Basal expression and induction of glutamate decarboxylase and GABA in excitatory granule cells of the rat and monkey hippocampal dentate gyrus

Structure, biophysics, and circuit function of a "giant" cortical presynaptic terminal

The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and "flash-and-freeze" electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.

KangPiLao decoction modulates cognitive and emotional disorders in rats with central fatigue through the GABA/Glu pathway

Background: Central fatigue (CF) is a subjective sense of tiredness associated with cognitive and memory disorders, accompanied by reduced physical endurance and negative emotions, such as anxiety and depression. Disease progression and prognosis with regards to CF have been unfavorable and possibly contribute to dementia, schizophrenia, and other diseases. Additionally, effective treatments for CF are lacking. KangPiLao decoction (KPLD) has been widely applied in clinical treatment and is composed of six Chinese herbal medicines, some of which have confirmed anti-fatigue effects. While glutamic acid (Glu) is the main excitatory transmitter in the central nervous system (CNS), gamma-aminobutyric acid (GABA) is the major inhibitory transmitter. Both are involved in emotional, cognitive, and memory functions. This research was designed to explore how KPLD regulates cognitive and emotional disorders in rats with CF and to identify the relationship between the regulatory effect and the GABA/Glu pathway.

Methods: The compounds comprising KPLD were analyzed using high-performance liquid chromatography-mass spectrometry. Sixty Wistar rats were randomly divided into six groups. The modified multiple platform method was used to induce CF. Cognitive, emotional, and fatigue states were evaluated by performing behavioral tests (Morris water maze [MWM], open-field test [OFT], and grip strength test). Histomorphology, western blotting, immunohistochemistry, and RT-qPCR were performed to investigate protein and mRNA expression levels in the hippocampus and prefrontal cortexes involved in the GABA/Glu pathway.

Results: Rats with CF exhibited impaired spatial cognition and increased negative emotions in the MWM and OFT. KPLD enabled the improvement of these symptoms, especially in the high-concentration group. Western blotting and RT-qPCR demonstrated that the expression of GABAARα1, GABAARγ2, GABABR1, and GAD67 in rats with CF was higher, whereas GAT-1 and NMDAR2B were lower in the hippocampus and prefrontal cortex. KPLD decreased the expression of GABAARα1, GABABR1, GABAARγ2, and GAD67 in the hippocampus and prefrontal cortex and enhanced the expression of NR2B in the prefrontal cortex.

Conclusion: KPLD significantly improved cognitive and emotional disorders in rats with CF by regulating the GABA/Glu pathway. Overall, KPLD may be a promising candidate for developing a drug for treating CF.

The Aerial Parts of Bupleurum Chinense DC. Aromatic Oil Attenuate Kainic Acid-Induced Epilepsy-Like Behavior and Its Potential Mechanisms

The aerial parts of Bupleurum Chinense DC. aromatic oil (BAO) were a well-known Chinese herbal medicine plant extract used to treat epilepsy. This study aimed to explore the therapeutic effect of BAO on kainic acid- (KA-) induced epileptic rats and the possible mechanism of its antiepileptic effect. The composition and content of BAO were analyzed by GC-MS, and BAO was administered orally to alleviate the epileptic behavior induced by KA brain injection. The behavior of epileptic rats was determined by Racine grading criteria. And hematoxylin-eosin staining (HE), Nissl staining, immunohistochemistry, Elisa, Western blot, and other methods were used to study the antiepileptic mechanism of BAO, and the possible mechanism was verified by the epileptic cell model of hippocampal neurons induced by the low-Mg2+ extracellular fluid. BAO was mainly composed of terpenoids and aliphatic compounds. And BAO could improve KA-induced epilepsy-like behavior, neuroinflammation, and neurotransmitter abnormalities in the hippocampus. Furthermore, BAO could regulate the expression of GABA, NMDAR1, Notch1, and MAP2 to improve the symptoms of epilepsy. These results were also validated at the cellular level. These results indicated that BAO could alleviate the epilepsy-like behavior through the action of the Notch/NMDAR/GABA pathway.

Psychotic Symptoms as the Initial Presentation of a Long-Lasting Misdiagnosed Anti-GAD65 Autoimmune Encephalitis: An Emblematic Case and Literature Review

OBJECTIVE

To present a long-lasting misdiagnosed case of anti-GAD65 autoimmune encephalitis (AE) and promote the early identification of reversible psychotic symptoms in AE.

METHODS

The case report was generated through detailed assessment of clinical characteristics, cerebral magnetic resonance images, and laboratory results. Meanwhile, a literatures review related to the topic was conducted.

RESULTS

Psychotic symptoms could be presented in the early stage of anti-GAD65 autoimmune encephalitis. Even though there exists a transdisciplinary gap that hinder the timely recognition of early psychiatric symptoms as components of organic disease, a few strategies could be introduced to enable the earlier recognition and appropriate treatment.

CONCLUSIONS

Our report intends to raise awareness to promote the early identification of immune-mediated "symptomatic" forms of psychosis.

Different changes in pre- and postsynaptic components in the hippocampal CA1 subfield after transient global cerebral ischemia

To date, ischemia-induced damage to dendritic spines has attracted considerable attention, while the possible effects of ischemia on presynaptic components has received relatively less attention. To further examine ischemia-induced changes in pre- and postsynaptic specializations in the hippocampal CA1 subfield, we modeled global cerebral ischemia with two-stage 4-vessel-occlusion in rats, and found that three postsynaptic markers, microtubule-associated protein 2 (MAP2), postsynaptic density protein 95 (PSD95), and filamentous F-actin (F-actin), were all substantially decreased in the CA1 subfield after ischemia/reperfusion (I/R). Although no significant change was detected in synapsin I, a presynaptic marker, in the CA1 subfield at the protein level, confocal microscopy revealed that the number and size of synapsin I puncta were significantly changed in the CA1 stratum radiatum after I/R. The size of synapsin I puncta became slightly, but significantly reduced on Day 1.5 after I/R. From Days 2 to 7 after I/R, the number of synapsin I puncta became moderately decreased, while the size of synapsin I puncta was significantly increased. Interestingly, some enlarged puncta of synapsin I were observed in close proximity to the dendritic shafts of CA1 pyramidal cells. Due to the more substantial decrease in the number of F-actin puncta, the ratio of synapsin I/F-actin puncta was significantly increased after I/R. The decrease in synapsin I puncta size in the early stage of I/R may be the result of excessive neurotransmitter release due to I/R-induced hyperexcitability in CA3 pyramidal cells, while the increase in synapsin I puncta in the later stage of I/R may reflect a disability of synaptic vesicle release due to the loss of postsynaptic contacts.

Improved Method for Efficient Generation of Functional Neurons from Murine Neural Progenitor Cells

Neuronal culture was used to investigate neuronal function in physiological and pathological conditions. Despite its inevitability, primary neuronal culture remained a gold standard method that requires laborious preparation, intensive training, and animal resources. To circumvent the shortfalls of primary neuronal preparations and efficiently give rise to functional neurons, we combine a neural stem cell culture method with a direct cell type-conversion approach. The lucidity of this method enables the efficient preparation of functional neurons from mouse neural progenitor cells on demand. We demonstrate that induced neurons (NPC-iNs) by this method make synaptic connections, elicit neuronal activity-dependent cellular responses, and develop functional neuronal networks. This method will provide a concise platform for functional neuronal assessments. This indeed offers a perspective for using these characterized neuronal networks for investigating plasticity mechanisms, drug screening assays, and probing the molecular and biophysical basis of neurodevelopmental and neurodegenerative diseases.

