Dynamic equilibrium of neurotransmitter transporters: not just for reuptake anymore

Tonic extrasynaptic GABA(A) receptor currents control gonadotropin-releasing hormone neuron excitability in the mouse

It is well established that the GABA(A) receptor plays an important role in regulating the electrical excitability of GnRH neurons. Two different modes of GABA(A) receptor signaling exist: one mediated by synaptic receptors generating fast (phasic) postsynaptic currents and the other mediated by extrasynaptic receptors generating a persistent (tonic) current. Using GABA(A) receptor antagonists picrotoxin, bicuculline methiodide, and gabazine, which differentiate between phasic and tonic signaling, we found that ∼50% of GnRH neurons exhibit an approximately 15-pA tonic GABA(A) receptor current in the acute brain slice preparation. The blockade of either neuronal (NO711) or glial (SNAP-5114) GABA transporter activity within the brain slice revealed the presence of tonic GABA signaling in ∼90% of GnRH neurons. The GABA(A) receptor δ subunit is only found in extrasynaptic GABA(A) receptors. Using single-cell RT-PCR, GABA(A) receptor δ subunit mRNA was identified in GnRH neurons and the δ subunit-specific agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol was found to activate inward currents in GnRH neurons. Perforated-patch clamp studies showed that 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol exerted the same depolarizing or hyperpolarizing effects as GABA on juvenile and adult GnRH neurons and that tonic GABA(A) receptor signaling regulates resting membrane potential. Together, these studies reveal the presence of a tonic GABA(A) receptor current in GnRH neurons that controls their excitability. The level of tonic current is dependent, in part, on neuronal and glial GABA transporter activity and mediated by extrasynaptic δ subunit-containing GABA(A) receptors.

Differential distribution of proteins regulating GABA synthesis and reuptake in axon boutons of subpopulations of cortical interneurons

Subclasses of γ-aminobutyric acid (GABA) interneurons differentially influence cortical network activity. The contribution of differences in GABA synthesis and reuptake in axon boutons to cell type-specific functions is unknown. GABA is synthesized within boutons by glutamic acid decarboxylase 65 (GAD65) and GAD67, while GAT1 is responsible for GABA reuptake. Using an imaging methodology capable of determining the colocalization frequency of different immunocytochemical labels in the same bouton and the quantification of the fluorescence intensity of each label in these same structures, we assessed the bouton levels of GAD65, GAD67, and GAT1 in parvalbumin-expressing chandelier (PV(ch)) and basket (PV(b)) neurons and cannabinoid 1 receptor-expressing basket (CB1r(b)) neurons in the monkey prefrontal cortex. We show that PV(ch) boutons almost exclusively contained GAD67, relative to GAD65, whereas CB1r(b) boutons contained mostly GAD65. In contrast, both GAD65 and GAD67 were easily detected in PV(b) boutons. Furthermore, in comparison with PV(ch) boutons, CB1r(b) boutons expressed low to undetectable levels of GAT1. Our findings provide a new basis for the distinctive functional roles of these perisomatic-innervating interneurons in cortical circuits. In addition, they strongly suggest that altered levels of GAD67 or GAD65, as seen in some psychiatric diseases, would have cell type-specific consequences on the modulation of GABA neurotransmission.

Differential localization and function of GABA transporters, GAT-1 and GAT-3, in the rat globus pallidus

