Correlation between anticonvulsant activity and inhibitory action on glial gamma-aminobutyric acid uptake of the highly selective mouse gamma-aminobutyric acid transporter 1 inhibitor 3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole and its N-alkylated analogs

Astrocytes regulate inhibitory neurotransmission through GABA uptake, metabolism, and recycling

Synaptic regulation of the primary inhibitory neurotransmitter γ-aminobutyric acid (GABA) is essential for brain function. Cerebral GABA homeostasis is tightly regulated through multiple mechanisms and is directly coupled to the metabolic collaboration between neurons and astrocytes. In this essay, we outline and discuss the fundamental roles of astrocytes in regulating synaptic GABA signaling. A major fraction of synaptic GABA is removed from the synapse by astrocytic uptake. Astrocytes utilize GABA as a metabolic substrate to support glutamine synthesis. The astrocyte-derived glutamine is subsequently transferred to neurons where it serves as the primary precursor of neuronal GABA synthesis. The flow of GABA and glutamine between neurons and astrocytes is collectively termed the GABA-glutamine cycle and is essential to sustain GABA synthesis and inhibitory signaling. In certain brain areas, astrocytes are even capable of synthesizing and releasing GABA to modulate inhibitory transmission. The majority of oxidative GABA metabolism in the brain takes place in astrocytes, which also leads to synthesis of the GABA-related metabolite γ-hydroxybutyric acid (GHB). The physiological roles of endogenous GHB remain unclear, but may be related to regulation of tonic inhibition and synaptic plasticity. Disrupted inhibitory signaling and dysfunctional astrocyte GABA handling are implicated in several diseases including epilepsy and Alzheimer's disease. Synaptic GABA homeostasis is under astrocytic control and astrocyte GABA uptake, metabolism, and recycling may therefore serve as relevant targets to ameliorate pathological inhibitory signaling.

Structural basis of GABA reuptake inhibition

γ-Aminobutyric acid (GABA) transporter 1 (GAT1)¹ regulates neuronal excitation of the central nervous system by clearing the synaptic cleft of the inhibitory neurotransmitter GABA upon its release from synaptic vesicles. Elevating the levels of GABA in the synaptic cleft, by inhibiting GABA reuptake transporters, is an established strategy to treat neurological disorders, such as epilepsy². Here we determined the cryo-electron microscopy structure of full-length, wild-type human GAT1 in complex with its clinically used inhibitor tiagabine³, with an ordered part of only 60 kDa. Our structure reveals that tiagabine locks GAT1 in the inward-open conformation, by blocking the intracellular gate of the GABA release pathway, and thus suppresses neurotransmitter uptake. Our results provide insights into the mixed-type inhibition of GAT1 by tiagabine, which is an important anticonvulsant medication. Its pharmacodynamic profile, confirmed by our experimental data, suggests initial binding of tiagabine to the substrate-binding site in the outward-open conformation, whereas our structure presents the drug stalling the transporter in the inward-open conformation, consistent with a two-step mechanism of inhibition⁴. The presented structure of GAT1 gives crucial insights into the biology and pharmacology of this important neurotransmitter transporter and provides blueprints for the rational design of neuromodulators, as well as moving the boundaries of what is considered possible in single-particle cryo-electron microscopy of challenging membrane proteins.

Extensive astrocyte metabolism of γ-aminobutyric acid (GABA) sustains glutamine synthesis in the mammalian cerebral cortex

Synaptic transmission is closely linked to brain energy and neurotransmitter metabolism. However, the extent of brain metabolism of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and the relative metabolic contributions of neurons and astrocytes, are yet unknown. The present study was designed to investigate the functional significance of brain GABA metabolism using isolated mouse cerebral cortical slices and slices of neurosurgically resected neocortical human tissue of the temporal lobe. By using dynamic isotope labeling, with [¹⁵ N]GABA and [U-¹³ C]GABA as metabolic substrates, we show that both mouse and human brain slices exhibit a large capacity for GABA metabolism. Both the nitrogen and the carbon backbone of GABA strongly support glutamine synthesis, particularly in the human cerebral cortex, indicative of active astrocytic GABA metabolism. This was further substantiated by pharmacological inhibition of the primary astrocytic GABA transporter subtype 3 (GAT3), by (S)-SNAP-5114 or 1-benzyl-5-chloro-2,3-dihydro-1H-indole-2,3-dione (compound 34), leading to significant reductions in oxidative GABA carbon metabolism. Interestingly, this was not the case when tiagabine was used to specifically inhibit GAT1, which is predominantly found on neurons. Finally, we show that acute GABA exposure does not directly stimulate glycolytic activity nor oxidative metabolism in cultured astrocytes, but can be used as an additional substrate to enhance uncoupled respiration. These results clearly show that GABA is actively metabolized in astrocytes, particularly for the synthesis of glutamine, and challenge the current view that synaptic GABA homeostasis is maintained primarily by presynaptic recycling.

Synthesis and biological evaluation of α- and β-hydroxy substituted amino acid derivatives as potential mGAT1–4 inhibitors

In this study, we report the synthesis and biological evaluation of a variety of α- and β-hydroxy substituted amino acid derivatives as potential amino acid subunits in inhibitors of GABA uptake transporters (GATs). In order to ensure that the test compounds adopt a binding pose similar to that presumed for related larger GAT inhibitors, lipophilic residues were introduced either at the amino nitrogen atom or at the alcohol function. Several of the synthesized compounds were found to exhibit similar inhibitory activity at the GAT subtypes mGAT2, mGAT3, and mGAT4, respectively, as compared with the reference N-butylnipecotic acid. Hence, these compounds might serve as starting point for future developments of more complex GAT inhibitors.

