Uptake and metabolism of gamma-aminobutyric acid by neurones and glial cells

A comparative review on the well-studied GAT1 and the understudied BGT-1 in the brain

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Its homeostasis is maintained by neuronal and glial GABA transporters (GATs). The four GATs identified in humans are GAT1 (SLC6A1), GAT2 (SLC6A13), GAT3 (SLC6A11), and betaine/GABA transporter-1 BGT-1 (SLC6A12) which are all members of the solute carrier 6 (SLC6) family of sodium-dependent transporters. While GAT1 has been investigated extensively, the other GABA transporters are less studied and their role in CNS is not clearly defined. Altered GABAergic neurotransmission is involved in different diseases, but the importance of the different transporters remained understudied and limits drug targeting. In this review, the well-studied GABA transporter GAT1 is compared with the less-studied BGT-1 with the aim to leverage the knowledge on GAT1 to shed new light on the open questions concerning BGT-1. The most recent knowledge on transporter structure, functions, expression, and localization is discussed along with their specific role as drug targets for neurological and neurodegenerative disorders. We review and discuss data on the binding sites for Na⁺, Cl^-, substrates, and inhibitors by building on the recent cryo-EM structure of GAT1 to highlight specific molecular determinants of transporter functions. The role of the two proteins in GABA homeostasis is investigated by looking at the transport coupling mechanism, as well as structural and kinetic transport models. Furthermore, we review information on selective inhibitors together with the pharmacophore hypothesis of transporter substrates.

Glycine Transporter 2: Mechanism and Allosteric Modulation

Neurotransmitter sodium symporters (NSS) are a subfamily of SLC6 transporters responsible for regulating neurotransmitter signalling. They are a major target for psychoactive substances including antidepressants and drugs of abuse, prompting substantial research into their modulation and structure-function dynamics. Recently, a series of allosteric transport inhibitors have been identified, which may reduce side effect profiles, compared to orthosteric inhibitors. Allosteric inhibitors are also likely to provide different clearance kinetics compared to competitive inhibitors and potentially better clinical outcomes. Crystal structures and homology models have identified several allosteric modulatory sites on NSS including the vestibule allosteric site (VAS), lipid allosteric site (LAS) and cholesterol binding site (CHOL1). Whilst the architecture of eukaryotic NSS is generally well conserved there are differences in regions that form the VAS, LAS, and CHOL1. Here, we describe ligand-protein interactions that stabilize binding in each allosteric site and explore how differences between transporters could be exploited to generate NSS specific compounds with an emphasis on GlyT2 modulation.

Reconstitution of GABA, Glycine and Glutamate Transporters

In contrast to water soluble enzymes which can be purified and studied while in solution, studies of solute carrier (transporter) proteins require both that the protein of interest is situated in a phospholipid membrane and that this membrane forms a closed compartment. An additional challenge to the study of transporter proteins has been that the transport depends on the transmembrane electrochemical gradients. Baruch I. Kanner understood this early on and first developed techniques for studying plasma membrane vesicles. This advanced the field in that the experimenter could control the electrochemical gradients. Kanner, however, did not stop there, but started to solubilize the membranes so that the transporter proteins were taken out of their natural environment. In order to study them, Kanner then had to find a way to reconstitute them (reinsert them into phospholipid membranes). The scope of the present review is both to describe the reconstitution method in full detail as that has never been done, and also to reveal the scientific impact that this method has had. Kanner's later work is not reviewed here although that also deserves a review because it too has had a huge impact.

Functional Characterization of the γ-Aminobutyric Acid Transporter from Mycobacterium smegmatis MC2 155 Reveals Sodium-Driven GABA Transport

The spread of multidrug-resistant tuberculosis increases its global health impact in humans. As there is transmission both to and from animals, the spread of the disease also increases its effects in a broad range of animal species.

Characterizing the mycobacterial transporters involved in the uptake and/or catabolism of host-derived nutrients required by mycobacteria may identify novel drug targets against tuberculosis. Here, we identify and characterize a member of the amino acid-polyamine-organocation superfamily, a potential γ-aminobutyric acid (GABA) transport protein, GabP, from Mycobacterium smegmatis. The protein was expressed to a level allowing its purification to homogeneity, and size exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) analysis of the purified protein showed that it was dimeric. We showed that GabP transported γ-aminobutyric acid both in vitro and when overexpressed in E. coli. Additionally, transport was greatly reduced in the presence of β-alanine, suggesting it could be either a substrate or inhibitor of GabP. Using GabP reconstituted into proteoliposomes, we demonstrated that γ-aminobutyric acid uptake is driven by the sodium gradient and is stimulated by membrane potential. Molecular docking showed that γ-aminobutyric acid binds MsGabP, another Mycobacterium smegmatis putative GabP, and the Mycobacterium tuberculosis homologue in the same manner. This study represents the first expression, purification, and characterization of an active γ-aminobutyric acid transport protein from mycobacteria.