GABA transporter subtype 1 (GAT-1) and GABA transporter subtype 3 (GAT-3) are the main transporters that regulate inhibitory GABAergic transmission in the mammalian brain through GABA reuptake. In this study, we characterized the ultrastructural localizations and determined the respective roles of these transporters in regulating evoked inhibitory postsynaptic currents (eIPSCs) in globus pallidus (GP) neurons after striatal stimulation. In the young and adult rat GP, GAT-1 was preferentially expressed in unmyelinated axons, whereas GAT-3 was almost exclusively found in glial processes. Except for rare instances of GAT-1 localization, neither of the two transporters was significantly expressed in GABAergic terminals in the rat GP. 1-(4,4-Diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) (10 μm), a GAT-1 inhibitor, significantly prolonged the decay time, but did not affect the amplitude, of eIPSCs induced by striatal stimulation (15-20 V). On the other hand, the semi-selective GAT-3 inhibitor 1-(2-[tris(4-methoxyphenyl)methoxy]ethyl)-(S)-3-piperidinecarboxylic acid (SNAP 5114) (10 μm) increased the amplitude and prolonged the decay time of eIPSCs. The effects of transporter blockade on the decay time and amplitude of eIPSCs were further increased when both inhibitors were applied together. Furthermore, SKF 89976A or SNAP 5114 blockade also increased the amplitude and frequency of spontaneous IPSCs, but did not affect miniature IPSCs. Significant GABA(A) receptor-mediated tonic currents were induced in the presence of high concentrations of both SKF 89976A (30 μm) and SNAP 5114 (30 μm). In conclusion, these data indicate that GAT-1 and GAT-3 represent different target sites through which GABA reuptake may subserve complementary regulation of GABAergic transmission in the rat GP.

Profound desensitization by ambient GABA limits activation of δ-containing GABAA receptors during spillover

High-affinity extrasynaptic GABA(A) receptors (GABA(A)Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA(A)Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA(A)Rs. We found that δ subunit-containing GABA(A)Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" δ subunit-containing GABA(A)Rs exhibited profound desensitization, rendering them insensitive to brief synaptic- or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

Neuronal responses below firing threshold for subthreshold cross-modal enhancement

Multisensory integration (such as somatosensation-vision, gustation-olfaction) could occur even between subthreshold stimuli that in isolation do not reach perceptual awareness. For example, when a somatosensory (subthreshold) stimulus is delivered within a close spatiotemporal congruency, a visual (subthreshold) stimulus evokes a visual percept. Cross-modal enhancement of visual perception is maximal when the somatosensory stimulation precedes the visual one by tens of milliseconds. This rapid modulatory response would not be consistent with a top-down mechanism acting through higher-order multimodal cortical areas, but rather a direct interaction between lower-order unimodal areas. To elucidate the neuronal mechanisms of subthreshold cross-modal enhancement, we simulated a neural network model. In the model, lower unimodal (X, Y) and higher multimodal (M) networks are reciprocally connected by bottom-up and top-down axonal projections. The lower networks are laterally connected with each other. A pair of stimuli was presented to the lower networks, whose respective intensities were too weak to induce salient neuronal activity (population response) when presented alone. Neurons of the Y network were slightly depolarized below firing threshold when a cross-modal stimulus was presented alone to the X network. This allowed the Y network to make a rapid (within tens of milliseconds) population response when presented with a subsequent congruent stimulus. The reaction speed of the Y network was accelerated, provided that the top-down projections were strengthened. We suggest that a subthreshold (nonpopulation) response to a cross-modal stimulus, acting through interaction between lower (primary unisensory) areas, may be essential for a rapid suprathreshold (population) response to a congruent stimulus that follows. Top-down influences on cross-modal enhancement may be faster than expected, accelerating reaction speed to input, in which ongoing-spontaneous subthreshold excitation of lower-order unimodal cells by higher-order multimodal cells may play an active role.

GATMD: γ-aminobutyric acid transporter mutagenesis database

Since the cloning of the first γ-aminobutyric acid (GABA) transporter (GAT1; SLC6A1) from rat brain in 1990, more than 50 published studies have provided structure-function information on investigator-designed rat and mouse GAT1 mutants. To date, more than 200 of 599 GAT1 residues have been subjected to mutagenesis experiments by substitution with different amino acids, and the resulting transporter functional properties have significantly advanced our understanding of the mechanism of Na+- and Cl⁻-coupled GABA transport by this important member of the neurotransmitter:sodium symporter family. Moreover, many studies have addressed the functional consequences of amino acid deletion or insertion at various positions along the primary sequence. The enormity of this growing body of structure-function information has prompted us to develop GABA Transporter Mutagenesis Database (GATMD), a web-accessible, relational database of manually annotated biochemical, functional and pharmacological data reported on GAT1-the most intensely studied GABA transporter isoform. As of the last update of GATMD, 52 GAT1 mutagenesis papers have yielded 3360 experimental records, which collectively contain a total of ∼100 000 annotated parameters. Database URL: http://physiology.sci.csupomona.edu/GATMD/