Pharmacological Characterization of a Betaine/GABA Transporter 1 (BGT1) Inhibitor Displaying an Unusual Biphasic Inhibition Profile and Anti-seizure Effects

Focal epileptic seizures can in some patients be managed by inhibiting γ-aminobutyric acid (GABA) uptake via the GABA transporter 1 (GAT1) using tiagabine (Gabitril®). Synergistic anti-seizure effects achieved by inhibition of both GAT1 and the betaine/GABA transporter (BGT1) by tiagabine and EF1502, compared to tiagabine alone, suggest BGT1 as a target in epilepsy. Yet, selective BGT1 inhibitors are needed for validation of this hypothesis. In that search, a series of BGT1 inhibitors typified by (1R,2S)-2-((4,4-bis(3-methylthiophen-2-yl)but-3-en-yl)(methyl)amino)cyclohexanecarboxylic acid (SBV2-114) was developed. A thorough pharmacological characterization of SBV2-114 using a cell-based [³H]GABA uptake assay at heterologously expressed BGT1, revealed an elusive biphasic inhibition profile with two IC₅₀ values (4.7 and 556 μM). The biphasic profile was common for this structural class of compounds, including EF1502, and was confirmed in the MDCK II cell line endogenously expressing BGT1. The possibility of two binding sites for SBV2-114 at BGT1 was assessed by computational docking studies and examined by mutational studies. These investigations confirmed that the conserved residue Q299 in BGT1 is involved in, but not solely responsible for the biphasic inhibition profile of SBV2-114. Animal studies revealed anti-seizure effects of SBV2-114 in two mouse models, supporting a function of BGT1 in epilepsy. However, as SBV2-114 is apparent to be rather non-selective for BGT1, the translational relevance of this observation is unknown. Nevertheless, SBV2-114 constitutes a valuable tool compound to study the molecular mechanism of an emerging biphasic profile of BGT1-mediated GABA transport and the putative involvement of two binding sites for this class of compounds.

Structural and molecular aspects of betaine-GABA transporter 1 (BGT1) and its relation to brain function

ɣ-aminobutyric-acid (GABA) functions as the principal inhibitory neurotransmitter in the central nervous system. Imbalances in GABAergic neurotransmission are involved in the pathophysiology of various neurological diseases such as epilepsy, Alzheimer's disease and stroke. GABA transporters (GATs) facilitate the termination of GABAergic signaling by transporting GABA together with sodium and chloride from the synaptic cleft into presynaptic neurons and surrounding glial cells. Four different GATs have been identified that all belong to the solute carrier 6 (SLC6) transporter family: GAT1-3 (SLC6A1, SLC6A13, SLC6A11) and betaine/GABA transporter 1 (BGT1, SLC6A12). BGT1 has emerged as an interesting target for treating epilepsy due to animal studies that reported anticonvulsant effects for the GAT1/BGT1 selective inhibitor EF1502 and the BGT1 selective inhibitor RPC-425. However, the precise involvement of BGT1 in epilepsy remains elusive because of its controversial expression levels in the brain and the lack of highly selective and potent tool compounds. This review gathers the current structural and functional knowledge on BGT1 with emphasis on brain relevance, discusses all available compounds, and tries to shed light on the molecular determinants driving BGT1 selectivity. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Persistent Overexposure to N-Methyl-D-Aspartate (NMDA) Calcium-Dependently Downregulates Glutamine Synthetase, Aquaporin 4, and Kir4.1 Channel in Mouse Cortical Astrocytes

Astrocytes express N-methyl-d-aspartate (NMDA) receptor (NMDAR) but its functions in these cells are not well defined. This study shows that the sustained exposure (8–72 h) of mouse astrocytes to NMDA decreases the expression of the functional astroglia-specific proteins, glutamine synthetase (GS), and the water channel protein aquaporin-4 (AQP4) and also reduces GS activity. Similar to rat astrocytes (Obara-Michlewska et al. Neurochem Int 88:20–25, 2015), the exposure of mouse astrocytes to NMDA also decreased the expression of the inward rectifying potassium channel Kir4.1. NMDA failed to elicit the effects in those cells incubated in the absence of Ca²⁺ and in those in which the GluN1 subunit of the NMDAR was silenced with GluN1 siRNA. The downregulation of GS, AQP4, and Kir4.1 observed in vitro may reflect NMDAR-mediated alterations of astrocytic functions noted in central nervous system pathologies associated with increased glutamate (Glu) release and excitotoxic tissue damage.

Delineation of the Role of Astroglial GABA Transporters in Seizure Control

Studies of GABA transport in neurons and astrocytes have provided evidence that termination of GABA as neurotransmitter is brought about primarily by active transport into the presynaptic, GABAergic nerve endings. There is, however, a considerable transport capacity in the astrocytes surrounding the synaptic terminals, a transport which may limit the availability of transmitter GABA leading to a higher probability of seizure activity governed by the balance of excitatory and inhibitory neurotransmission. Based on this it was hypothesized that selective inhibition of astrocytic GABA transport might prevent such seizure activity. A series of GABA analogs of restricted conformation were synthesized and in a number of collaborative investigations between Prof. Steve White at the University of Utah and medicinal chemists and pharmacologists at the School of Pharmacy and the University of Copenhagen, Denmark, GABA analogs with exactly this pharmacological property were identified. The most important analogs identified were N-methyl-exo-THPO (N-methyl-3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole) and its lipophilic analog EF-1502 ((RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) both of which turned out to be potent anticonvulsants in animal models of epilepsy.