IMPORTANCE The spread of multidrug-resistant tuberculosis increases its global health impact in humans. As there is transmission both to and from animals, the spread of the disease also increases its effects in a broad range of animal species. Identifying new mycobacterial transporters will enhance our understanding of mycobacterial physiology and, furthermore, provides new drug targets. Our target protein is the gene product of msmeg_6196, annotated as GABA permease, from Mycobacterium smegmatis strain MC² 155. Our current study demonstrates it is a sodium-dependent GABA transporter that may also transport β-alanine. As GABA may well be an essential nutrient for mycobacterial metabolism inside the host, this could be an attractive target for the development of new drugs against tuberculosis.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

Formation, speciation and toxicity of CX3R-type disinfection by-products (DBPs) from chlor(am)ination of 2,4-diaminobutyric acid (DAB)

2,4-diaminobutyric acid (DAB), a newly identified algal toxins in water, pose a great threat to human health. DAB may react with chlorine or chloramine to produce CX₃R-type disinfection by-products (DBPs) during water treatment processes. This study mainly investigated the formation and speciation of DBPs from chlor(am)ination of DAB. The results revealed that haloacetic acids (HAAs), trihalomethanes (THMs) and haloacetonitriles (HANs) were the main kinds of CX₃R-type DBPs generated from DAB during chlor(am)ination, of which dichloroacetic acid yielded the highest. The formation and total toxicity of four CX₃R-type DBPs from DAB during chloramination was significantly lower than that during chlorination at each Cl₂:N molar ratio. However, more formation of Br-THMs and I-THMs were observed during chloramination in the presence of Br^-/I^-. Futhermore, the effects of chlor(am)ine dosage, solution pH, reaction time, and the concentration of Br^- and I^- on the formation and speciation of CX₃R-type DBPs were also evaluated during chlor(am)ination. The plausible formation pathways of CX₃R-type DBPs from DAB were proposed and verified by theoretical calculation. The quantum chemistry calculations indicate that 1N in DAB and 8N in 2,4-diaminochlorobutyric acid (C₄H₉O₂N₂Cl) were more likely to be attacked by electrophiles, supporting the proposed pathway schemes.

Microglial expression of GAT-1 in the cerebral cortex

Microglial cells are the immune cells of the brain that, by sensing the microenvironment, permit a correct brain development and function. They communicate with other glial cells and with neurons, releasing and responding to a number of molecules that exert effects on surrounding cells. Among these, neurotransmitters and, in particular, gamma-aminobutyric acid (GABA) has recently gained interest in this context. We demonstrated the expression of GABA transporter 1 (GAT-1) in microglial cells both in soma and cell processes. We show that microglial cell treatment with 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid hydrochloride (NNC-711), a potent and selective GAT-1 inhibitor, significantly reduced Na⁺ -dependent GABA uptake. On the other hand, GABA uptake was significantly increased by cell treatment with (S)-1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-3-piperidinecarboxylic acid (SNAP-5114), a GAT-2/3 inhibitor, and this effect was completely blocked by the botulinum toxin BoNT/C1, that specifically cleaves and inactives syntaxin 1A (STX1A). Overall, these findings show that microglial cells express GAT-1 and indicate that STX1A plays an important role in the regulation of GAT-1-dependent GABA uptake in microglia.

Decomposition of β-N-methylamino-L-alanine (BMAA) and 2,4-diaminobutyric acid (DAB) during chlorination and consequent disinfection byproducts formation

β-N-methylamino-L-alanine (BMAA) and 2,4-diaminobutyric acid (DAB) are two newly identified algal toxins, and they may react with chlorine to undergo decomposition and generate disinfection byproducts (DBPs) during pre-chlorination as well as chlorine disinfection. In this study, the decomposition of BMAA and DAB during chlorination and the consequent DBPs formation were investigated. The BMAA and DAB concentrations in source waters were determined, the decomposition kinetics of BMAA and DAB and the formation of DBPs during chlorination were studied, the formation pathways of DBPs from BMAA and DAB were explored, and the factors which may affect the decomposition and DBPs formation were examined. The results revealed that BMAA and DAB were commonly detected in source waters from Taihu Lake, and the highest level of BMAA reached 230.8 ng/L, while the concentrations of DAB were generally around 2.0 ng/L. The decomposition of BMAA and DAB during chlorination both followed pseudo-first-order decay while the decomposition rate constant of DAB was significantly higher than that of BMAA. Trihalomethanes (THMs), haloacetic acids (HAAs), and haloacetonitriles (HANs) were all generated during the chlorination of BMAA and DAB with relatively high yields. Notably, the THMs, HAAs, and HANs yields of each carbon atom from BMAA and DAB were significantly higher than that from other organic precursors, and the formation of HANs from DAB was significantly higher than that from BMAA. The formation pathways of DBPs from BMAA and DAB were tentatively proposed and verified through theoretical calculations. Of note, the proposed formation pathways of THMs and HAAs from BMAA were similar to that from DAB, while the proposed formation pathways of HANs from BMAA and DAB showed some differences. Chlorine dose, pH and temperature all affected the decomposition of BMAA and DAB and DBPs formation during chlorination.