Insufficient augmentation of ambient GABA responsible for age-related cognitive deficit

Age-related degeneration of intracortical inhibition could underlie declines in cognitive function during senescence. Based on a hypothesis that a decrease in basal concentration of ambient (extrasynaptic) GABA with aging leads to depressing intracortical inhibition, we investigated how the basal concentration affects stimulus-evoked activity (as signal), ongoing-spontaneous activity (as noise) of neurons and their (signal-to-noise) ratio S/N. We simulated a neural network model equipped with a GABA transport system that regulates ambient GABA concentration in a neuronal activity-dependent manner. An increase in basal concentration augmented ambient GABA, increased GABA-mediated inhibitory current, and depressed ongoing-spontaneous activity while still keeping stimulus-evoked activity. This led to S/N improvement, for which it was necessary for the reversal potential of GABA transporter to be close to the resting potential of neurons. Above the resting potential, ongoing-spontaneous activity was predominantly enhanced due to excessive GABA-uptake from the extracellular space by transporters. Below the resting potential, stimulus-evoked activity was predominantly depressed, caused by excessive GABA-release. We suggest that the insufficient augmentation of ambient GABA due to a decrease in its basal concentration may be one of the possible causes of cognitive deficit with aging, increasing ongoing-spontaneous neuronal activity as noise. GABA transporter may contribute to improving S/N, provided that its reversal potential is close to the resting potential.

GABA acts as a ligand chaperone in the early secretory pathway to promote cell surface expression of GABAA receptors

GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in brain. The fast inhibitory effect of GABA is mediated through the GABA(A) receptor, a postsynaptic ligand-gated chloride channel. We propose that GABA can act as a ligand chaperone in the early secretory pathway to facilitate GABA(A) receptor cell surface expression. Forty-two hours of GABA treatment increased the surface expression of recombinant receptors expressed in HEK 293 cells, an effect accompanied by an increase in GABA-gated chloride currents. In time-course experiments, a 1h GABA exposure, followed by a 5h incubation in GABA-free medium, was sufficient to increase receptor surface expression. A shorter GABA exposure could be used in HEK 293 cells stably transfected with the GABA transporter GAT-1. In rGAT-1HEK 293 cells, the GABA effect was blocked by the GAT-1 inhibitor NO-711, indicating that GABA was acting intracellularly. The effect of GABA was prevented by brefeldin A (BFA), an inhibitor of early secretory pathway trafficking. Coexpression of GABA(A) receptors with the GABA synthetic enzyme glutamic acid decarboxylase 67 (GAD67) also resulted in an increase in receptor surface levels. GABA treatment failed to promote the surface expression of GABA binding site mutant receptors, which themselves were poorly expressed at the surface. Consistent with an intracellular action of GABA, we show that GABA does not act by stabilizing surface receptors. Furthermore, GABA treatment rescued the surface expression of a receptor construct that was retained within the secretory pathway. Lastly, the lipophilic competitive antagonist (+)bicuculline promoted receptor surface expression, including the rescue of a secretory pathway-retained receptor. Our results indicate that a neurotransmitter can act as a ligand chaperone in the early secretory pathway to regulate the surface expression of its receptor. This effect appears to rely on binding site occupancy, rather than agonist-induced structural changes, since chaperoning is observed with both an agonist and a competitive antagonist.