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.

Glial GABA Transporters as Modulators of Inhibitory Signalling in Epilepsy and Stroke

Imbalances in GABA-mediated tonic inhibition are involved in several pathophysiological conditions. A classical way of controlling tonic inhibition is through pharmacological intervention with extrasynaptic GABA_A receptors that sense ambient GABA and mediate a persistent GABAergic conductance. An increase in tonic inhibition may, however, also be obtained indirectly by inhibiting glial GABA transporters (GATs). These are sodium-coupled membrane transport proteins that normally act to terminate GABA neurotransmitter action by taking up GABA into surrounding astrocytes. The aim of the review is to provide an overview of glial GATs in regulating tonic inhibition, especially in epilepsy and stroke. This entails a comprehensive summary of changes known to occur in GAT expression levels and signalling following epileptic and ischemic insults. Further, we discuss the accumulating pharmacological evidence for targeting GATs in these diseases.

A New Computational Model for Neuro-Glio-Vascular Coupling: Astrocyte Activation Can Explain Cerebral Blood Flow Nonlinear Response to Interictal Events

Developing a clear understanding of the relationship between cerebral blood flow (CBF) response and neuronal activity is of significant importance because CBF increase is essential to the health of neurons, for instance through oxygen supply. This relationship can be investigated by analyzing multimodal (fMRI, PET, laser Doppler…) recordings. However, the important number of intermediate (non-observable) variables involved in the underlying neurovascular coupling makes the discovery of mechanisms all the more difficult from the sole multimodal data. We present a new computational model developed at the population scale (voxel) with physiologically relevant but simple equations to facilitate the interpretation of regional multimodal recordings. This model links neuronal activity to regional CBF dynamics through neuro-glio-vascular coupling. This coupling involves a population of glial cells called astrocytes via their role in neurotransmitter (glutamate and GABA) recycling and their impact on neighboring vessels. In epilepsy, neuronal networks generate epileptiform discharges, leading to variations in astrocytic and CBF dynamics. In this study, we took advantage of these large variations in neuronal activity magnitude to test the capacity of our model to reproduce experimental data. We compared simulations from our model with isolated epileptiform events, which were obtained in vivo by simultaneous local field potential and laser Doppler recordings in rats after local bicuculline injection. We showed a predominant neuronal contribution for low level discharges and a significant astrocytic contribution for higher level discharges. Besides, neuronal contribution to CBF was linear while astrocytic contribution was nonlinear. Results thus indicate that the relationship between neuronal activity and CBF magnitudes can be nonlinear for isolated events and that this nonlinearity is due to astrocytic activity, highlighting the importance of astrocytes in the interpretation of regional recordings.

Glutamatergic Mechanisms Associated with Seizures and Epilepsy

Epilepsy is broadly characterized by aberrant neuronal excitability. Glutamate is the predominant excitatory neurotransmitter in the adult mammalian brain; thus, much of past epilepsy research has attempted to understand the role of glutamate in seizures and epilepsy. Seizures induce elevations in extracellular glutamate, which then contribute to excitotoxic damage. Chronic seizures can alter neuronal and glial expression of glutamate receptors and uptake transporters, further contributing to epileptogenesis. Evidence points to a shared glutamate pathology for epilepsy and other central nervous system (CNS) disorders, including depression, which is often a comorbidity of epilepsy. Therapies that target glutamatergic neurotransmission are available, but many have met with difficulty because of untoward adverse effects. Better understanding of this system has generated novel therapeutic targets that directly and indirectly modulate glutamatergic signaling. Thus, future efforts to manage the epileptic patient with glutamatergic-centric treatments now hold greater potential.

The subcellular localization of GABA transporters and its implication for seizure management

The ability to modulate the synaptic GABA levels has been demonstrated by using the clinically effective and selective GAT1 inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid]. N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502) which not only inhibits GAT1 like tiagabine but also BGT1 has been shown to modulate extrasynaptic GABA levels. The simultaneous inhibition of synaptic and extrasynaptic GABA transporters using tiagabine and EF1502, respectively has been demonstrated to exert a synergistic anticonvulsant effect in several seizure models in mice. The pharmacological profile of these and similar compounds has been thoroughly investigated in in vitro systems, comparing the GAT subtype selectivity with the ability to inhibit GABA uptake in primary cultures of neurons and astrocytes. However, an exact explanation has not yet been found. In the present study, the ability of GATs to form homo and/or heterodimers was investigated as well as to which membrane micro environment the GATs reside. To investigate dimerization of GATs, fusion proteins of GATs tagged with either yellow fluorescent protein or cerulean fluorescent protein were made and fluorescence resonance energy transfer (FRET) was measured. It was found that GATs form both homo- and hetero-dimers in N2A and HEK-293 cells. Microdomain localization of GATs as investigated by detergent resistant membrane fractions after treatment of tissue with Brij-98 or Triton X-100 revealed that BGT1 and GAT1 mostly localize to non-membrane rafts independent of the detergent used. However, GAT3 localizes to membrane rafts when using Brij-98. Taken together, these results suggest that the observed hetero dimerization of GATs in the FRET study is unlikely to have functional implications since the GATs are located to very different cellular compartments and cell types.