Norman Bowery's discoveries about extrasynaptic and asynaptic GABA systems and their significance

Before discovering the GABA-B receptor, Norman Bowery completed a series of studies on an extrasynaptic or asynaptic "GABA system" in the rat superior cervical sympathetic ganglion. First, he discovered an uptake system for GABA in neuroglial cells in the ganglia and in peripheral nerves, with a different substrate specificity than that in neurons. Second, he showed that accumulated GABA in sympathetic glial cells was metabolized to succinate by a transaminase enzyme. Third, he provided detailed structure-activity information about compounds activating an extrasynaptic GABA-A receptor on neurons in the rat sympathetic ganglion. Fourth, he showed that some amino acid substrates for the neuroglial transporter could indirectly stimulate neurons by releasing GABA from adjacent glial cells, and that GABA could also be released from neuroglial cells by membrane depolarization. In this review, these discoveries are briefly described and updated and some of their implications assessed. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Effect of Dimerization on the Dynamics of Neurotransmitter:Sodium Symporters

Dimerization is a common feature among the members of the neurotransmitter:sodium symporter (NSS) family of membrane proteins. Yet, the effect of dimerization on the mechanism of action of NSS members is not fully understood. In this study, we examined the collective dynamics of two members of the family, leucine transporter (LeuT) and dopamine transporter (DAT), to assess the significance of dimerization in modulating the functional motions of the monomers. We used to this aim the anisotropic network model (ANM), an efficient and robust method for modeling the intrinsic motions of proteins and their complexes. Transporters belonging to the NSS family are known to alternate between outward-facing (OF) and inward-facing (IF) states, which enables the uptake and release of their substrate (neurotransmitter) respectively, as the substrate is transported from the exterior to the interior of the cell. In both LeuT and DAT, dimerization is found to alter the collective motions intrinsically accessible to the individual monomers in favor of the functional transitions (OF ↔ IF), suggesting that dimerization may play a role in facilitating transport.

GAT-1 mediated GABA uptake in rat oligodendrocytes

Stimulated by the results of a recent paper on the effects of tiagabine, a selective inhibitor of the main GABA transporter GAT-1, on oligodendrogenesis, we verified the possibility that GAT-1 may be expressed in oligodendrocytes using immunocytochemical methods and functional assays. Light microscopic analysis of the subcortical white matter of all animals revealed the presence of numerous GAT-1+ cells of different size (from 3 to 29 µm) and morphology. An electron microscope analysis revealed that, besides fibrous astrocytes and interstitial neurons, GAT-1 immunoreactivity was present in immature and mature oligodendrocytes. Co-localization studies between GAT-1 and markers specific for oligodendrocytes (NG2 and RIP) showed that about 12% of GAT-1 positive cells in the white matter were immature oligodendrocytes, while about 15% were mature oligodendrocytes. In vitro functional assays showed that oligodendrocytes exhibit tiagabine-sensitive Na⁺ -dependent GABA uptake. Although relationships between GABA and oligodendrocytes have been known for many years, this is the first demonstration that GAT-1 is expressed in oligodendrocytes. The present results on the one hand definitely closes the era of "neuronal" and "glial" GABA transporters, on the other they suggest that oligodendrocytes may contribute to pathophysiology of the several diseases in which GAT-1 have been implicated to date. GLIA 2017;65:514-522.

Astrocytic GABA Transporters: Pharmacological Properties and Targets for Antiepileptic Drugs

Inactivation of GABA-mediated neurotransmission is achieved by high-affinity transporters located at both GABAergic neurons and the surrounding astrocytes. Early studies of the pharmacological properties of neuronal and glial GABA transporters suggested that different types of transporters might be expressed in the two cell types, and such a scenario was confirmed by the cloning of four distinctly different GABA transporters from a number of different species. These GABA-transport entities have been extensively characterized using a large number of GABA analogues of restricted conformation, and several of these compounds have been shown to exhibit pronounced anticonvulsant activity in a variety of animal seizure models. As proof of concept of the validity of this drug development approach, one GABA-transport inhibitor, tiagabine, has been developed as a clinically active antiepileptic drug. This review provides a detailed account of efforts to design new subtype-selective GABA-transport inhibitors aiming at identifying novel antiepileptic drug candidates.

Energy landscape of LeuT from molecular simulations

The bacterial sodium-coupled leucine transporter (LeuT) has been broadly used as a structural model for understanding the structure-dynamics-function of mammalian neurotransmitter transporters as well as other solute carriers that share the same fold (LeuT fold), as the first member of the family crystallographically resolved in multiple states: outward-facing open, outward-facing occluded, and inward-facing open. Yet, a complete picture of the energy landscape of (sub)states visited along the LeuT transport cycle has been elusive. In an attempt to visualize the conformational spectrum of LeuT, we performed extensive simulations of LeuT dimer dynamics in the presence of substrate (Ala or Leu) and co-transported Na(+) ions, in explicit membrane and water. We used both conventional molecular dynamics (MD) simulations (with Anton supercomputing machine) and a recently introduced method, collective MD, that takes advantage of collective modes of motions predicted by the anisotropic network model. Free energy landscapes constructed based on ∼40 μs trajectories reveal multiple substates occluded to the extracellular (EC) and/or intracellular (IC) media, varying in the levels of exposure of LeuT to EC or IC vestibules. The IC-facing transmembrane (TM) helical segment TM1a shows an opening, albeit to a smaller extent and in a slightly different direction than that observed in the inward-facing open crystal structure. The study provides insights into the spectrum of conformational substates and paths accessible to LeuT and highlights the differences between Ala- and Leu-bound substates.