Postdepolarization potentiation of GABAA receptors: a novel mechanism regulating tonic conductance in hippocampal neurons

Ambient GABA in the brain activates GABA(A) receptors to produce tonic inhibition. Membrane potential influences both GABA transport and GABA(A) receptors and could thereby regulate tonic inhibition. We investigated the voltage dependence of tonic currents in cultured rat hippocampal neurons using patch-clamp techniques. Tonic GABA(A) conductance increased with depolarization from 15 +/- 3 pS/pF at -80 mV to 29 +/- 5 pS/pF at -40 mV. Inhibition of vesicular or nonvesicular GABA release did not prevent voltage-dependent increases of tonic conductance. Currents evoked with exogenous GABA (1 mum) were outwardly rectifying, similar to tonic currents caused by endogenous GABA. These results indicate that the voltage-dependent increase of tonic conductance was attributable to intrinsic GABA(A) receptor properties rather than an elevation of ambient GABA. After transient depolarization to +40 mV, endogenous tonic currents measured at -60 mV were increased by 75 +/- 17%. This novel form of tonic current modulation, termed postdepolarization potentiation (PDP), recovered with a time constant of 63 s, was increased by exogenous GABA and inhibited by GABA(A) receptor antagonists. Measurements of E(GABA) showed PDP was caused by increased conductance and not a change in the anion gradient. To assess the functional significance of PDP, we used voltage-clamp waveforms that replicated epileptiform activity. PDP was produced by this pathophysiological depolarization. These data show that depolarization produces prolonged potentiation of tonic conductance attributable to voltage-dependent properties of GABA(A) receptors. These properties are well suited to limit excitability during pathophysiological depolarization accompanied by rises in ambient GABA, such as occur during seizures and ischemia.

Localization and function of GABA transporters in the globus pallidus of parkinsonian monkeys

The GABA transporters GAT-1 and GAT-3 are abundant in the external and internal segments of the globus pallidus (GPe and GPi, respectively). We have shown that pharmacological blockade of either of these transporters results in decreased neuronal firing, and in elevated levels of extracellular GABA in normal monkeys. We now studied whether the electrophysiologic and biochemical effects of local intra-pallidal injections of GAT-1 and GAT-3 blockers, or the subcellular localization of these transporters, are altered in monkeys rendered parkinsonian by the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The subcellular localization of the transporters in GPe and GPi, studied with electron microscopy immunoperoxidase, was similar to that found in normal animals: i.e., GAT-3 immunoreactivity was mostly confined to glial processes, while GAT-1 labeling was expressed in unmyelinated axons and glial processes. A combined injection/recording device was used to record the extracellular activity of single neurons in GPe and GPi, before, during and after administration of small volumes (1microl) of either the GAT-1 inhibitor, SKF-89976A hydrochloride (720ng), or the GAT-3 inhibitor, (S)-SNAP-5114 (500ng). In GPe, the effects of GAT-1 or GAT-3 blockade were similar to those seen in normal monkeys. However, unlike the findings in the normal state, the firing of most neurons was not affected by blockade of either transporter in GPi. These results suggest that, after dopaminergic depletion, the functions of GABA transporters are altered in GPi; without major changes in their subcellular localization.

Ionic dysregulations typical of ischemia provoke release of glycine and GABA by multiple mechanisms

Energy deprivation during ischemia causes dysregulations of ions, particularly sodium, potassium and calcium. Under these conditions, release of neurotransmitters is often enhanced and can occur by multiple mechanisms. The aim of this work was to characterize the modes of exit of glycine and GABA from nerve endings exposed to stimuli known to reproduce some of the ionic changes typical of ischemic conditions. Their approach was chosen instead of application of ischemic conditions because the release evoked during ischemia is mechanistically too heterogeneous. Mouse hippocampus and spinal cord synaptosomes, pre-labeled with [(3)H]glycine or [(3)H]GABA, were exposed in superfusion to 50 mM KCl or to 10 microM veratridine. The evoked overflows differed greatly between the two transmitters and between the two regions examined. Significant portions of the K(+)- and the veratridine-evoked overflows occurred by classical exocytosis. Carrier-mediated release of GABA, but not of glycine, was evoked by high K(+); GABA and, less so, glycine were released through transporter reversal by veratridine. External calcium-dependent overflows were only in part sensitive to omega-conotoxins; significant portions occurred following reversal of the plasmalemmal Na(+)/Ca(2+) exchanger. Finally, a relevant contribution to the overall transmitter overflows came from cytosolic calcium originating through the mitochondrial Na(+)/Ca(2+) exchanger. To conclude, ionic dysregulations typical of ischemia cause neurotransmitter release by heterogeneous mechanisms that differ depending on the transmitters and the CNS regions examined.