Evaluation of anticonvulsant actions of dibromophenyl enaminones using in vitro and in vivo seizure models

Epilepsy and other seizure disorders are not adequately managed with currently available drugs. We recently synthesized a series of dibromophenyl enaminones and demonstrated that AK6 and E249 were equipotent to previous analogs but more efficacious in suppressing neuronal excitation. Here we examined the actions of these lead compounds on in vitro and in vivo seizure models. In vitro seizures were induced in the hippocampal slice chemically (zero Mg2+ buffer and picrotoxin) and electrically using patterned high frequency stimulation (HFS) of afferents. In vivo seizures were induced in rats using the 6 Hz and the maximal electroshock models. AK6 (10 µM) and E249 (10 µM) depressed the amplitude of population spikes recorded in area CA1 of the hippocampus by -50.5±4.3% and -40.1±3.1% respectively, with partial recovery after washout. In the zero Mg2+ model, AK6 (10 µM) depressed multiple population spiking (mPS) by -59.3±6.9% and spontaneous bursts (SBs) by -65.9±7.2% and in the picrotoxin-model by -43.3±7.2% and -50.0±8.3%, respectively. Likewise, E249 (10 µM) depressed the zero-Mg2+-induced mPS by -48.8±9.5% and SBs by -55.8±15.5%, and in the picrotoxin model by -37.1±5.5% and -56.5±11.4%, respectively. They both suppressed post-HFS induced afterdischarges and SBs. AK6 and E249 dose-dependently protected rats in maximal electroshock and 6 Hz models of in vivo seizures after 30 min pretreatment. Their level of protection in both models was similar to that obtained with phenytoin Finally, while AK6 had no effect on locomotion in rats, phenytoin significantly decreased locomotion. AK6 and E249, suppressed in vitro and in vivo seizures to a similar extent. Their in vivo activities are comparable with but not superior to phenytoin. The most efficacious, AK6 produced no locomotor suppression while phenytoin did. Thus, AK6 and E249 may be excellent candidates for further investigation as potential agents for the treatment of epilepsy syndromes with possibly less CNS side effects.

Pharmacological identification of a guanidine-containing β-alanine analogue with low micromolar potency and selectivity for the betaine/GABA transporter 1 (BGT1)

The γ-aminobutyric acid (GABA) transporters (GATs) are key membrane transporter proteins involved in the termination of GABAergic signaling at synapses in the mammalian brain and proposed drug targets in neurological disorders such as epilepsy. To date, four different GAT subtypes have been identified: GAT1, GAT2, GAT3 and the betaine/GABA transporter 1 (BGT1). Owing to the lack of potent and subtype selective inhibitors of the non-GAT1 GABA transporters, the physiological role and therapeutic potential of these transporters remain to be fully understood. Based on bioisosteric replacement of the amino group in β-alanine or GABA, a series of compounds was generated, and their pharmacological activity assessed at human GAT subtypes. Using a cell-based [(3)H]GABA uptake assay, several selective inhibitors at human BGT1 were identified. The guanidine-containing compound 9 (2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic acid hydrochloride) displayed more than 250 times greater potency than the parent compound β-alanine at BGT1 and is thus the most potent inhibitor reported to date for this subtype (IC50 value of 2.5 µM). In addition, compound 9 displayed about 400, 16 and 40 times lower inhibitory potency at GAT1, GAT2 and GAT3, respectively. Compound 9 was shown to be a substrate for BGT1 and to have an overall similar pharmacological profile at the mouse orthologue. Compound 9 constitutes an interesting pharmacological tool for specifically investigating the cellular pharmacology of BGT1 and is the first small-molecule substrate identified with such a high selectivity for BGT1 over the three other GAT subtypes.

Structural analysis of the positive AMPA receptor modulators CX516 and Me-CX516 in complex with the GluA2 ligand-binding domain

Positive allosteric modulators of the ionotropic glutamate receptor A2 (GluA2) can serve as lead compounds for the development of cognitive enhancers. Several benzamide-type (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor modulators such as aniracetam, CX516 and CX614 have been shown to inhibit the deactivation of AMPA receptors with a less pronounced effect on desensitization. Despite CX516 being an extensively investigated AMPA receptor modulator and one of the few clinically evaluated compounds, the binding mode of CX516 to AMPA receptors has not been reported. Here, the structures of a GluA2 ligand-binding domain mutant in complex with CX516 and the 3-methylpiperidine analogue of CX516 (Me-CX516) are reported. The structures show that the binding modes of CX516 and Me-CX516 are similar to those of aniracetam and CX614 and that there is limited space for substitution at the piperidine ring of CX516. The results therefore support that CX516, like aniracetam and CX614, modulates deactivation of AMPA receptors.