Maintaining the presynaptic glutamate supply for excitatory neurotransmission

Glutamate released from synapses during excitatory neurotransmission must be rapidly recycled to maintain neuronal communication. This review evaluates data from physiological experiments at hippocampal CA3 to CA1 synapses and the calyx of Held synapse in the brainstem to analyze quantitatively the rates of release and resupply of glutamate required to sustain neurotransmission. We calculate that, without efficient recycling, the presynaptic glutamate supply will be exhausted within about a minute of normal synaptic activity. We also discuss replenishment of the presynaptic pool by diffusion from the soma, direct uptake of glutamate back into the presynaptic terminal, and uptake of glutamate precursor molecules. Diffusion of glutamate from the soma is calculated to be fast enough to resupply presynaptic glutamate in the hippocampus but not at the calyx of Held. However, because the somatic cytoplasm will also quickly run out of glutamate and synapses can function continually even if the presynaptic axon is severed, mechanisms other than diffusion must be present to resupply glutamate for release. Direct presynaptic uptake of glutamate is not present at the calyx of Held but may play a role in glutamate recycling in the hippocampus. Alternatively, glutamine or tricarboxylic acid cycle intermediates released from glia can serve as a precursor for glutamate in synaptic terminals, and we calculate that the magnitude of presynaptic glutamine uptake is sufficient to supply enough glutamate to sustain neurotransmission. The nature of these mechanisms, their relative abundance, and the co-ordination between them remain areas of intensive investigation.

Increases in the numerical density of GAT-1 positive puncta in the barrel cortex of adult mice after fear conditioning

Three days of fear conditioning that combines tactile stimulation of a row of facial vibrissae (conditioned stimulus, CS) with a tail shock (unconditioned stimulus, UCS) expands the representation of “trained” vibrissae, which can be demonstrated by labeling with 2-deoxyglucose in layer IV of the barrel cortex. We have also shown that functional reorganization of the primary somatosensory cortex (S1) increases GABAergic markers in the hollows of “trained” barrels of the adult mouse. This study investigated how whisker-shock conditioning (CS+UCS) affected the expression of puncta of a high-affinity GABA plasma membrane transporter GAT-1 in the barrel cortex of mice 24 h after associative learning paradigm. We found that whisker-shock conditioning (CS+UCS) led to increase expression of neuronal and astroglial GAT-1 puncta in the “trained” row compared to controls: Pseudoconditioned, CS-only, UCS-only and Naïve animals. These findings suggest that fear conditioning specifically induces activation of systems regulating cellular levels of the inhibitory neurotransmitter GABA.

Validation of the analytical procedure for the determination of the neurotoxin β-N-methylamino-L-alanine in complex environmental samples

The neurotoxic l-2-amino-3-methylaminopropionic acid (BMAA) was hypothesized to be involved in sporadic cases of amyotrophic lateral sclerosis (ALS). Studies highlighting a possible implication of environmental factors in the incidence of sporadic ALS have become more numerous over recent years. Over the past years, the most widely used method for quantifying BMAA was based on the derivatization of this polar and basic molecule with a fluorescent compound (6-aminoquinolonyl-N-hydroxysuccinimidyl, 6-AQC). This derivatization allows the retention of the conjugate by reversed-phase liquid chromatography and its detection by fluorescence. Nevertheless, recent findings have shown that this method applied to complex samples may cause false positive responses. We therefore developed an analytical procedure for the determination of underivatized BMAA at trace level in complex environmental matrices (river water, cyanobacteria and biofilm) using solid-phase extraction (SPE) based on mixed mode sorbent to concentrate and clean up real samples. Analyzes were performed by hydrophilic interaction chromatography (HILIC) coupled to electrospray ionization and tandem mass spectrometry used in multiple reaction monitoring scan mode. Analytical procedures were validated for the different natural samples using the total error approach. BMAA can be quantified by these reliable and highly selective analytical methods in a range of only a few ng mL(-1) in river water and a few ng mg(-1) dry weight in cyanobacteria and biofilm matrices.

GABA and Glutamate Transporters in Brain

The mammalian genome contains four genes encoding GABA transporters (GAT1, slc6a1; GAT2, slc6a13; GAT3, slc6a11; BGT1, slc6a12) and five glutamate transporter genes (EAAT1, slc1a3; EAAT2, slc1a2; EAAT3, slc1a1; EAAT4, slc1a6; EAAT5, slc1a7). These transporters keep the extracellular levels of GABA and excitatory amino acids low and provide amino acids for metabolic purposes. The various transporters have different properties both with respect to their transport functions and with respect to their ability to act as ion channels. Further, they are differentially regulated. To understand the physiological roles of the individual transporter subtypes, it is necessary to obtain information on their distributions and expression levels. Quantitative data are important as the functional capacity is limited by the number of transporter molecules. The most important and most abundant transporters for removal of transmitter glutamate in the brain are EAAT2 (GLT-1) and EAAT1 (GLAST), while GAT1 and GAT3 are the major GABA transporters in the brain. EAAT3 (EAAC1) does not appear to play a role in signal transduction, but plays other roles. Due to their high uncoupled anion conductance, EAAT4 and EAAT5 seem to be acting more like inhibitory glutamate receptors than as glutamate transporters. GAT2 and BGT1 are primarily expressed in the liver and kidney, but are also found in the leptomeninges, while the levels in brain tissue proper are too low to have any impact on GABA removal, at least in normal young adult mice. The present review will provide summary of what is currently known and will also discuss some methodological pitfalls.