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

Alteration of Ambient GABA by Phasic and Tonic Neuronal Activation

Neurons of primary auditory cortex (AI) emit spikes (action potentials) in two distinct manners, responding to sounds in an onset or a sustained manner. The former AI neurons are called phasic cells and the latter tonic cells. The phasic cells generate spikes for a brief time period (less than hundreds of milliseconds) at the onset of an auditory stimulus (e.g., a tone frequency sound), and the tonic cells continuously generate spikes throughout the stimulation period. Simulating a neural network model of AI, we investigated whether and how the onset discharges influence the sustained discharges that are believed to play a central role in encoding auditory information. Onset discharges, triggered by a phasic input, briefly excited GABAergic interneurons and transiently increased the level of ambient GABA, which was immediately recognized by extrasynaptic GABAa receptors and provided inhibitory currents into neurons. The transient alteration of ambient GABA allowed tonic cells to respond selectively to a tonic input. The timing of phasic input relative to a tonic one had a great impact on the responsiveness of tonic cells. We found optimal timing for the best selective responsiveness: phasic input preceding tonic input by several tens of milliseconds. Offset discharges induced by a secondary input to phasic cells, applied at the end of the tonic input period, suddenly terminated the sustained discharges and allowed the network to return rapidly to the ongoing-spontaneous neuronal state. We suggest that the transporter-mediated alteration of ambient GABA, triggered by onset discharges, may improve the response property of AI neurons. Offset discharges may have a role in resetting AI neurons so that they can prepare for the next auditory input.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

Increase of GABAA receptor-mediated tonic inhibition in dentate granule cells after traumatic brain injury

Traumatic brain injury (TBI) can result in altered inhibitory neurotransmission, hippocampal dysfunction, and cognitive impairments. GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) and tonic (extrasynaptic) whole cell currents were recorded in control rat hippocampal dentate granule cells (DGCs) and at 90days after controlled cortical impact (CCI). At 34 degrees C, in CCI DGCs, sIPSC frequency and amplitude were unchanged, whereas mIPSC frequency was decreased (3.10+/-0.84Hz, n=16, and 2.44+/-0.67Hz, n=7, p<0.05). At 23 degrees C, 300nM diazepam increased peak amplitude of mIPSCs in control and CCI DGCs, but the increase was 20% higher in control (26.81+/-2.2pA and 42.60+/-1.22pA, n=9, p=0.031) compared to CCI DGCs (33.46+/-2.98pA and 46.13+/-1.09pA, n=10, p=0.047). At 34 degrees C, diazepam did not prolong decay time constants (6.59+/-0.12ms and 6.62+/-0.98ms, n=9, p=0.12), the latter suggesting that CCI resulted in benzodiazepine-insensitive pharmacology in synaptic GABA(A) receptors (GABA(A)Rs). In CCI DGCs, peak amplitude of mIPSCs was inhibited by 100microM furosemide (51.30+/-0.80pA at baseline and 43.50+/-5.30pA after furosemide, n=5, p<0.001), a noncompetitive antagonist of GABA(A)Rs with an enhanced affinity to alpha4 subunit-containing receptors. Potentiation of tonic current by the GABA(A)R delta subunit-preferring competitive agonist THIP (1 and 3microM) was increased in CCI DGCs (47% and 198%) compared to control DGCs (13% and 162%), suggesting the presence of larger tonic current in CCI DGCs; THIP (1microM) had no effect on mIPSCs. Taken together, these results demonstrate alterations in synaptic and extrasynaptic GABA(A)Rs in DGCs following CCI.