Altered Parvalbumin Basket Cell Terminals in the Cortical Visuospatial Working Memory Network in Schizophrenia

BACKGROUND

Visuospatial working memory (vsWM), which is commonly impaired in schizophrenia, involves information processing across the primary visual cortex, association visual cortex, posterior parietal cortex, and dorsolateral prefrontal cortex (DLPFC). Within these regions, vsWM requires inhibition from parvalbumin-expressing basket cells (PVBCs). Here, we analyzed indices of PVBC axon terminals across regions of the vsWM network in schizophrenia.

METHODS

For 20 matched pairs of subjects with schizophrenia and unaffected comparison subjects, tissue sections from the primary visual cortex, association visual cortex, posterior parietal cortex, and DLPFC were immunolabeled for PV, the 65- and 67-kDa isoforms of glutamic acid decarboxylase (GAD65 and GAD67) that synthesize GABA (gamma-aminobutyric acid), and the vesicular GABA transporter. The density of PVBC terminals and of protein levels per terminal was quantified in layer 3 of each cortical region using fluorescence confocal microscopy.

RESULTS

In comparison subjects, all measures, except for GAD65 levels, exhibited a caudal-to-rostral decline across the vsWM network. In subjects with schizophrenia, the density of detectable PVBC terminals was significantly lower in all regions except the DLPFC, whereas PVBC terminal levels of PV, GAD67, and GAD65 proteins were lower in all regions. A composite measure of inhibitory strength was lower in subjects with schizophrenia, although the magnitude of the diagnosis effect was greater in the primary visual, association visual, and posterior parietal cortices than in the DLPFC.

CONCLUSIONS

In schizophrenia, alterations in PVBC terminals across the vsWM network suggest the presence of a shared substrate for cortical dysfunction during vsWM tasks. However, regional differences in the magnitude of the disease effect on an index of PVBC inhibitory strength suggest region-specific alterations in information processing during vsWM tasks.

Rare presentation of anti-GAD-65 antibody-positive autoimmune encephalitis and simultaneous onset of type 1 diabetes mellitus in a paediatric patient

A 16-year-old female patient presented with subacute onset of headaches, changes in acute mental status, expressive aphasia and auditory hallucinations. New oedema and enhancement of the temporal lobe were seen on brain MRI, with correlating subclinical seizures seen on electroencephalogram. Simultaneously, our patient was diagnosed with new-onset type 1 diabetes mellitus, with positive anti-glutamic acid decarboxylase (anti-GAD-65) antibodies in the serum. Cerebrospinal fluid studies remained negative, including anti-GAD-65 antibodies. Clinical remission was achieved with corticosteroids and intravenous immunoglobulins.

Going the Extra (Synaptic) Mile: Excitotoxicity as the Road Toward Neurodegenerative Diseases

Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

BDNF Expression in Cortical GABAergic Interneurons

Brain-derived neurotrophic factor (BDNF) is a major neuronal growth factor that is widely expressed in the central nervous system. It is synthesized as a glycosylated precursor protein, (pro)BDNF and post-translationally converted to the mature form, (m)BDNF. BDNF is known to be produced and secreted by cortical glutamatergic principal cells (PCs); however, it remains a question whether it can also be synthesized by other neuron types, in particular, GABAergic interneurons (INs). Therefore, we utilized immunocytochemical labeling and reverse transcription quantitative PCR (RT-qPCR) to investigate the cellular distribution of proBDNF and its RNA in glutamatergic and GABAergic neurons of the mouse cortex. Immunofluorescence labeling revealed that mBDNF, as well as proBDNF, localized to both the neuronal populations in the hippocampus. The precursor proBDNF protein showed a perinuclear distribution pattern, overlapping with the rough endoplasmic reticulum (ER), the site of protein synthesis. RT-qPCR of samples obtained using laser capture microdissection (LCM) or fluorescence-activated cell sorting (FACS) of hippocampal and cortical neurons further demonstrated the abundance of BDNF transcripts in both glutamatergic and GABAergic cells. Thus, our data provide compelling evidence that BDNF can be synthesized by both principal cells and INs of the cortex.

Localization of phosphotyrosine adaptor protein ShcD/SHC4 in the adult rat central nervous system

Background

Mammalian Shc (Src homology and collagen) proteins comprise a family of four phosphotyrosine adaptor molecules which exhibit varied spatiotemporal expression and signaling functions. ShcD is the most recently discovered homologue and it is highly expressed in the developing central nervous system (CNS) and adult brain. Presently however, its localization within specific cell types of mature neural structures has yet to be characterized.

Results

In the current study, we examine the expression profile of ShcD in the adult rat CNS using immunohistochemistry, and compare with those of the neuronally enriched ShcB and ShcC proteins. ShcD shows relatively widespread distribution in the adult brain and spinal cord, with prominent levels of staining throughout the olfactory bulb, as well as in sub-structures of the cerebellum and hippocampus, including the subgranular zone. Co-localization studies confirm the expression of ShcD in mature neurons and progenitor cells. ShcD immunoreactivity is primarily localized to axons and somata, consistent with the function of ShcD as a cytoplasmic adaptor. Regional differences in expression are observed among neural Shc proteins, with ShcC predominating in the hippocampus, cerebellum, and some fiber tracts. Interestingly, ShcD is uniquely expressed in the olfactory nerve layer and in glomeruli of the main olfactory bulb.

Conclusions

Together our findings suggest that ShcD may provide a distinct signaling contribution within the olfactory system, and that overlapping expression of ShcD with other Shc proteins may allow compensatory functions in the brain.

Predictive value of high titer of GAD65 antibodies in a case of limbic encephalitis

We report the case of a 42-year-old woman who presented with vertigo and migraine and rapidly developed cognitive decline and seizures. Both serum and cerebro-spinal fluid samples showed high titer of anti-glutamic acid decarboxylase (anti-GAD65) antibodies (998,881 IU/ml and 54,687 IU/ml respectively). Limbic encephalitis was diagnosed and high dose steroids treatment started. During one-year follow-up, without further immunomodulatory therapy, the patient became seizure free, and cognitive functions returned to normal. Serum anti-GAD65 antibodies titer decreased significantly but remained elevated (192,680 IU/ml). We discuss the prognostic and pathogenic value of high titer anti-GAD65 antibodies and its variations in a case of autoimmune limbic encephalitis.