Glutamate and GABA synthesis, release, transport and metabolism as targets for seizure control

The synthesis, release, reuptake, and metabolism of the excitatory and inhibitory neurotransmitters glutamate and GABA, respectively, are tightly controlled. Given the role that these two neurotransmitters play in normal and abnormal neurotransmission, it is important to consider the processes whereby they are regulated. This brief review is focused entirely on the metabolic aspects of glutamate and GABA synthesis and neurotransmission. It describes in limited detail the synthesis, release, reuptake, metabolism, cellular compartmentation and pharmacology of the glutamatergic and GABAergic synapse. This review also provides a summary and brief description of the pathologic and phenotypic features of the various genetic animal models that have been developed in an effort to provide a greater understanding of the role that each of the aforementioned metabolic processes plays in controlling excitatory and inhibitory neurotransmission and how their use will hopefully facilitate the development of safer and more efficacious therapies for the treatment of epilepsy and other neurological disorders.

GABA transporters as targets for new drugs

GABA, the principal inhibitory neurotransmitter in the cerebral cortex, maintains the inhibitory tone that counterbalances neuronal excitation. The identification and subsequent development of GABA-transport inhibitors has shown the important role that GABA transporters play in the control of the CNS. To date, four GABA transporters have been cloned (GAT1-4). Compounds that inhibit GABA uptake are targets for epilepsy treatment. Currently, they are also being investigated for other possible indications such as the treatment of psychosis, general anxiety and sleep disorders, drug addiction, acute and chronic pain. These and other issues are discussed in this article.

Expression of GABAergic receptors in mouse taste receptor cells

Background

Multiple excitatory neurotransmitters have been identified in the mammalian taste transduction, with few studies focused on inhibitory neurotransmitters. Since the synthetic enzyme glutamate decarboxylase (GAD) for gamma-aminobutyric acid (GABA) is expressed in a subset of mouse taste cells, we hypothesized that other components of the GABA signaling pathway are likely expressed in this system. GABA signaling is initiated by the activation of either ionotropic receptors (GABA_A and GABA_C) or metabotropic receptors (GABA_B) while it is terminated by the re-uptake of GABA through transporters (GATs).

Methodology/Principal Findings

Using reverse transcriptase-PCR (RT-PCR) analysis, we investigated the expression of different GABA signaling molecules in the mouse taste system. Taste receptor cells (TRCs) in the circumvallate papillae express multiple subunits of the GABA_A and GABA_B receptors as well as multiple GATs. Immunocytochemical analyses examined the distribution of the GABA machinery in the circumvallate papillae. Both GABA_A-and GABA_B- immunoreactivity were detected in the peripheral taste receptor cells. We also used transgenic mice that express green fluorescent protein (GFP) in either the Type II taste cells, which can respond to bitter, sweet or umami taste stimuli, or in the Type III GAD67 expressing taste cells. Thus, we were able to identify that GABAergic receptors are expressed in some Type II and Type III taste cells. Mouse GAT4 labeling was concentrated in the cells surrounding the taste buds with a few positively labeled TRCs at the margins of the taste buds.

Conclusions/Significance

The presence of GABAergic receptors localized on Type II and Type III taste cells suggests that GABA is likely modulating evoked taste responses in the mouse taste bud.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

Regulation of excitation by GABA neurotransmission: focus on metabolism and transport

The vast majority of excitatory synapses in the central nervous system (CNS) utilize glutamate as the neurotransmitter. The level of excitation appears to be under regulatory control by the major inhibitory neurotransmitter GABA, which is synthesized from glutamate by its decarboxylation catalysed by glutamate decarboxylase (GAD). The inactivation of GABA is brought about by high affinity GABA transporters located in the presynaptic GABAergic neurons as well as surrounding astrocytes and subsequently GABA may be metabolized by GABA-transaminase (GABA-T) ultimately allowing the carbon skeleton to enter the tricarboxylic acid (TCA) cycle for oxidative metabolism. In the presynaptic GABAergic neuron, GABA taken up seems, however, preferentially to enter the vesicular GABA pool and hence it is recycled as a transmitter. It has become clear that compounds acting as inhibitors at either the transporters or GABA-T are capable of regulating the inhibitory tonus thus controlling excitation. This has led to development of clinically efficatious antiepileptic drugs. This paper shall review recent progress in targeting these pharmacological entities.

Cloning and Characterization of a Functional Human γ-Aminobutyric Acid (GABA) Transporter, Human GAT-2

Plasma membrane gamma-aminobutyric acid (GABA) transporters act to terminate GABA neurotransmission in the mammalian brain. Intriguingly four distinct GABA transporters have been cloned from rat and mouse, whereas only three functional homologs of these transporters have been cloned from human. The aim of this study therefore was to search for this fourth missing human transporter. Using a bioinformatics approach, we successfully identified and cloned the full-length cDNA of a so far uncharacterized human GABA transporter (GAT). The predicted protein displays high sequence similarity to rat GAT-2 and mouse GAT3, and in accordance with the nomenclature for rat GABA transporters, we therefore refer to the transporter as human GAT-2. We used electrophysiological and cell-based methods to demonstrate that this protein is a functional transporter of GABA. The transport was saturable and dependent on both Na(+) and Cl(-). Pharmacologically the transporter is distinct from the other human GABA transporters and similar to rat GAT-2 and mouse GAT3 with high sensitivity toward GABA and beta-alanine. Furthermore the GABA transport inhibitor (S)-SNAP-5114 displayed some inhibitory activity at the transporter. Expression analysis by reverse transcription-PCR showed that GAT-2 mRNA is present in human brain, kidney, lung, and testis. The finding of the human GAT-2 demonstrates for the first time that the four plasma membrane GABA transporters identified in several mammalian species are all conserved in human. Furthermore the availability of human GAT-2 enables the use of all human clones of the GABA transporters in drug development programs and functional characterization of novel inhibitors of GABA transport.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