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the 'what' of brain function, the 'how' of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the 'how' that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic-extrasynaptic receptor trafficking, neuron-glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.

Immunocytochemical evidence that monkey rod bipolar cells use GABA

Certain bipolar cells in most species immunostain for GABA or its synthesizing enzyme glutamic acid decarboxylase. However, it is unknown whether they actually release GABA and, if so, from which cellular compartment and by what release mechanism. We investigated these questions in monkey retina where rod bipolar cells immunostain for GABA. We found that rod bipolar cells immunostain for one isoform of GAD (GAD65) in their somas, dendrites and axon terminals. Near the fovea, the somatic stain of rod bipolar cells is weaker than that of horizontal cells but, at the periphery, it is stronger. Staining for the vesicular GABA transporter in monkey rod bipolar cells is negative. However, staining for the GABA transporter GAT3 is positive in the soma and primary dendrites (but not in the axon terminals). Staining for GAT3 is also positive in horizontal cells. Double staining of rod bipolar cells and the alpha subunit of the GABAA receptor reveals scarce GABAA puncta that appose rod bipolar dendrites. We conclude that monkey rod bipolar cells use GABA and discuss the possibility that they tonically release GABA from their dendrites using a reverse action of GAT3.

Neurotransmitter transporters expressed in glial cells as regulators of synapse function

Synaptic neurotransmission at high temporal and spatial resolutions requires efficient removal and/or inactivation of presynaptically released transmitter to prevent spatial spreading of transmitter by diffusion and allow for fast termination of the postsynaptic response. This action must be carefully regulated to result in the fine tuning of inhibitory and excitatory neurotransmission, necessary for the proper processing of information in the central nervous system. At many synapses, high-affinity neurotransmitter transporters are responsible for transmitter deactivation by removing it from the synaptic cleft. The most prevailing neurotransmitters, glutamate, which mediates excitatory neurotransmission, as well as GABA and glycine, which act as inhibitory neurotransmitters, use these uptake systems. Neurotransmitter transporters have been found in both neuronal and glial cells, thus suggesting high cooperativity between these cell types in the control of extracellular transmitter concentrations. The generation and analysis of animals carrying targeted disruptions of transporter genes together with the use of selective inhibitors have allowed examining the contribution of individual transporter subtypes to synaptic transmission. This revealed the predominant role of glial expressed transporters in maintaining low extrasynaptic neurotransmitter levels. Additionally, transport activity has been shown to be actively regulated on both transcriptional and post-translational levels, which has important implications for synapse function under physiological and pathophysiological conditions. The analysis of these mechanisms will enhance not only our understanding of synapse function but will reveal new therapeutic strategies for the treatment of human neurological diseases.

Administration of neurotoxic doses of MDMA reduces sensitivity to ethanol and increases GAT-1 immunoreactivity in mice striatum

RATIONALE

Mice with reduced dopamine activity following neurotoxic doses of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') consume more ethanol (EtOH) and show greater preference for EtOH. In keeping with human studies and other animal models where alcohol consumption and preference are also high, MDMA treatment will reduce sensitivity to certain physiological effects of EtOH.

OBJECTIVE

We have examined the sensitivity to the acute effects of EtOH in MDMA-lesioned mice and the effects of EtOH on striatal gamma-aminobutyric acid (GABA) accumulation and expression of GABA subtype-1 transporter (GAT-1).

METHODS

C57BL/6J mice were injected with neurotoxic MDMA (30 mg/kg, three times, every 3 h, i.p.). Seven days later, mice were given EtOH (3 g/kg, i.p.) to determine the loss of righting response and the development of rapid tolerance to the hypothermic effect of EtOH. The effect of EtOH on the striatal accumulation of GABA after inhibiting GABA transaminase and on GAT-1 immunoreactivity was also determined.