Hippocampal GAD67 Transduction Using rAAV8 Regulates Epileptogenesis in EL Mice

Gene therapy has been employed as a therapeutic approach for intractable focal epilepsies. Considering the potential of focal GABAergic neuromodulation in regulating epileptogenesis, the GABA-producing enzyme, γ-aminobutyric acid decarboxylase 67 (GAD67), is highly suitable for epilepsy therapy. The EL/Suz (EL) mouse is a model of multifactorial temporal lobe epilepsy. In the present study, we examined focal gene transduction in epileptic EL mice using recombinant adeno-associated virus serotype 8 (rAAV8) expressing human GAD67 to enhance GABA-mediated neural inhibition. Eight-week-old mice were bilaterally injected with rAAV8-GFP or rAAV8-GAD67 in the hippocampal CA3 region. After four weeks, the GAD67-transduced EL mice, but not the rAAV-GFP-treated EL mice, exhibited a significant reduction in seizure generation. The GAD67-mediated depression became stable after 14 weeks. The excitability of the CA3 region was markedly reduced in the GAD67-transduced EL mice, consistent with the results of the Ca²⁺ imaging using hippocampal slices. In addition, downregulation of c-Fos expression was observed in GAD67-transduced hippocampi. Our findings showed that rAAV8-GAD67 induced significant changes in the GABAergic system in the EL hippocampus. Thus, rAAV8-mediated GAD67 gene transfer is a promising therapeutic strategy for the treatment of epilepsies.

Distribution of FMRFamide-related peptides and co-localization with glutamate in Cupiennius salei, an invertebrate model system

FMRFamide-related proteins have been described in both vertebrate and invertebrate nervous systems and have been suggested to play important roles in a variety of physiological processes. One proposed function is the modulation of signal transduction in mechanosensory neurons and their associated behavioral pathways in the Central American wandering spider Cupiennius salei; however, little is known about the distribution and abundance of FMRFamide-related proteins (FaRPs) within this invertebrate system. We employ immunohistochemistry, Hoechst nuclear stain and confocal microscopy of serial sections to detect, characterize and quantify FMRFamide-like immunoreactive neurons throughout all ganglia of the spider brain and along leg muscle. Within the different ganglia, between 3.4 and 12.6% of neurons showed immunolabeling. Among the immunoreactive cells, weakly and strongly labeled neurons could be distinguished. Between 71.4 and 81.7% of labeled neurons showed weak labeling, with 18.3 to 28.6% displaying strong labeling intensity. Among the weakly labeled neurons were characteristic motor neurons that have previously been shown to express ɣ-aminobutyric acid or glutamate. Ultrastructural investigations of neuromuscular junctions revealed mixed presynaptic vesicle populations including large electron-dense vesicles characteristic of neuropeptides. Double labeling for glutamate and FaRPs indicated that a subpopulation of neurons may co-express both neuroactive compounds. Our findings suggest that FaRPs are expressed throughout all ganglia and that different neurons have different expression levels. We conclude that FaRPs are likely utilized as neuromodulators in roughly 8% of neurons in the spider nervous system and that the main transmitter in a subpopulation of these neurons is likely glutamate.

Autoimmune encephalitis associated with glutamic acid decarboxylase antibodies: a case series

Antibodies against glutamic acid decarboxylase (GAD) are associated with various neurologic conditions described in patients including stiff person syndrome, cerebellar ataxia, refractory epilepsy, limbic and extralimbic encephalitis. GAD antibodies-related limbic encephalitis cases are well described; reports of extralimbic involvement are limited. We describe four cases of GAD antibody-related autoimmune encephalitis. Three of them had extralimbic involvement and only one had limbic encephalitis.

PV Interneurons: Critical Regulators of E/I Balance for Prefrontal Cortex-Dependent Behavior and Psychiatric Disorders

Elucidating the prefrontal cortical microcircuit has been challenging, given its role in multiple complex behaviors, including working memory, cognitive flexibility, attention, social interaction and emotional regulation. Additionally, previous methodological limitations made it difficult to parse out the contribution of certain neuronal subpopulations in refining cortical representations. However, growing evidence supports a fundamental role of fast-spiking parvalbumin (PV) GABAergic interneurons in regulating pyramidal neuron activity to drive appropriate behavioral responses. Further, their function is heavily diminished in the prefrontal cortex (PFC) in numerous psychiatric diseases, including schizophrenia and autism. Previous research has demonstrated the importance of the optimal balance of excitation and inhibition (E/I) in cortical circuits in maintaining the efficiency of cortical information processing. Although we are still unraveling the mechanisms of information representation in the PFC, the E/I balance seems to be crucial, as pharmacological, chemogenetic and optogenetic approaches for disrupting E/I balance induce impairments in a range of PFC-dependent behaviors. In this review, we will explore two key hypotheses. First, PV interneurons are powerful regulators of E/I balance in the PFC, and help optimize the representation and processing of supramodal information in PFC. Second, diminishing the function of PV interneurons is sufficient to generate an elaborate symptom sequelae corresponding to those observed in a range of psychiatric diseases. Then, using this framework, we will speculate on whether this circuitry could represent a platform for the development of therapeutic interventions in disorders of PFC function.

The distribution of cholinergic neurons and their co-localization with FMRFamide, in central and peripheral neurons of the spider Cupiennius salei

The spider Cupiennius salei is a well-established model for investigating information processing in arthropod sensory systems. Immunohistochemistry has shown that several neurotransmitters exist in the C. salei nervous system, including GABA, glutamate, histamine, octopamine and FMRFamide, while electrophysiology has found functional roles for some of these transmitters. There is also evidence that acetylcholine (ACh) is present in some C. salei neurons but information about the distribution of cholinergic neurons in spider nervous systems is limited. Here, we identify C. salei genes that encode enzymes essential for cholinergic transmission: choline ACh transferase (ChAT) and vesicular ACh transporter (VAChT). We used in-situ hybridization with an mRNA probe for C. salei ChAT gene to locate somata of cholinergic neurons in the central nervous system and immunohistochemistry with antisera against ChAT and VAChT to locate these proteins in cholinergic neurons. All three markers labeled similar, mostly small neurons, plus a few mid-sized neurons, in most ganglia. In the subesophageal ganglia, labeled neurons are putative efferent, motor or interneurons but the largest motor and interneurons were unlabeled. Groups of anti-ChAT labeled small neurons also connect the optic neuropils in the spider protocerebrum. Differences in individual cell labeling intensities were common, suggesting a range of ACh expression levels. Double-labeling found a subpopulation of anti-VAChT-labeled central and mechanosensory neurons that were also immunoreactive to antiserum against FMRFamide-like peptides. Our findings suggest that ACh is an important neurotransmitter in the C. salei central and peripheral nervous systems.

Long-lasting memory deficits in mice withdrawn from cocaine are concomitant with neuroadaptations in hippocampal basal activity, GABAergic interneurons and adult neurogenesis

Cocaine addiction disorder is notably aggravated by concomitant cognitive and emotional pathology that impedes recovery. We studied whether a persistent cognitive/emotional dysregulation in mice withdrawn from cocaine holds a neurobiological correlate within the hippocampus, a limbic region with a key role in anxiety and memory but that has been scarcely investigated in cocaine addiction research. Mice were submitted to a chronic cocaine (20 mg/kg/day for 12 days) or vehicle treatment followed by 44 drug-free days. Some mice were then assessed on a battery of emotional (elevated plus-maze, light/dark box, open field, forced swimming) and cognitive (object and place recognition memory, cocaine-induced conditioned place preference, continuous spontaneous alternation) behavioral tests, while other mice remained in their home cage. Relevant hippocampal features [basal c-Fos activity, GABA⁺, parvalbumin (PV)⁺ and neuropeptide Y (NPY)⁺ interneurons and adult neurogenesis (cell proliferation and immature neurons)] were immunohistochemically assessed 73 days after the chronic cocaine or vehicle protocol. The cocaine-withdrawn mice showed no remarkable exploratory or emotional alterations but were consistently impaired in all the cognitive tasks. All the cocaine-withdrawn groups, independent of whether they were submitted to behavioral assessment or not, showed enhanced basal c-Fos expression and an increased number of GABA⁺ cells in the dentate gyrus. Moreover, the cocaine-withdrawn mice previously submitted to behavioral training displayed a blunted experience-dependent regulation of PV⁺ and NPY⁺ neurons in the dentate gyrus, and neurogenesis in the hippocampus. Results highlight the importance of hippocampal neuroplasticity for the ingrained cognitive deficits present during chronic cocaine withdrawal.