GABA: Homeostatic and pharmacological aspects

The central nervous system (CNS) operates by a fine-tuned balance between excitatory and inhibitory signalling. In this context, the inhibitory neurotransmission may be of particular interest as it has been suggested that such neuronal pathways may constitute 'command pathways' and the principle of 'dis-inhibition' leading ultimately to excitation may play a fundamental role (Roberts, E. (1974). Adv. Neurol., 5: 127-143). The neurotransmitter responsible for this signalling is gamma-aminobutyrate (GABA) which was first discovered in the CNS as a curious amino acid (Roberts, E., Frankel, S. (1950). J. Biol. Chem., 187: 55-63) and later proposed as an inhibitory neurotransmitter (Curtis, D.R., Watkins, J.C. (1960). J. Neurochem., 6: 117-141; Krnjevic, K., Schwartz, S. (1967). Exp. Brain Res., 3: 320-336). The present review will describe aspects of GABAergic neurotransmission related to homeostatic mechanisms such as biosynthesis, metabolism, release and inactivation. Additionally, pharmacological and therapeutic aspects of this will be discussed.

Characterization of a novel tonic gamma-aminobutyric acidA receptor-mediated inhibition in magnocellular neurosecretory neurons and its modulation by glia

In addition to mediating conventional quantal synaptic transmission (also known as phasic inhibition), gamma-aminobutyric acidA (GABAA) receptors have been recently shown to underlie a slower, persistent form of inhibition (tonic inhibition). Using patch-clamp electrophysiology and immunohistochemistry, we addressed here whether a GABAA receptor-mediated tonic inhibition is present in supraoptic nucleus (SON) neurosecretory neurons; identified key modulatory mechanisms, including the role of glia; and determined its functional role in controlling SON neuronal excitability. Besides blocking GABAA-mediated inhibitory postsynaptic currents, the GABAA receptor blockers bicuculline and picrotoxin caused an outward shift in the holding current (I(tonic)), both in oxytocin and vasopressin neurons. Conversely, the high-affinity antagonist gabazine selectively blocked inhibitory postsynaptic currents. Under basal conditions, I(tonic) was independent on the degree of synaptic activity but was strongly modulated by the activity GABA transporters (GATs), mostly the GAT3 isoform, found here to be localized in SON glial cells/processes. Extracellular activation of GABAergic afferents evoked a small gabazine-insensitive, bicuculline-sensitive current, which was enhanced by GAT blockade. These results suggest that I(tonic) may be activated by spillover of GABA during conditions of strong and/or synchronous synaptic activity. Blockade of I(tonic) increased input resistance, induced membrane depolarization and firing activity, and enhanced the input-output function of SON neurons. In summary, our results indicate that GABAA receptors, possibly of different molecular configuration and subcellular distribution, mediate synaptic and tonic inhibition in SON neurons. The latter inhibitory modality plays a major role in modulating SON neuronal excitability, and its efficacy is modulated by the activity of glial GATs.

A novel selective γ-aminobutyric acid transport inhibitor demonstrates a functional role for GABA transporter subtype GAT2/BGT-1 in the CNS

The system of GABA transporters in neural cells constitutes an efficient mechanism for terminating inhibitory GABAergic neurotransmission. This transport system is an important therapeutical target in epileptic disorders, but potentially also in other neurological disorders. Thus, selective intervention in GABA uptake has been the subject of extensive research for several decades. In a series of lipophilic diaromatic derivatives of (RS)-3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole (exo-THPO), N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502) turned out to be an equipotent inhibitor at the mouse transporters GAT1 and GAT2 (BGT-1) but inactive at GAT3 and GAT4. This novel pharmacological profile among GABA uptake inhibitors prompted a thorough investigation of the in vivo properties of this compound. These investigations have for the first time demonstrated a functional role for GABA transporter subtype GAT2/BGT-1, which points to the therapeutic relevance of inhibiting this transporter subtype. An overview of the development and characterisation of EF1502 is presented here.

Supersensitivity of P2X receptors in cerebrocortical cell cultures after in vitro ischemia

Neuronally enriched primary cerebrocortical cultures were exposed to glucose-free medium saturated with argon (in vitro ischemia) instead of oxygen (normoxia). Ischemia did not alter P2X7 receptor mRNA, although serum deprivation clearly increased it. Accordingly, P2X7 receptor immunoreactivity (IR) of microtubuline-associated protein 2 (MAP2)-IR neurons or of glial fibrillary acidic protein (GFAP)-IR astrocytes was not affected; serum deprivation augmented the P2X7 receptor IR only in the astrocytic, but not the neuronal cell population. However, ischemia markedly increased the ATP- and 2'-3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP)-induced release of previously incorporated [3H]GABA. Both Brilliant Blue G and oxidized ATP inhibited the release of [3H]GABA caused by ATP application; the Brilliant Blue G-sensitive, P2X7 receptor-mediated fraction, was much larger after ischemia than after normoxia. Whereas ischemic stimulation failed to alter the amplitude of ATP- and BzATP-induced small inward currents recorded from a subset of non-pyramidal neurons, BzATP caused a more pronounced increase in the frequency of miniature inhibitory postsynaptic currents (mIPSCs) after ischemia than after normoxia. Brilliant Blue G almost abolished the effect of BzATP in normoxic neurons. Since neither the amplitude of mIPSCs nor that of the muscimol-induced inward currents was affected by BzATP, it is assumed that BzATP acts at presynaptic P2X7 receptors. Finally, P2X7 receptors did not enhance the intracellular free Ca2+ concentration either in proximal dendrites or in astrocytes, irrespective of the normoxic or ischemic pre-incubation conditions. Hence, facilitatory P2X7 receptors may be situated at the axon terminals of GABAergic non-pyramidal neurons. When compared with normoxia, ischemia appears to markedly increase P2X7 receptor-mediated GABA release, which may limit the severity of the ischemic damage. At the same time we did not find an accompanying enhancement of P2X7 mRNA or protein expression, suggesting that receptors may become hypersensitive because of an increased efficiency of their transduction pathways.