RESULTS

Mice pre-treated with a neurotoxic dose of MDMA were less sensitive to the sedative-hypnotic effect of acute EtOH and exhibited alterations in the development of rapid tolerance to the hypothermic effect of EtOH. These animals showed an increase in striatal GAT-1 immunoreactivity. EtOH reduced GABA concentration in the striatum of non-lesioned mice, an effect not observed in MDMA-lesioned mice.

CONCLUSION

These findings indicate that mice with a MDMA-induced dopaminergic lesion show increased expression of striatal GAT-1 that may contribute to the lower sensitivity to EtOH-induced sedative effects and the resistance to the development of rapid tolerance to hypothermia produced by EtOH.

Long-lasting intrinsic optical changes observed in the neurointermediate lobe of the mouse pituitary reflect volume changes in cells of the pars intermedia

BACKGROUND/AIMS

Complex intrinsic optical changes (light scattering) are readily observed in the neurointermediate lobe of the mouse pituitary gland following electrical stimulation of the infundibular stalk. Our laboratory has previously identified three distinct phases within the light scattering signal: two rapid responses to action potential stimulation and a long duration recovery. The rapid light scattering signals, restricted to the neurohypophysial portion (posterior pituitary) of the neurointermediate lobe, consist of an E-wave and an S-wave that reflect excitation and secretion, respectively. The E-wave has the approximate shape of the action potential and includes voltage- and current-related components and is independent of Ca(2+) entry. The S-wave is related to Ca(2+) entry and exocytosis. The slow recovery phase of the light scattering signal, which we designated the R-wave, is less well characterized.

METHODS

Using high temporal resolution light scattering measurements, we monitored intrinsic optical changes in the neurointermediate lobe of the mouse pituitary gland. Pharmacological interventions during the measurements were employed.

RESULTS

The data presented here provide optical and pharmacological evidence suggesting that the R-wave, which comprises signals from the posterior pituitary as well as from the pars intermedia, mirrors volume changes in pars intermedia cells following a train of stimuli applied to the infundibular stalk. These volume changes were blocked by the GABA-receptor antagonists bicuculline and picrotoxin, and were mimicked by direct application of GABA in the absence of electrical stimulation.

CONCLUSIONS

These results emphasize the importance of central GABAergic projections into the neurointermediate lobe, and the potential role of GABA in effecting hormone release from the pars intermedia.

Inhibitors of the gamma-aminobutyric acid transporter 1 (GAT1) do not reveal a channel mode of conduction

We expressed the gamma-aminobutyric acid (GABA) transporter GAT1 (SLC6A1) in Xenopus laevis oocytes and performed GABA uptake experiments under voltage clamp at different membrane potentials as well as in the presence of the specific GAT1 inhibitors SKF-89976A and NO-711. In the absence of the inhibitors, GAT1 mediated the inward translocation of 2 net positive charges across the plasma membrane for every GABA molecule transported into the cell. This 2:1 charge flux/GABA flux ratio was the same over a wide range of membrane potentials from -110 mV to +10 mV. Moreover, when GABA-evoked (500 microM) currents were measured at -50 and -90 mV, neither SKF-89976A (5 and 25 microM) nor NO-711 (2 microM) altered the 2:1 charge flux/GABA flux ratio. The results are not consistent with previous hypotheses that (i) GABA evokes an uncoupled channel-mediated current in GAT1, and (ii) GAT1 inhibitors block the putative uncoupled current gated by GABA. Rather, the results suggest tight coupling of GAT1-mediated charge flux and GABA flux.

