata and dendritic processes of multipolar GABAergic interneurons, recognized sites for the clustering of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate glutamate receptors by Narp. We propose that VGLUT2 and Narp induction by excitation-transcription coupling leads to increased glutamatergic transmission at synapses on GABAergic inhibitory feedback neurons as part of a coordinated program of Ca(2+)-signal transcription involved in mechanisms of homeostatic plasticity after prolonged hyperactivity.

Diminished perisomatic GABAergic terminals on cortical neurons adjacent to amyloid plaques

One of the main pathological hallmarks of Alzheimer's disease (AD) is the accumulation of plaques in the cerebral cortex, which may appear either in the neuropil or in direct association with neuronal somata. Since different axonal systems innervate the dendritic (mostly glutamatergic) and perisomatic (mostly GABAergic) regions of neurons, the accumulation of plaques in the neuropil or associated with the soma might produce different alterations to synaptic circuits. We have used a variety of conventional light, confocal and electron microscopy techniques to study their relationship with neuronal somata in the cerebral cortex from AD patients and APP/PS1 transgenic mice. The main finding was that the membrane surfaces of neurons (mainly pyramidal cells) in contact with plaques lack GABAergic perisomatic synapses. Since these perisomatic synapses are thought to exert a strong influence on the output of pyramidal cells, their loss may lead to the hyperactivity of the neurons in contact with plaques. These results suggest that plaques modify circuits in a more selective manner than previously thought.

Heterogeneity of glutamatergic and GABAergic release machinery in cerebral cortex: analysis of synaptogyrin, vesicle-associated membrane protein, and syntaxin

To define whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the nature and amount of proteins they express, we studied the degree of co-localization of synaptogyrin (SGYR) 1 and 3, vesicle-associated membrane protein (VAMP) 1 and 2, syntaxin (STX) 1A and 1B in vesicular glutamate transporter (VGLUT)1-, VGLUT2- and vesicular GABA transporter (VGAT)-positive (+) puncta and synaptic vesicles in the rat cerebral cortex. Co-localization studies showed that SGYR1 and 3 were expressed in about 90% of VGLUT1+, 70% of VGLUT2+ and 80% of VGAT+ puncta; VAMP1 was expressed in approximately 45% of VGLUT1+, 55% of VGLUT2+, and 80% of VGAT+ puncta; VAMP2 in about 95% of VGLUT1+, 75% of VGLUT2+, and 80% of VGAT+ puncta; STX1A in about 65% of VGLUT1+, 30% of VGLUT2+, and 3% of VGAT+ puncta, and STX1B in approximately 45% of VGLUT1+, 35% of VGLUT2+, and 70% of VGAT+ puncta. Immunoisolation studies showed that while STX1A was completely segregated and virtually absent from VGAT synaptic vesicles, STX1B, VAMP1/VAMP2, SGYR1/SGYR3 showed a similar pattern with the highest expression in VGLUT1 immunoisolated vesicles and the lowest in VGAT immunoisolated vesicles. Moreover, we studied the localization of STX1B at the electron microscope and found that a population of axon terminals forming symmetric synapses were STX1B-positive.These results extend our previous observations on the differential expression of presynaptic proteins involved in neurotransmitter release in GABAergic and glutamatergic terminals and indicate that heterogeneity of glutamatergic and GABAergic release machinery can be contributed by both the presence or absence of a given protein in a nerve terminal and the amount of protein expressed by synaptic vesicles.