Gamma-aminobutyric acid and mefloquine-induced seizures in mice

The role of GABAergic mechanism in the convulsant effect of mefloquine was investigated in mice. Mefloquine dose dependently induced tonic seizures in mice. Aminooxyacetic acid, diaminobutyric acid and muscimol significantly protected mice against mefloquine-induced seizures by significantly delaying the onset and decreasing the incidence of the seizures. Bicuculline and picrotoxin significantly enhanced the seizure producing effect of mefloquine and also significantly antagonised the protective effect of muscimol against the seizures. Phenobarbitone and diazepam effectively protected mice against mefloquine-induced seizures. Phenytoin did not alter mefloquine-induced seizures. These data indicate that GABA mechanisms might be involved in seizures produced by mefloquine in mice.

Effect of Hyperosmotic Conditions on the Expression of the Betaine-GABA-Transporter (BGT-1) in Cultured Mouse Astrocytes

The adaptation of cells to hyperosmotic conditions involves accumulation of organic osmolytes to achieve osmotic equilibrium and maintenance of cell volume. The Na+ and Cl(-)-coupled betaine/GABA transporter, designated BGT-1, is responsible for the cellular accumulation of betaine and has been proposed to play a role in osmoregulation in the brain. BGT-1 is also called GAT2 (GABA transporter 2) when referring to the mouse transporter homologue. Using Western Blotting the expression of the mouse GAT2 protein was investigated in astrocyte primary cultures exposed to a growth medium made hyperosmotic (353+/-2.5 mosmol/kg) by adding sodium chloride. A polyclonal anti-BGT-1 antibody revealed the presence of two characteristic bands at 69 and 138 kDa. When astrocytes were grown for 24 h under hyperosmotic conditions GAT2 protein was up-regulated 2-4-fold compared to the level of the isotonic control. Furthermore, the expected dimer of GAT2 was also up-regulated after 24 h under the hyperosmotic conditions. The [3H]GABA uptake was examined in the hyperosmotic treated astrocytes, and characterized using different selective GABA transport inhibitors. The up-regulation of GAT2 protein was not affecting total GABA uptake but the hyperosmotic condition did change total GABA uptake possibly involving GAT1. Immunocytochemical studies revealed cell membrane localization of GAT2 throughout astroglial processes. Taken together, these results indicate that astroglial GAT2 expression and function may be regulated by hyperosmolarity in cultured mouse astrocytes, suggesting a role of GAT2 in osmoregulation in neural cells.

Pharmacological and biochemical aspects of GABAergic neurotransmission: pathological and neuropsychobiological relationships

1. The GABAergic neurotransmission has been implicated in the modulation of many neural networks in forebrain, midbrain and hindbrain, as well as, in several neurological disorders. 2. The complete comprehension of GABA system neurochemical properties and the search for approaches in identifying new targets for the treatment of neural diseases related to GABAergic pathway are of the extreme relevance. 3. The present review will be focused on the pharmacology and biochemistry of the GABA metabolism, GABA receptors and transporters. In addition, the pathological and psychobiological implications related to GABAergic neurotransmission will be considered.

Blockade of the membranal GABA transporter potentiates GABAergic responses evoked in pyramidal cells by mossy fiber activation after seizures

The presence of a neurotransmitter within a synaptic terminal is normally due to its local synthesis. However, uptake of the neurotransmitter from the extracellular milieu by a specific membranal transporter can also explain its intraterminal accumulation. A specific vesicular transporter carries the neurotransmitter into synaptic vesicles, enabling it to be released in a calcium-dependent manner when the terminal is invaded by an action potential. Under certain circumstances, a neurotransmitter can also be released by the reversal in the direction of its membranal transporter, in a calcium-independent manner. Interestingly, gamma-aminobutyric acid (GABA) can be released in this manner after epileptic activity. With intracellular recordings, in this work we show that in rats subjected to seizures, but not in naive rats, mossy fiber stimulation in the presence of glutamate receptor blockers produces bicuculline-sensitive inhibitory postsynaptic potentials (IPSPs) in pyramidal cells. The blockade of the membranal GABA transporter (GAT-1) strongly enhances the amplitude and decay time of the IPSPs in both high and low extracellular calcium concentrations. This electrophysiological evidence, together with previous neurochemical and immunohistological data, show that GAT-1 contributes to the termination of the synaptic action of mossy fiber-released GABA and rules out its involvement in depolarization-dependent GABA release.