Dentate gyrus autonomous ictal activity in the status epilepticus rat model of epilepsy

The dentate gyrus (DG) as part of the hippocampal formation is believed to serve as a gatekeeper with strong inhibitory properties against uncontrolled propagation of neuronal activity from the entorhinal cortex and neocortical structures. In temporal lobe epilepsy, the DG becomes hyperexcitable and loses its gate function, enabling propagation of ictal activity into downstream structures such as CA3 and CA1 areas. Furthermore, the DG, apart from facilitating propagation, may also be able to autonomously generate ictal activity, but this point has remained open so far. To tackle this question, we used intrinsic optical imaging in combination with electrophysiological recordings in brain slice preparations from rats in which status epilepticus had been induced electrically several weeks prior to measurements. Upon omission of Mg⁺⁺ from the artificial cerebrospinal fluid, in 15 out of 33 slices (45.4%) from 9 out of 13 epileptic animals (69.2%), spontaneous and autonomous ictal activity, mostly seizure-like events (SLE), was observed in the DG. This activity manifested independently from SLE generated in adjacent cortices and never occurred in slices from control animals. SLE generated in the DG differed from those with origin in the entorhinal or temporal cortex by longer latency to the first event after Mg⁺⁺ omission (p<0.001), a higher SLE frequency (p<0.05), higher amplitude (p<0.001) and a longer SLE duration (p<0.05). We conclude that in epilepsy, the DG, in addition to facilitated gating of activity from upstream structures, can serve as an independent generator of ictal activity.

γ-aminobutyric acid as a metabolite: Interpreting magnetic resonance spectroscopy experiments

The current rise in the prevalence of magnetic resonance spectroscopy experiments to measure γ-aminobutyric acid in the living human brain is an exciting and productive area of research. As research spreads into clinical populations and cognitive research, it is important to fully understand the source of the magnetic resonance spectroscopy signal and apply appropriate interpretation to the results of the experiments. γ-aminobutyric acid is present in the brain not only as a neurotransmitter, but also in high intracellular concentrations, both as a transmitter precursor and a metabolite. γ-aminobutyric acid concentrations measured by magnetic resonance spectroscopy are not necessarily implicated in neurotransmission and therefore may reflect a very different brain activity to that commonly suggested. In this perspective, we examine some of the considerations to be taken in the interpretation of any γ-aminobutyric acid signal measured by magnetic resonance spectroscopy.

Co-existence of Functionally Different Vesicular Neurotransmitter Transporters

The vesicular transmitter transporters VGLUT, VGAT, VMAT2 and VAChT, define phenotype and physiological properties of neuronal subtypes. VGLUTs concentrate the excitatory amino acid glutamate, VGAT the inhibitory amino acid GABA, VMAT2 monoamines, and VAChT acetylcholine (ACh) into synaptic vesicle (SV). Following membrane depolarization SV release their content into the synaptic cleft. A strict segregation of vesicular transporters is mandatory for the precise functioning of synaptic communication and of neuronal circuits. In the last years, evidence accumulates that subsets of neurons express more than one of these transporters leading to synaptic co-release of different and functionally opposing transmitters and modulation of synaptic plasticity. Synaptic co-existence of transporters may change during pathological scenarios in order to ameliorate misbalances in neuronal activity. In addition, evidence increases that transporters also co-exist on the same vesicle providing another layer of regulation. Generally, vesicular transmitter loading relies on an electrochemical gradient ΔμH⁺ driven by the proton ATPase rendering the lumen of the vesicle with respect to the cytosol positive (Δψ) and acidic (ΔpH). While the activity of VGLUT mainly depends on the Δψ component, VMAT, VGAT and VAChT work best at a high ΔpH. Thus, a vesicular synergy of transporters depending on the combination may increase or decrease the filling of SV with the principal transmitter. We provide an overview on synaptic co-existence of vesicular transmitter transporters including changes in the excitatory/inhibitory balance under pathological conditions. Additionally, we discuss functional aspects of vesicular synergy of transmitter transporters.

The plastic neurotransmitter phenotype of the hippocampal granule cells and of the moss in their messy fibers

The granule cells (GCs) and their axons, the mossy fibers (MFs), make synapses with interneurons in the hilus and CA3 area of the hippocampus and with pyramidal cells of CA3, each with distinct anatomical and functional characteristics. Many features of synaptic communication observed at the MF synapses are not usually observed in most cortical synapses, and thus have drawn the attention of many groups studying different aspects of the transmission of information. One particular aspect of the GCs, that makes their study unique, is that they express a dual glutamatergic-GABAergic phenotype and several groups have contributed to the understanding of how two neurotransmitters of opposing actions can act on a single target when simultaneously released. Indeed, the GCs somata and their mossy fibers express in a regulated manner glutamate and GABA, GAD, VGlut and VGAT, all markers of both phenotypes. Finally, their activation provokes both glutamate-R-mediated and GABA-R-mediated synaptic responses in the postsynaptic cell targets and even in the MFs themselves. The developmental and activity-dependent expression of these phenotypes seems to follow a "logical" way to maintain an excitation-inhibition balance of the dentate gyrus-to-CA3 communication.

Reduction in NPY-positive neurons and dysregulation of excitability in young senescence-accelerated mouse prone 8 (SAMP8) hippocampus precede the onset of cognitive impairment

The senescence-accelerated mouse prone 8 (SAMP8) strain is considered a neurodegeneration model showing age-related cognitive deficits with little physical impairment. Young SAMP8 mice, however, exhibit signs of disturbances in development such as marked hyperactivity and reduced anxiety well before the onset of cognitive impairment. As the key enzyme in local regulation of thyroid hormone (TH) signaling, type 2 deiodinase, was significantly reduced in the SAMP8 hippocampus relative to that of the normally aging SAM-resistant 1 (SAMR1), we used these two strains to compare the development of the hippocampal GABAergic system, which is known to be strongly affected by hypothyroidism. Among GABAergic components, neuronal K+ /Cl- co-transporter 2 was down-regulated in SAMP8 transiently at 2 weeks. Although distribution of total GABAergic neurons was similar in both strains, 22-30% reduction was observed in the neuropeptide Y (NPY)-positive subpopulation of GABAergic neurons in SAMP8. Electrophysiological studies on hippocampal slices obtained at 4 weeks revealed that epileptiform activity, induced by high-frequency stimulation, lasted four times longer in SAMP8 compared with SAMR1, indicating a dysregulation of excitability that may be linked to the behavioral abnormalities of young SAMP8 and to neurodegeneration later on in life. Local attenuation of TH signaling may thus impact the normal development of the GABAergic system.