Parawixin2, a novel non-selective GABA uptake inhibitor from Parawixia bistriata spider venom, inhibits pentylenetetrazole-induced chemical kindling in rats

The aims of the present work were to investigate the effects of the repeated administration of Parawixin2 (2-amino-5-ureidopentanamide; formerly FrPbAII), a novel GABA and glycine uptake inhibitor, in rats submitted to PTZ-induced kindling. Wistar rats were randomly divided in groups (n=6-8) for different treatments. Systemic injections of PTZ were administered every 48 h in the dose of 33 mg/kg; i.p., that is sufficient to induce fully kindled seizures in saline i.c.v. treated rats in a short period of time (28 days). Treatments in two types of positive controls (diazepam - DZP and nipecotic acid - NA groups) consisted in daily systemic injections of DZP (2mg/kg; i.p.) or i.c.v. injections of NA (12 μg/μL), while in experimental groups in daily i.c.v. injections of different doses of Parawixin2 (0.15; 0.075; 0.015 μg/μL). Seizures were analyzed using the Lamberty & Klitgaard score and kindling was considered as established after at least three consecutive seizures of score 4 or 5. Cumulative seizure scores for each group were analyzed using repeated measures of ANOVA followed by Tukey test. PTZ induced 4 and 5-score seizures after 12 injections in saline treated rats, whereas daily injection of Parawixin2 inhibited the onset of seizures in a dose dependent manner. Also, the challenging administration of PTZ did not raise seizure score in animals treated with the highest dose of Parawixin2 or those treated with DZP or NA. These findings together with previous data from our laboratory show that Parawixin2 could be a useful probe to design new antiepileptic drugs.

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.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

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.

(1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115), a potent γ-aminobutyric acid aminotransferase inactivator for the treatment of cocaine addiction

Vigabatrin, a GABA aminotransferase (GABA-AT) inactivator, is used to treat infantile spasms and refractory complex partial seizures and is in clinical trials to treat addiction. We evaluated a novel GABA-AT inactivator (1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115, compound 1) and observed that it does not exhibit other GABAergic or off-target activities and is rapidly and completely orally absorbed and eliminated. By use of in vivo microdialysis techniques in freely moving rats and microPET imaging techniques, 1 produced similar inhibition of cocaine-induced increases in extracellular dopamine and in synaptic dopamine in the nucleus accumbens at (1)/(300) to (1)/(600) the dose of vigabatrin. It also blocks expression of cocaine-induced conditioned place preference at a dose (1)/(300) that of vigabatrin. Electroretinographic (ERG) responses in rats treated with 1, at doses 20-40 times higher than those needed to treat addiction in rats, exhibited reductions in ERG responses, which were less than the reductions observed in rats treated with vigabatrin at the same dose needed to treat addiction in rats. In conclusion, 1 can be administered at significantly lower doses than vigabatrin, which suggests a potential new treatment for addiction with a significantly reduced risk of visual field defects.

Association of a synonymous GAT3 polymorphism with antiepileptic drug pharmacoresistance

It would be likely that the genetic variants of the GTA3 gene encoding GAT-3, an astrocytic GABA transporter, may alter gamma-aminobutyric acid (GABA) neurotransmission in the synaptic cleft in the epileptic brain and cause antiepileptic drugs (AEDs) pharmacoresistance. A candidate gene association analysis with fine mapping was performed to dissect the genetic contributions of GAT3 to AEDs pharmacoresistance. Two independent case sample sets were recruited (Samples 1 and 2), and each set was divided into two groups (drug-resistant and drug-responsive) according to the treatment outcomes with AEDs. Sample1 (n=400) was used for the initial exploratory stage of the study and sample 2 (n=435) was used for confirmation of the genetic association in the replication stage of the study. A GAT3 polymorphism (GAT3 c.1572 C>T, rs2272400) was nominally associated with AEDs pharmacoresistance (P(CC) vs P(CT/TT)=0.012, P(allelic)=0.01). The odds ratio (OR) for AED pharmacoresistance was 1.6 (95% confidence interval (CI), 1.11-2.24; P=0.01) in the additive models of inheritance. The statistical significance remained after we adjusted for a confounding factor, the etiology of epilepsy, at 0.012 (adjusted OR: 1.73, 95% CI: 1.13-2.67) and used Bonferroni's correction for multiple comparisons at 0.048. Importantly, the positive association of c.1572 T was reproduced in the replication stage (P(allelic)=0.037, joint P-value of the replication=0.001). The results suggest that GAT3 c.1572T may be one of the contributing factors with a modest effect on AEDs pharmacoresistance in the epileptic brain, shed light on a better understanding of the underlying mechanisms and serve as an impetus for new avenues of treatment for AEDs pharmacoresistance.

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.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Selective GABA transporter inhibitors tiagabine and EF1502 exhibit mechanistic differences in their ability to modulate the ataxia and anticonvulsant action of the extrasynaptic GABA(A) receptor agonist gaboxadol

Modulation of the extracellular levels of GABA via inhibition of the synaptic GABA transporter GAT1 by the clinically effective and selective GAT1 inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid; Gabitril] has proven to be an effective treatment strategy for focal seizures. Even though less is known about the therapeutic potential of other GABA transport inhibitors, previous investigations have demonstrated that N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502), which, like tiagabine, is inactive on GABA(A) receptors, inhibits both GAT1 and the extrasynaptic GABA and betaine transporter BGT1, and exerts a synergistic anticonvulsant effect when tested in combination with tiagabine. In the present study, the anticonvulsant activity and motor impairment associated with systemic administration of gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol), which, at the doses used in this study (i.e., 1-5 mg/kg) selectively activates extrasynaptic α4-containing GABA(A) receptors, was determined alone and in combination with either tiagabine or EF1502 using Frings audiogenic seizure-susceptible and CF1 mice. EF1502, when administered in combination with gaboxadol, resulted in reduced anticonvulsant efficacy and Rotarod impairment associated with gaboxadol. In contrast, tiagabine, when administered in combination with gaboxadol, did not modify the anticonvulsant action of gaboxadol or reverse its Rotarod impairment. Taken together, these results highlight the mechanistic differences between tiagabine and EF1502 and support a functional role for BGT1 and extrasynaptic GABA(A) receptors.