Inhibition of gamma-aminobutyric acid uptake: anatomy, physiology and effects against epileptic seizures

The transport of gamma-aminobutyric (GABA) limits the overspill from the synaptic cleft and serves to maintain a constant extracellular level of GABA. Two transporters, GABA transporter-1 (GAT-1) and GAT-3, are the most likely candidates for regulating GABA transport in the brain. Drugs acting either selectively or nonselectively at GATs exert distinct anticonvulsant effects, presumably because of distinct regions of action. Here I shall give a brief review of the localization and physiology of GATs and describe effects of selective and nonselective inhibitors thereof in different animal models of epilepsy.

Changes in rectal temperature and ECoG spectral power of sensorimotor cortex elicited in conscious rabbits by i.c.v. injection of GABA, GABA(A) and GABA(B) agonists and antagonists

1. In order to ascertain whether both GABA(A) and GABA(B), or only GABA(B) receptors, directly modulate thermoregulation in conscious rabbits, GABA(A)/GABA(B) agonist and antagonist agents were injected intracerebroventricularly in conscious rabbits while monitoring changes in rectal temperature (RT), gross motor behaviour (GMB) and electrocorticogram (ECoG) power spectra (ps) from sensorimotor cortices. 2. GABA (48 micromol), nipecotic acid (50 nmol), THIP (60 nmol), muscimol (18 nmol) and baclofen (8 nmol) induced hypothermia (-deltaRTmax values of 1.70+/-0.1, 1.4+/-0.2, 1.0+/-0.4, 1.1+/-0.2 and 1.6+/-0.3 degrees C, respectively), accompanied by inhibition of GMB and ECoG synchronization. THIP increased ps at delta frequency band (1.1-3.3 Hz), while GABA, nipecotic acid, muscimol and baclofen did the same at both delta and (4.6-6.5 Hz) frequency bands. ECoG ps changes were concomitant or even preceded hypothermia. 3. Bicuculline (1.8 nmol) induced hyperthermia (deltaRTmax 1.2+/-0.5 degrees C) and slight excitation of GMB, while CGP35348 (1.2 micromol) did not affect RT nor GMB. Both compounds did not affect ECoG ps. 4. Bicuculline potentiated muscimol-induced hypothermia, inhibition of GMB and synchronization of ECoG, while CGP35348 fully antagonized these effects. 5. In conclusion, the present results, while confirming the prevailing role of GABA(B), also outline a direct involvement of GABA(A) receptors in the central mechanisms of thermoregulation. Ascending inhibition towards discrete cortical areas controlling muscular activity and thermogenesis may result from GABA receptor activation in neurones proximal to the ventricles, thus contributing to hypothermia, although hypothermia-induced reduction of neuronal activity of these cortical areas cannot be ruled out.

The GABAergic phenotype of the “glutamatergic” granule cells of the dentate gyrus

The granule cells of the dentate gyrus (DG), origin of the mossy fibers (MFs), have been considered to be glutamatergic. However, data obtained with different experimental approaches in recent years may be calling for a redefinition of their phenotype. Although they indeed release glutamate for fast neurotransmission, immunohistological and molecular biology evidence has revealed that these glutamatergic cells also express GABAergic markers. The granule cell expression of a GABAergic phenotype is developmentally regulated. Electrophysiological studies reveal that during the first 3 weeks of age, mossy fiber stimulation provokes monosynaptic fast inhibitory transmission mediated by GABA, besides the monosynaptic excitatory glutamatergic transmission, onto their targets in CA3. After this age, mossy fiber GABAergic transmission abruptly disappears and the GABAergic markers are undetected. In the adult, the GABAergic markers are upregulated and GABA-mediated transmission emerges after induction of hyperexcitability. The simultaneous glutamate- and GABA-mediated signals share the same plastic and pharmacological characteristics that correspond to neurotransmission of mossy fiber origin. This intriguing evidence gives rise to two fundamental points of discussion. The first is the plausible fact that glutamate and GABA, two neurotransmitters of opposing actions, are coreleased from the mossy fibers. The second relates to its functional implications that can be immediately inferred, as the dentate gyrus can exert direct GABA-mediated excitatory actions early in life and inhibitory actions in young and adult hippocampus. This evidence poses the need to reevaluate and reinterpret some aspects of the physiology of the mossy fiber pathway under normal and pathological conditions. This work reviews the recent evidence that supports the assumption that glutamate and GABA can be coreleased from a single pathway, the mossy fibers, and makes some considerations about its functional implications.

Hyperalgesic effects of gamma-aminobutyric acid transporter I in mice

The present study focused on the involvement of gamma-aminobutyric acid transporter I (GAT1) in pain. We found that GABA uptake was increased in mouse spinal cord at 20 min and 120 min after formalin injection and in mouse brain at 120 min, but not 20 min, after formalin injection. In addition, the antinociceptive effects of GAT1-selective inhibitors were examined using assays of thermal (tail-flick) and chemical (formalin and acetic acid) nociception in C57BL/6J mice. The GAT1-selective inhibitors, ethyl nipecotate and NO-711, exhibited significant antinociceptive effects in these nociceptive assays. To study further the effects of GAT1 on pain, we used two kinds of GAT1-overexpressing transgenic mice (under the control of a CMV promoter or a NSE promoter) to examine the nociceptive responses in these mice. In the thermal, formalin, and acetic acid assays, both kinds of transgenic mice displayed significant hyperalgesia after nociceptive stimuli. In addition, the micro opioid receptor antagonist naloxone had no influence on nociceptive responses in wild-type and transgenic mice. The results indicate that GAT1 is involved in the regulation of pain processes, and point to the possibility of developing analgesic drugs that target GAT1 other than opioid receptors.