The role of TRPC6 in seizure susceptibility and seizure-related neuronal damage in the rat dentate gyrus

Transient receptor potential canonical channel-6 (TRPC6) forms Ca(2+)-permeable non-selective cation channels in neurons. Although TRPC6 plays an important role in neurite outgrowth and neuronal survival during development, TRPC6 expression profiles available to identify distinctive hippocampal neuronal damage and hippocampal excitability in epilepsy are less defined. As compared to normal animals, TRPC6 expression was down-regulated in chronic epileptic rats showing spontaneous recurrent seizures. TRPC6 knockdown increased seizure susceptibility, excitability ratio and paired-pulse inhibition in the dentate gyrus (DG) of normal animals. Furthermore, TRPC6 knockdown promoted programmed neuronal necrosis in dentate granule cells, but prevented it in CA1 and CA3 neurons following status epilepticus. The present data suggest for the first time that TRPC6 may inhibit seizure susceptibility and neuronal vulnerability in the rat DG.

Viral and transgenic reporters and genetic analysis of adult neurogenesis

Stem and progenitor cells of the developing and adult brain can be effectively identified and manipulated using reporter genes, introduced into transgenic reporter mouse lines or recombinant viruses. Such reporters rely on an ever-increasing variety of fluorescent proteins and a continuously expanding list of regulatory elements and of mouse lines engineered for cell- or time-specific recombination. An important extension of stem-cell-based genetic strategies is an opportunity to explore the properties of newly generated neurons and their contribution to synaptic plasticity. Here, we review available strategies for marking and quantifying various classes of stem and progenitor cells in the adult brain, genetically tracing their progeny, and studying the properties of stem cells and new neurons. We compare various experimental approaches to labeling and investigating stem cells and their progeny and discuss caveats and limitations inherent to each approach.

Limbic encephalitis with antibodies to glutamic acid decarboxylase presenting with brainstem symptoms

Limbic encephalitis (LE) is a neurological syndrome that may present in association with cancer, infection, or as an isolate clinical condition often accompanying autoimmune disorders. LE associated with glutamic acid decarboxylase antibodies (anti-GAD) is rare in children. Here, we characterized the clinical and laboratory features of a patient presenting with brainstem involvement with non-paraneoplastic LE associated with anti-GAD antibodies. In our patient, after plasma exchange, we determined a dramatic improvement of the neurological deficits.

Lower glutamic acid decarboxylase 65-kDa isoform messenger RNA and protein levels in the prefrontal cortex in schizoaffective disorder but not schizophrenia

BACKGROUND

Altered gamma-aminobutyric acid (GABA) signaling in the prefrontal cortex (PFC) has been associated with cognitive dysfunction in patients with schizophrenia and schizoaffective disorder. Levels of the GABA-synthesizing enzyme glutamic acid decarboxylase 67-kDa isoform (GAD67) in the PFC have been consistently reported to be lower in patients with these disorders, but the status of the second GABA-synthesizing enzyme, glutamic acid decarboxylase 65-kDa isoform (GAD65), remains unclear.

METHODS

GAD65 messenger RNA (mRNA) levels were quantified in PFC area 9 by quantitative polymerase chain reaction from 62 subjects with schizophrenia or schizoaffective disorder and 62 matched healthy comparison subjects. In a subset of subject pairs, GAD65 relative protein levels were quantified by confocal immunofluorescence microscopy.

RESULTS

Mean GAD65 mRNA levels were 13.6% lower in subjects with schizoaffective disorder but did not differ in subjects with schizophrenia relative to their matched healthy comparison subjects. In the subjects with schizoaffective disorder, mean GAD65 protein levels were 19.4% lower and were correlated with GAD65 mRNA levels. Lower GAD65 mRNA and protein levels within subjects with schizoaffective disorder were not attributable to factors commonly comorbid with the diagnosis.

CONCLUSIONS

In concert with previous studies, these findings suggest that schizoaffective disorder is associated with lower levels of both GAD65 and GAD67 mRNA and protein in the PFC, whereas subjects with schizophrenia have lower mean levels of only GAD67 mRNA and protein. Because cognitive function is generally better preserved in patients with schizoaffective disorder relative to patients with schizophrenia, these findings may support an interpretation that GAD65 downregulation provides a homeostatic response complementary to GAD67 downregulation that serves to reduce inhibition in the face of lower PFC network activity.

Fine Tuning of Synaptic Plasticity and Filtering by GABA Released from Hippocampal Autaptic Granule Cells

The functional consequence of γ-aminobutyric acid (GABA) release at mossy fiber terminals is still a debated topic. Here, we provide multiple evidence of GABA release in cultured autaptic hippocampal granule cells. In ∼50% of the excitatory autaptic neurons, GABA, VGAT, or GAD67 colocalized with vesicular glutamate transporter 1-positive puncta, where both GABAB and GABAA receptors (Rs) were present. Patch-clamp recordings showed a clear enhancement of autaptic excitatory postsynaptic currents in response to the application of the GABABR antagonist CGP58845 only in neurons positive to the selective granule cell marker Prox1, and expressing low levels of GAD67. Indeed, GCP non-responsive excitatory autaptic neurons were both Prox1- and GAD67-negative. Although the amount of released GABA was not sufficient to activate functional postsynaptic GABAARs, it effectively activated presynaptic GABABRs that maintain a tonic "brake" on the probability of release and on the size of the readily releasable pool and contributed to resting potential hyperpolarization possibly through extrasynaptic GABAAR activation. The autocrine inhibition exerted by GABABRs on glutamate release enhanced both paired-pulse facilitation and post-tetanic potentiation. Such GABABR-mediated changes in short-term plasticity confer to immature granule cells the capability to modulate their filtering properties in an activity-dependent fashion, with remarkable consequences on the dynamic behavior of neural circuits.

Glutamic acid decarboxylase 1 alternative splicing isoforms: characterization, expression and quantification in the mouse brain

Background

GABA has important functions in brain plasticity related processes like memory, learning, locomotion and during the development of the nervous system. It is synthesized by the glutamic acid decarboxylase (GAD). There are two isoforms of GAD, GAD1 and GAD2, which are encoded by different genes. During embryonic development the transcription of GAD1 mRNA is regulated by alternative splicing and several alternative transcripts were distinguished in human, mouse and rat. Despite the fact that the structure of GAD1 gene has been extensively studied, knowledge of its exact structural organization, alternative promoter usage and splicing have remained incomplete.

Results

In the present study we report the identification and characterization of novel GAD1 splicing isoforms (GenBank: KM102984, KM102985) by analyzing genomic and mRNA sequence data using bioinformatics, cloning and sequencing. Ten mRNA isoforms are generated from GAD1 gene locus by the combined actions of utilizing different promoters and alternative splicing of the coding exons. Using RT-PCR we found that GAD1 isoforms share similar pattern of expression in different mouse tissues and are expressed early during development. Quantitative RT-PCR was used to investigate the expression of GAD1 isoforms and GAD2 in olfactory bulb, cortex, medial and lateral striatum, hippocampus and cerebellum of adult mouse. Olfactory bulb showed the highest expression of GAD1 transcripts. Isoforms 1/2 are the most abundant forms. Their expression is significantly higher in the lateral compared to the medial striatum. Isoforms 3/4, 5/6, 7/8 and 9/10 are barely detectable in all investigated regions except of the high expression in olfactory bulb. When comparing GAD1 expression with GAD2 we found that Isoforms 1/2 are the predominant isoforms. In situ hybridization confirmed the predominant expression of Isoforms 7/8 and 9/10 in the olfactory bulb and revealed their weak expression in hippocampus, cerebellum and some other areas known to express GAD1.