GABA transport inhibitors and seizure protection: the past and future

Since it was first reported approximately 40 years ago that putative amino acid neurotransmitters, including GABA, would likely be inactivated by synaptic high-affinity transporters, there has been an exponential increase in interest in delineating the pharmacological characteristics of these transporters. During the 1980s and 1990s a large series of publications was devoted to a detailed characterization of neuronal and astroglial GABA transporters demonstrating important differences between these, a notion that turned out to be of relevance for the development of anticonvulsants targeting GABA transporters. The cloning era, leading to the identification of four proteins capable of transporting GABA across plasma membranes, has further boosted this research. Ultimately the clinically active antiepileptic drug, tiagabine, was developed and it was established that its mechanism of action involved inhibition of the GABA transporter-1 (GAT1). Current and future research is directed towards a better understanding of how extrasynaptic GABA receptors may be regulated via manipulation of extrasynaptic GABA levels, possibly involving extrasynaptic GABA transporters, most likely non-GAT1 transporters.

Homology modelling of the GABA transporter and analysis of tiagabine binding

A homology model of the human GABA transporter (GAT-1) based on the recently reported crystal structures of the bacterial leucine transporter from Aquifex aeolicus (LeuT) was developed. The stability of the resulting model embedded in a membrane environment was analyzed by extensive molecular dynamics (MD) simulations. Based on docking studies and subsequent MD simulations of three compounds, the endogenous ligand GABA and two potent inhibitors, (R)-nipecotic acid and the anti-epilepsy drug tiagabine, various binding modes were identified and are discussed. Whereas GABA and (R)-nipecotic acid, which are both substrates, are stabilised with residues located deep inside the occluded state binding pocket (including residues Tyr 60 and Ser 396), tiagabine, which contains a large aliphatic side chain, is stabilised in a binding mode that extends from the substrate binding pocket (i.e., stabilised by Phe 294) to the extracellular vestibule, where the side chain is stabilised by aliphatic residues. The tiagabine binding mode, reaching from the substrate binding site to the extracellular vestibule, forces the side chain of Phe 294 to adopt a distinct conformation from that found in the occluded conformation of the transporter. Hence, in presence of tiagabine, GAT-1 is constrained in an open-to-out conformation. Our results may be of particular interest for the design of new GAT-1 inhibitors.

The micro-architecture of the cerebral cortex: functional neuroimaging models and metabolism

In order to interpret/integrate data obtained with different functional neuroimaging modalities (e.g. fMRI, EEG/MEG, PET/SPECT, fNIRS), forward-generative models of a diversity of brain mechanisms at the mesoscopic level are considered necessary. For the cerebral cortex, the brain structure with possibly the most relevance for functional neuroimaging, a variety of such biophysical models has been proposed over the last decade. The development of technological tools to investigate in vitro the physiological, anatomical and biochemical principles at the microscopic scale in comparative studies formed the basis for such theoretical progresses. However, with the most recent introduction of systems to record electrical (e.g. miniaturized probes chronically/acutely implantable in the brain), optical (e.g. two-photon laser scanning microscopy) and atomic nuclear spectral (e.g. nuclear magnetic resonance spectroscopy) signals using living laboratory animals, the field is receiving even greater attention. Major advances have been achieved by combining such sophisticated recording systems with new experimental strategies (e.g. transgenic/knock-out animals, high resolution stereotaxic manipulation systems for probe-guidance and cellular-scale chemical-delivery). Theoreticians may now be encouraged to re-consider previously formulated mesoscopic level models in order to incorporate important findings recently made at the microscopic scale. In this series of reviews, we summarize the background at the microscopic scale, which we suggest will constitute the foundations for upcoming representations at the mesoscopic level. In this first part, we focus our attention on the nerve ending particles in order to summarize basic principles and mechanisms underlying cellular metabolism in the cerebral cortex. It will be followed by two parts highlighting major features in its organization/working-principles to regulate both cerebral blood circulation and neuronal activity, respectively. Contemporary theoretical models for functional neuroimaging will be revised in the fourth part, with particular emphasis in their applications, advantages/limitations and future prospects.

Inhibition of the betaine-GABA transporter (mGAT2/BGT-1) modulates spontaneous electrographic bursting in the medial entorhinal cortex (mEC)

Disruptions in GABAergic neurotransmission have been implicated in numerous CNS disorders, including epilepsy and neuropathic pain. Selective inhibition of neuronal and glial GABA transporter subtypes may offer unique therapeutic options for regaining balance between inhibitory and excitatory systems. The ability of two GABA transport inhibitors to modulate inhibitory tone via inhibition of mGAT1 (tiagabine) or mGAT2/BGT-1 (N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-4-(methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), also known as EF1502) was evaluated using an in vitro model of spontaneous interictal-like bursting (SB). SBs were recorded extracellularly in combined mEC-HC horizontal brain slices (400 microm; 31+/-1 degrees C) obtained from KA-treated rats. Slice recordings demonstrated that EF1502 exhibited a concentration-dependent reduction in SB frequency. EF1502 significantly reduced SB rate to 32% of control at the 30 microM concentration, while reducing the area and duration of SB activity to 60% and 46% of control, respectively, at the 10 microM concentration. In contrast, the GAT1 selective inhibitor tiagabine (3, 10, and 30 microM) was unable to significantly reduce the frequency of SB activity in the mEC, despite significantly reducing both the duration (51% of control) and area (58% of control) of the SB at concentrations as low as 3 microM. The ability of EF1502, but not tiagabine, to inhibit SBs in the mEC suggests that this in vitro model of pharmacoresistant SB activity is useful to differentiate between novel anticonvulsants with similar mechanisms of action and suggests a therapeutic potential for non-GAT1 transport inhibitors.