Conclusions

Generation of ten splicing isoforms of GAD1 was described including two so far uncharacterized transcripts. GAD1 splicing isoforms producing the shorter, enzymatically inactive GAD25 protein are expressed at very low level in adult mouse brain except in the olfactory bulb that is associated with neurogenesis and synaptic plasticity even during adulthood.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2202-15-114) contains supplementary material, which is available to authorized users.

Glycolysis in energy metabolism during seizures

Studies have shown that glycolysis increases during seizures, and that the glycolytic metabolite lactic acid can be used as an energy source. However, how lactic acid provides energy for seizures and how it can participate in the termination of seizures remains unclear. We reviewed possible mechanisms of glycolysis involved in seizure onset. Results showed that lactic acid was involved in seizure onset and provided energy at early stages. As seizures progress, lactic acid reduces the pH of tissue and induces metabolic acidosis, which terminates the seizure. The specific mechanism of lactic acid-induced acidosis involves several aspects, which include lactic acid-induced inhibition of the glycolytic enzyme 6-diphosphate kinase-1, inhibition of the N-methyl-D-aspartate receptor, activation of the acid-sensitive 1A ion channel, strengthening of the receptive mechanism of the inhibitory neurotransmitter γ-minobutyric acid, and changes in the intra- and extracellular environment.

Dual-transmitter neurons: functional implications of co-release and co-transmission

Co-transmission, the ability of a neuron to release multiple transmitters, has long been recognized in selected circuits. However, the release of multiple primary neurotransmitters from a single neuron is only beginning to be appreciated. Here we consider recent examples of co-transmission as well as co-release-the packaging of multiple neurotransmitters into a single vesicle. The properties associated with each mode of release greatly enhance the possible action of such neurons within circuits. The functional importance of dual- (or multi-) transmitter neurons extends beyond actions on postsynaptic receptors, due in part to differential spatial and temporal profiles of each neurotransmitter. Recent evidence also suggests that the dual-transmitter phenotype can be dynamically regulated during development and following injury or disease.

Anti-glutamic Acid decarboxylase antibody associated limbic encephalitis in a child: expanding the spectrum of pediatric inflammatory brain diseases

Anti-glutamic acid decarboxylase directed antibodies are a rare cause of autoimmune limbic encephalitis that is relatively resistant to immunotherapy. Here we report a 15-year-old boy with nonparaneoplastic, anti-glutamic acid decarboxylase limbic encephalitis presenting with subacute headache, memory disturbance, psychiatric symptoms, and seizures. At onset, his memory disturbance manifested as transient global amnesia-like episodes. Clinical remission was achieved with rituximab, intravenous immunoglobulin, and corticosteroids.

GABA and glutamate pathways are spatially and developmentally affected in the brain of Mecp2-deficient mice

Proper brain functioning requires a fine-tuning between excitatory and inhibitory neurotransmission, a balance maintained through the regulation and release of glutamate and GABA. Rett syndrome (RTT) is a rare genetic disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene affecting the postnatal brain development. Dysfunctions in the GABAergic and glutamatergic systems have been implicated in the neuropathology of RTT and a disruption of the balance between excitation and inhibition, together with a perturbation of the electrophysiological properties of GABA and glutamate neurons, were reported in the brain of the Mecp2-deficient mouse. However, to date, the extent and the nature of the GABA/glutamate deficit affecting the Mecp2-deficient mouse brain are unclear. In order to better characterize these deficits, we simultaneously analyzed the GABA and glutamate levels in Mecp2-deficient mice at 2 different ages (P35 and P55) and in several brain areas. We used a multilevel approach including the quantification of GABA and glutamate levels, as well as the quantification of the mRNA and protein expression levels of key genes involved in the GABAergic and glutamatergic pathways. Our results show that Mecp2-deficient mice displayed regional- and age-dependent variations in the GABA pathway and, to a lesser extent, in the glutamate pathway. The implication of the GABA pathway in the RTT neuropathology was further confirmed using an in vivo treatment with a GABA reuptake inhibitor that significantly improved the lifespan of Mecp2-deficient mice. Our results confirm that RTT mouse present a deficit in the GABAergic pathway and suggest that GABAergic modulators could be interesting therapeutic agents for this severe neurological disorder.

Electrophysiological characterization of granule cells in the dentate gyrus immediately after birth

Granule cells (GCs) in the dentate gyrus are generated mainly postnatally. Between embryonic day 10 and 14, neural precursors migrate from the primary dentate matrix to the dentate gyrus where they differentiate into neurons. Neurogenesis reaches a peak at the end of the first postnatal week and it is completed at the end of the first postnatal month. This process continues at a reduced rate throughout life. Interestingly, immediately after birth, GCs exhibit a clear GABAergic phenotype. Only later they integrate the classical glutamatergic trisynaptic hippocampal circuit. Here, whole cell patch clamp recordings, in current clamp mode, were performed from immature GCs, intracellularly loaded with biocytin (in hippocampal slices from P0 to P3 old rats) in order to compare their morphological characteristics with their electrophysiological properties. The vast majority of GCs were very immature with small somata, few dendritic branches terminating with small varicosities and growth cones. In spite of their immaturity their axons reached often the cornu ammonis 3 area. Immature GCs generated, upon membrane depolarization, either rudimentary sodium spikes or more clear overshooting action potentials that fired repetitively. They exhibited also low threshold calcium spikes. In addition, most spiking neurons showed spontaneous synchronized network activity, reminiscent of giant depolarizing potentials (GDPs) generated in the hippocampus by the synergistic action of glutamate and GABA, both depolarizing and excitatory. This early synchronized activity, absent during adult neurogenesis, may play a crucial role in the refinement of local neuronal circuits within the developing dentate gyrus.

Thyroid hormone-dependent development of the GABAergic pre- and post-synaptic components in the rat hippocampus

Thyroid hormone (TH) plays essential roles in normal brain development mainly by regulating gene expression through binding to specific nuclear receptors which serve as transcription factors. Previous studies showed that perinatal deficiency of TH or impairment of its signaling severely affect brain development, especially the development of the γ-aminobutyric acid (GABA) system, but cellular and molecular targets of the hormone are only partly uncovered. In the present study, we focused on the developing rat hippocampus which was confirmed to be one of the regions highly sensitive to TH status, and found two new targets of the hormone among the pre- and post-synaptic components of the GABAergic system. One was glutamic acid decarboxylase 65 (GAD65), the protein level of which was reduced to less than 50% of control in the hippocampus of hypothyroid rats (obtained by administering 0.025% methimazole in drinking water to pregnant dams from gestational day 15 until 4 weeks postpartum) and recovered to control levels by daily thyroxine-replacement after birth. Reduction in GAD65 protein was correlated immunohistochemically with a 37% reduction in the number of GAD65-positive cells as well as a reduction in GAD65-positive processes. In contrast, the other GAD isotype, GAD67, was not affected by TH status. A subpopulation of GABAergic neurons containing parvalbumin was also confirmed to be highly dependent on TH status. The second target of thyroid hormone was neuron-specific K(+)/Cl(-) co-transporter, KCC2, which is responsible for switching of GABA action from excitatory to inhibitory. In the euthyroid hippocampus, a sharp rise of kcc2 expression was observed at postnatal day (PND)10 which was followed by a large increase in KCC2 protein at PND15. This transient rise in kcc2 expression was completely suppressed by hypothyroidism, resulting in nearly 80% reduction in KCC2 protein at PND15. These results indicate that the development of GABAergic terminals and the excitatory to inhibitory maturation of GABA signaling are strongly dependent on TH.

Mixed neurotransmission in the hippocampal mossy fibers

The hippocampal mossy fibers (MFs), the axons of the granule cells (GCs) of the dentate gyrus, innervate mossy cells and interneurons in the hilus on their way to CA3 where they innervate interneurons and pyramidal cells. Synapses on each target cell have distinct anatomical and functional characteristics. In recent years, the paradigmatic view of the MF synapses being only glutamatergic and, thus, excitatory has been questioned. Several laboratories have provided data supporting the hypothesis that the MFs can transiently release GABA during development and, in the adult, after periods of enhanced excitability. This transient glutamate-GABA co-transmission coincides with the transient up-regulation of the machinery for the synthesis and release of GABA in the glutamatergic GCs. Although some investigators have deemed this evidence controversial, new data has appeared with direct evidence of co-release of glutamate and GABA from single, identified MF boutons. However, this must still be confirmed by other groups and with other methodologies. A second, intriguing observation is that MF activation produced fast spikelets followed by excitatory postsynaptic potentials in a number of pyramidal cells, which, unlike the spikelets, underwent frequency potentiation and were strongly depressed by activation of metabotropic glutamate receptors. The spikelets persisted during blockade of chemical transmission and were suppressed by the gap junction blocker carbenoxolone. These data are consistent with the hypothesis of mixed electrical-chemical synapses between MFs and some pyramidal cells. Dye coupling between these types of principal cells and ultrastructural studies showing the co-existence of AMPA receptors and connexin 36 in this synapse corroborate their presence. A deeper consideration of mixed neurotransmission taking place in this synapse may expand our search and understanding of communication channels between different regions of the mammalian CNS.

Differential vulnerability of interneurons in the epileptic hippocampus

The loss of hippocampal interneurons has been considered as one reason for the onset of temporal lobe epilepsy (TLE) by shifting the excitation-inhibition balance. Yet, there are many different interneuron types which show differential vulnerability in the context of an epileptogenic insult. We used the intrahippocampal kainate (KA) mouse model for TLE in which a focal, unilateral KA injection induces status epilepticus (SE) followed by development of granule cell dispersion (GCD) and hippocampal sclerosis surrounding the injection site but not in the intermediate and temporal hippocampus. In this study, we characterized the loss of interneurons with respect to septotemporal position and to differential vulnerability of interneuron populations. To this end, we performed intrahippocampal recordings of the initial SE, in situ hybridization for glutamic acid decarboxylase 67 (GAD67) mRNA and immunohistochemistry for parvalbumin (PV) and neuropeptide Y (NPY) in the early phase of epileptogenesis at 2 days and at 21 days after KA injection, when recurrent epileptic activity and GCD have fully developed. We show that SE extended along the entire septotemporal axis of both hippocampi, but was stronger at distant sites than at the injection site. There was an almost complete loss of interneurons surrounding the injection site and expanding to the intermediate hippocampus already at 2 days but increasing until 21 days after KA. Furthermore, we observed differential vulnerability of PV- and NPY-expressing cells: while the latter were lost at the injection site but preserved at intermediate sites, PV-expressing cells were gone even at sites more temporal than GCD. In addition, we found upregulation of GAD67 mRNA expression in dispersed granule cells and of NPY staining in ipsilateral granule cells and ipsi- and contralateral mossy fibers. Our data thus indicate differential survival capacity of interneurons in the epileptic hippocampus and compensatory plasticity mechanisms depending on the hippocampal position.

Molecular and functional characterization of GAD67-expressing, newborn granule cells in mouse dentate gyrus

Dentate gyrus granule cells (GCs) have been suggested to synthesize both GABA and glutamate immediately after birth and under pathological conditions in the adult. Expression of the GABA synthesizing enzyme GAD67 by GCs during the first few weeks of postnatal development may then allow for transient GABA synthesis and synaptic release from these cells. Here, using the GAD67-EGFP transgenic strain G42, we explored the phenotype of GAD67-expressing GCs in the mouse dentate gyrus. We report a transient, GAD67-driven EGFP expression in differentiating GCs throughout ontogenesis. EGFP expression correlates with the expression of GAD and molecular markers of GABA release and uptake in 2–4 weeks post-mitotic GCs. These rather immature cells are able to fire action potentials (APs) and are synaptically integrated in the hippocampal network. Yet they show physiological properties that differentiate them from mature GCs. Finally, GAD67-expressing GCs express a specific complement of GABAA receptor subunits as well as distinctive features of synaptic and tonic GABA signaling. Our results reveal that GAD67 expression in dentate gyrus GCs is a transient marker of late differentiation that persists throughout life and the G42 strain may be used to visualize newborn GCs at a specific, well-defined differentiation stage.

Epilepsy-induced motility of differentiated neurons

Neuronal ectopia, such as granule cell dispersion (GCD) in temporal lobe epilepsy (TLE), has been assumed to result from a migration defect during development. Indeed, recent studies reported that aberrant migration of neonatal-generated dentate granule cells (GCs) increased the risk to develop epilepsy later in life. On the contrary, in the present study, we show that fully differentiated GCs become motile following the induction of epileptiform activity, resulting in GCD. Hippocampal slice cultures from transgenic mice expressing green fluorescent protein in differentiated, but not in newly generated GCs, were incubated with the glutamate receptor agonist kainate (KA), which induced GC burst activity and GCD. Using real-time microscopy, we observed that KA-exposed, differentiated GCs translocated their cell bodies and changed their dendritic organization. As found in human TLE, KA application was associated with decreased expression of the extracellular matrix protein Reelin, particularly in hilar interneurons. Together these findings suggest that KA-induced motility of differentiated GCs contributes to the development of GCD and establish slice cultures as a model to study neuronal changes induced by epileptiform activity.

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.

Astrocytic Control of Biosynthesis and Turnover of the Neurotransmitters Glutamate and GABA

Glutamate and GABA are the quantitatively major neurotransmitters in the brain mediating excitatory and inhibitory signaling, respectively. These amino acids are metabolically interrelated and at the same time they are tightly coupled to the intermediary metabolism including energy homeostasis. Astrocytes play a pivotal role in the maintenance of the neurotransmitter pools of glutamate and GABA since only these cells express pyruvate carboxylase, the enzyme required for de novo synthesis of the two amino acids. Such de novo synthesis is obligatory to compensate for catabolism of glutamate and GABA related to oxidative metabolism when the amino acids are used as energy substrates. This, in turn, is influenced by the extent to which the cycling of the amino acids between neurons and astrocytes may occur. This cycling is brought about by the glutamate/GABA - glutamine cycle the operation of which involves the enzymes glutamine synthetase (GS) and phosphate-activated glutaminase together with the plasma membrane transporters for glutamate, GABA, and glutamine. The distribution of these proteins between neurons and astrocytes determines the efficacy of the cycle and it is of particular importance that GS is exclusively expressed in astrocytes. It should be kept in mind that the operation of the cycle is associated with movement of ammonia nitrogen between the two cell types and different mechanisms which can mediate this have been proposed. This review is intended to delineate the above mentioned processes and to discuss quantitatively their relative importance in the homeostatic mechanisms responsible for the maintenance of optimal conditions for the respective neurotransmission processes to operate.