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.

Neuropharmacological profile of FrPbAII, purified from the venom of the social spider Parawixia bistriata (Araneae, Araneidae), in Wistar rats

The aims of the present study were to investigate the anticonvulsant activity and behavioral toxicity of FrPbAII using freely moving Wistar rats. Moreover, the effectiveness of this compound against chemical convulsants was compared to that of the inhibitor of the GABAergic uptake, nipecotic acid. Our results show that FrPbAII was effective against seizures induced by the i.c.v. injection of pilocarpine (ED(50) = 0.05 microg/animal), picrotoxin (ED(50) = 0.02 microg/animal), kainic acid (ED(50) = 0.2 microg/animal) and the systemic administration of PTZ (ED(50) = 0.03 microg/animal). The anticonvulsant effect of FrPbAII differed from that of nipecotic acid in potency, as the doses needed to block the seizures were more than 10 folds lower. Toxicity assays revealed that in the rotarod, the toxic dose of the FrPbAII is 1.33 microg/animal, and the therapeutic indexes were calculated for each convulsant. Furthermore, the spontaneous locomotor activity of treated animals was not altered when compared to control animals but differed from the animals treated with nipecotic acid. Still, FrPbAII did not induce changes in any of the behavioral parameters analyzed. Finally, when tested for cognitive impairments in the Morris water maze, the i.c.v. injection of FrPbAII did not alter escape latencies of treated animals. These findings indicate that the novel GABA uptake inhibitor is a potent anticonvulsant with mild side-effects when administered to Wistar rats.

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.

Anticonvulsant and anxiolytic activity of FrPbAII, a novel GABA uptake inhibitor isolated from the venom of the social spider Parawixia bistriata (Araneidae: Araneae)

This study was aimed at determining the effects of FrPbAII (174 Da), a novel isolated component from Parawixia bistriata spider venom, in the CNS of Wistar rats. Considering that FrPbAII inhibits the high affinity GABAergic uptake in a dose-dependent manner, its anxiolytic and anticonvulsant effects were analyzed in well-established animal models. Injection of FrPbAII in the rat hippocampus induced a marked anxiolytic effect, increasing the occupancy in the open arms of the elevated plus maze (EC(50)=0.09 microg/microl) and increasing the time spent in the lit area of the light-dark apparatus (EC(50)=0.03 microg/microl). Anxiolytic effects were also observed considering the number of entries in the open arms of the EPM and in the lit compartment of the light-dark box. Interestingly, when microinjected bilaterally in the SNPr of freely moving rats, FrPbAII (0.6 microg/microl) effectively prevented seizures induced by the unilateral GABAergic blockade of Area tempestas (bicuculline, 0.75 microg/microl). This anticonvulsant effect was similar to that evoked by muscimol (0.1 microg/microl) and baclofen (0.6 microg/microl), but differed from that of the specific GAT1 inhibitor, nipecotic acid (0.7 microg/microl). This difference could be accounted either for the parallel action of FrPbAII over glycinergic transporters or to an inspecific activity on GABAergic transporters. Data from the present investigation might be pointing to a novel compound with interesting and yet unexplored pharmacological potential.

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.

The Structure of a Mixed GluR2 Ligand-binding Core Dimer in Complex with (S)-Glutamate and the Antagonist (S)-NS1209

Ionotropic glutamate receptors (iGluRs) mediate fast synaptic transmission between cells of the central nervous system and are involved in various aspects of normal brain function. iGluRs are implicated in several brain disorders, e.g. in the high-frequency discharge of impulses during an epileptic seizure. (RS)-NS1209 functions as a competitive antagonist at 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionate receptors, and shows robust preclinical anticonvulsant and neuroprotective effects. This study explores 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionate receptor binding and selectivity of this novel class of antagonists. We present here the first X-ray structure of a mixed GluR2 ligand-binding core dimer, with the high-affinity antagonist (S)-8-methyl-5-(4-(N,N-dimethylsulfamoyl)phenyl)-6,7,8,9,-tetrahydro-1H-pyrrolo[3,2-h]-isoquinoline-2,3-dione-3-O-(4-hydroxybutyrate-2-yl)oxime [(S)-NS1209] in one protomer and the endogenous ligand (S)-glutamate in the other. (S)-NS1209 stabilises an even more open conformation of the D1 and D2 domains of the ligand-binding core than that of the apo structure due to steric hindrance. This is the first time ligand-induced hyperextension of the binding domains has been observed. (S)-NS1209 adopts a novel binding mode, including hydrogen bonding to Tyr450 and Gly451 of D1. Parts of (S)-NS1209 occupy new areas of the GluR2 ligand-binding cleft, and bind near residues that are not conserved among receptor subtypes. The affinities of (RS)-NS1209 at the GluR2 ligand-binding core as well as at GluR1-6 and mutated GluR1 and GluR3 receptors have been measured. Two distinct binding affinities were observed at the GluR3 and GluR4 receptors. In a functional in vitro assay, no difference in potency was observed between GluR2(Q)(o) and GluR3(o) receptors. The thermodynamics of binding of the antagonists (S)-NS1209, DNQX and (S)-ATPO to the GluR2 ligand-binding core have been determined by displacement isothermal titration calorimetry. The displacement of (S)-glutamate by all antagonists was shown to be driven by enthalpy.