Presynaptic but not postsynaptic GABA signaling at unitary mossy fiber synapses

Dentate gyrus granule cells have been suggested to corelease GABA and glutamate both in juvenile animals and under pathological conditions in adults. Although mossy fiber terminals (MFTs) are known to express glutamic acid decarboxylase (GAD) in early postnatal development, the functional role of GABA synthesis in MFTs remains controversial, and direct evidence for synaptic GABA release from MFTs is missing. Here, using GAD67-GFP transgenic mice, we show that GAD67 is expressed only in a population of immature granule cells in juvenile animals. We demonstrate that GABA can be released from these cells and modulate mossy fiber excitability through activation of GABAB autoreceptors. However, unitary postsynaptic currents generated by individual, GAD67-expressing granule cells are purely glutamatergic in all postsynaptic cell types tested. Thus GAD67 expression does not endow dentate gyrus granule cells with a full GABAergic phenotype and GABA primarily instructs the pre- rather than the postsynaptic element.

Rapid compensatory changes in the expression of EAAT-3 and GAT-1 transporters during seizures in cells of the CA1 and dentate gyrus

BACKGROUND

Epilepsy is a neurological disorder produced by an imbalance between excitatory and inhibitory neurotransmission, in which transporters of both glutamate and GABA have been implicated. Hence, at different times after local administration of the convulsive drug 4-aminopyridine (4-AP) we analyzed the expression of EAAT-3 and GAT-1 transporter proteins in cells of the CA1 and dentate gyrus.

METHODS

Dual immunofluorescence was used to detect the co-localization of transporters and a neuronal marker. In parallel, EEG recordings were performed and convulsive behavior was rated using a modified Racine Scale.

RESULTS

By 60 min after 4-AP injection, EAAT-3/NeuN co-labelling had increased in dentate granule cells and decreased in CA1 pyramidal cells. In the latter, this decrease persisted for up to 180 min after 4-AP administration. In both the DG and CA1, the number of GAT-1 labeled cells increased 60 min after 4-AP administration, although by 180 min GAT-1 labeled cells decreased in the DG alone. The increase in EAAT-3/NeuN colabelling in DG was correlated with maximum epileptiform activity and convulsive behavior.

CONCLUSIONS

These findings suggest that a compensatory mechanism exists to protect against acute seizures induced by 4-AP, whereby EAAT-3/NeuN cells is rapidly up regulated in order to enhance the removal of glutamate from the extrasynaptic space, and attenuating seizure activity.

GABA and glutamate are not colocalized in mossy fiber terminals of developing rodent hippocampus

It has been hypothesized that, in the developing rodent hippocampus, mossy fiber terminals release GABA together with glutamate. Here, we used transgenic glutamic acid decarboxylase-67 (GAD67)-GFP expressing mice and multi-label immunohistochemistry to address whether glutamatergic and GABAergic markers are colocalized. We demonstrate that in the dentate gyrus, interneurons positive for GABA/GAD are sparsely distributed along the edge of the hilus, in a different pattern from that of the densely packed granule cells. Co-staining for synaptophysin and vesicular glutamate transporter1 (VGLUT1) in postnatal day 14 brain sections from both mice and rats showed mossy fiber terminals as a group of large (2-5 μm in diameter) VGLUT1-positive excitatory presynaptic terminals in the stratum lucidum of area CA3a/b. Furthermore, co-staining for synaptophysin and vesicular GABA transporter (VGAT) revealed a group of small-sized (∼0.5 μm in diameter) inhibitory presynaptic terminals in the same area where identified mossy fiber terminals were present. The two types of terminals appeared to be mutually exclusive, and showed no colocalization. Thus, our results do not support the hypothesis that GABA is released as a neurotransmitter from mossy fiber terminals during development.

Co-release of glutamate and GABA from single, identified mossy fibre giant boutons

Several laboratories have provided immunohistochemical, molecular biological and electrophysiological evidence that the glutamatergic granule cells of the dentate gyrus can transiently express a GABAergic phenotype during development. Electrophysiological recordings on hippocampal slices obtained during this period have shown that stimulation of the mossy fibres (MFs) provokes simultaneous monosynaptic GABA(A) and glutamate receptor-mediated responses in their target cells,which have the pharmacological and physiological characteristics of MF neurotransmission. This evidence, although strongly supporting the hypothesis that MFs co-release glutamate and GABA, is indirect, as the extracellular stimulation used in slice experiments could activate fibres other than MFs. In this study, we show that selective stimulation of single, identified MF boutons (MFBs) attached to the apical dendrites of dissociated pyramidal cells of developing rats produced synaptic currents mediated by either glutamate receptors only or by both glutamate and GABA(A) receptors. By contrast, stimulation of MFBs of adult rats produced exclusively glutamate receptor-mediated responses. All responses evoked by stimulation of MFBs underwent strong frequency-dependent potentiation and were depressed by the activation of presynaptic metabotropic glutamate receptors. On the other hand, synaptic responses evoked by stimulation of interneuronal boutons located on the soma or on the basal dendrites of the same pyramidal cells were exclusively mediated by GABA(A) receptors, underwent frequency-dependent depression and were unaffected by mGluR agonists.We here demonstrate that the simultaneous glutamatergic and GABAergic responses evoked by MF stimulation in pyramidal cells of CA3 during development have a common origin in the giant MFBs.

Altered expression of brain monocarboxylate transporter 1 in models of temporal lobe epilepsy

Monocarboxylate transporter 1 (MCT1) facilitates the transport of monocarboxylate fuels (lactate, pyruvate and ketone bodies) and acidic drugs, such as valproic acid, across cell membranes. We recently reported that MCT1 is deficient on microvessels in the epileptogenic hippocampal formation in patients with medication-refractory temporal lobe epilepsy (TLE). To further define the role of MCT1 in the pathophysiology of TLE, we used immunohistochemistry and stereological analysis to localize and quantify the transporter in the hippocampal formation in three novel and highly relevant rat models of TLE and in nonepileptic control animals. One model utilizes methionine sulfoximine to induce brain glutamine synthetase deficiency and recurrent limbic seizures, while two models employ an episode of perforant pathway stimulation to cause epilepsy. MCT1 was lost on microvessels and upregulated on astrocytes in the hippocampal formation in all models of TLE. Notably, the loss of MCT1 on microvessels was not due to a reduction in microvessel density. The similarities in MCT1 expression among human subjects with TLE and several animal models of the disease strongly suggest a critical role of this molecule in the pathogenesis of TLE. We hypothesize that the downregulation of MCT1 may promote seizures via impaired uptake of ketone bodies and antiepileptic drugs by the epileptogenic brain. We also propose that the overexpression of MCT1 on astrocytes may lead to increased uptake or release of monocarboxylates by these cells, with important implications for brain metabolism and excitability. These hypotheses can now be rigorously tested in several animal models that replicate key features of human TLE.