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.

Selective inhibitors of GABA uptake: synthesis and molecular pharmacology of 4-N-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol analogues

A series of lipophilic diaromatic derivatives of the glia-selective GABA uptake inhibitor (R)-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol [(R)-exo-THPO, 4] were synthesized via reductive amination of 3-ethoxy-4,5,6,7-tetrahydrobenzo[d]isoxazol-4-one (9) or via N-alkylation of O-alkylatedracemic 4. The effects of the target compounds on GABA uptake mechanisms in vitro were measured using a rat brain synaptosomal preparation or primary cultures of mouse cortical neurons and glia cells (astrocytes), as well as HEK cells transfected with cloned mouse GABA transporter subtypes (GAT1-4). The activity against isoniazid-induced convulsions in mice after subcutaneous administration of the compounds was determined. All of the compounds were potent inhibitors of synaptosomal uptake the most potent compound being (RS)-4-[N-(1,1-diphenylbut-1-en-4-yl)amino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (17a, IC50 = 0.14 microM). The majority of the compounds showed a weak preference for glial, as compared to neuronal, GABA uptake. The highest degree of selectivity was 10-fold corresponding to the glia selectivity of (R)-N-methyl-exo-THPO (5). All derivatives showed a preference for the GAT1 transporter, as compared with GAT2-4, with the exception of (RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (28d), which quite surprisingly turned out to be more potent than GABA at both GAT1 and GAT2 subtypes. The GAT1 activity was shown to reside in (R)-28d whereas (R)-28d and (S)-28d contributed equally to GAT2 activity. This makes (S)-28d a GAT2 selective compound, and (R)-28d equally effective in inhibition of GAT1 and GAT2 mediated GABA transport. All compounds tested were effective as anticonvulsant reflecting that these compounds have blood-brain barrier permeating ability.

Bioisosteric Modifications of 2-Arylureidobenzoic Acids: Selective Noncompetitive Antagonists for the Homomeric Kainate Receptor Subtype GluR5

2-Arylureidobenzoic acids (AUBAs) have recently been presented as the first series of selective noncompetitive GluR5 antagonists. In this paper we have modified the acidic moiety of the AUBAs by introducing different acidic and neutral groups, and similarly, we have replaced the urea linker of the AUBAs with other structurally related linkers. Replacing the acid with neutral substituents led to inactive compounds in all instances, showing that an acidic moiety is necessary for activity. Replacing the carboxylic moiety in 2a with a sulfonic acid (5c) or a tetrazole ring (5d) improved the potency at GluR5 receptors (compounds 5c and 5d showed IC(50) values of 1.5 and 2.0 muM, respectively, compared to compound 2a with IC(50) = 4.8 muM). Compound 5c did not show improved in vivo activity in the ATPA rigidity test compared to 2a, whereas compound 5d was 4 times more potent than 2a. All compounds wherein the urea linker had been replaced showed lower or no activity. The results described extend the knowledge of structure-activity relationships for the AUBAs, and compound 5d may prove to be a good candidate for studying GluR5 receptors in vitro and in vivo.

First Demonstration of a Functional Role for Central Nervous System Betaine/γ-Aminobutyric Acid Transporter (mGAT2) Based on Synergistic Anticonvulsant Action among Inhibitors of mGAT1 and mGAT2

In a recent study, EF1502 [N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo [d]isoxazol-3-ol], which is an N-substituted analog of the GAT1-selective GABA uptake inhibitor exo-THPO (4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), was found to inhibit GABA transport mediated by both GAT1 and GAT2 in human embryonic kidney (HEK) cells expressing the mouse GABA transporters GAT1 to 4 (mGAT1-4). In the present study, EF1502 was found to possess a broad-spectrum anticonvulsant profile in animal models of generalized and partial epilepsy. When EF1502 was tested in combination with the clinically effective GAT1-selective inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid] or LU-32-176B [N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol], another GAT1-selective N-substituted analog of exo-THPO, a synergistic rather than additive anticonvulsant interaction was observed in the Frings audiogenic seizure-susceptible mouse and the pentylenetetrazol seizure threshold test. In contrast, combination of the two mGAT1-selective inhibitors, tiagabine and LU-32-176B, resulted in only an additive anticonvulsant effect. Importantly, the combination of EF1502 and tiagabine did not result in a greater than additive effect in the rotarod behavioral impairment test. In subsequent in vitro studies conducted in HEK-293 cells expressing the cloned mouse GAT transporters mGAT1 and mGAT2, EF1502 was found to noncompetitively inhibit both mGAT1 and the betaine/GABA transporter mGAT2 (K(i) of 4 and 5 muM, respectively). Furthermore, in a GABA release study conducted in neocortical neurons, EF1502 did not act as a substrate for the GABA carrier. Collectively, these findings support a functional role for mGAT2 in the control of neuronal excitability and suggest a possible utility for mGAT2-selective inhibitors in the treatment of epilepsy.