Brain regional distribution of GABA(A) receptors exhibiting atypical GABA agonism: roles of receptor subunits

The major inhibitory neurotransmitter in the brain, gamma-aminobutyric acid (GABA), has only partial efficacy at certain subtypes of GABA(A) receptors. To characterize these minor receptor populations in rat and mouse brains, we used autoradiographic imaging of t-butylbicyclophosphoro[(35)S]thionate ([(35)S]TBPS) binding to GABA(A) receptors in brain sections and compared the displacing capacities of 10mM GABA and 1mM 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a competitive GABA-site agonist. Brains from GABA(A) receptor alpha1, alpha4, delta, and alpha4+delta subunit knockout (KO) mouse lines were used to understand the contribution of these particular receptor subunits to "GABA-insensitive" (GIS) [(35)S]TBPS binding. THIP displaced more [(35)S]TBPS binding than GABA in several brain regions, indicating that THIP also inhibited GIS-binding. In these regions, GABA prevented the effect of THIP on GIS-binding. GIS-binding was increased in the cerebellar granule cell layer of delta KO and alpha4+delta KO mice, being only slightly diminished in that of alpha1 KO mice. In the thalamus and some other forebrain regions of wild-type mice, a significant amount of GIS-binding was detected. This GIS-binding was higher in alpha4 KO mice. However, it was fully abolished in alpha1 KO mice, indicating that the alpha1 subunit was obligatory for the GIS-binding in the forebrain. Our results suggest that native GABA(A) receptors in brain sections showing reduced displacing capacity of [(35)S]TBPS binding by GABA (partial agonism) minimally require the assembly of alpha1 and beta subunits in the forebrain and of alpha6 and beta subunits in the cerebellar granule cell layer. These receptors may function as extrasynaptic GABA(A) receptors.

Thermal hyperalgesia via supraspinal mechanisms in mice lacking glutamate decarboxylase 65

Gamma-aminobutyric acid, which is synthesized by two isoforms of glutamate decarboxylase (GAD), inhibits the transfer of nociceptive signals from primary afferent fibers to the central nervous system. However, the roles of a 65-kDa isoform of GAD (GAD65)-mediated GABA in nociceptive processing are less clear. This study tested whether partial reductions in GABAergic inhibitory tone by GAD65 gene knockout [GAD65(-/-)] would contribute to the regulation of pain threshold in mice. Experiments were performed on male wild-type (WT) mice and GAD65(-/-) mice. Acute nociception and inflammatory pain tests were compared between WT mice and GAD65(-/-) mice. GABA(A) receptor-mediated inhibitory postsynaptic currents were also examined by use of the whole-cell patch-clamp method in somatosensory cortical neurons in brain slices. In the hot plate test, which reflects supraspinal sensory integration, a significant reduction in the latency was observed for GAD65(-/-) mice. Intraperitoneal administration of the GABA transporter 1 inhibitor, 1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (C(21)H(22)N(2)O(3).HCl; NO-711), dose-dependently prolonged the latency in both genotypes, suggesting that GABA concentration contributes to acute thermal nociception. However, there was no genotype difference in responses to the tail-immersion test or the von Frey test, indicating that spinal reflex and mechanical nociception are kept intact in GAD65(-/-) mice. There was no genotype difference in responses to chemical inflammatory nociception (formalin test and carrageenan test). Although properties of the phasic component of inhibitory postsynaptic currents were similar in both genotypes, tonic inhibition was significantly reduced in GAD65(-/-) mice. These results support the hypothesis that GAD65-mediated GABA synthesis plays relatively small but significant roles in nociceptive processing via supraspinal mechanisms.

Glutamate uptake triggers transporter-mediated GABA release from astrocytes

Background

Glutamate (Glu) and γ-aminobutyric acid (GABA) transporters play important roles in regulating neuronal activity. Glu is removed from the extracellular space dominantly by glial transporters. In contrast, GABA is mainly taken up by neurons. However, the glial GABA transporter subtypes share their localization with the Glu transporters and their expression is confined to the same subpopulation of astrocytes, raising the possibility of cooperation between Glu and GABA transport processes.

Methodology/Principal Findings

Here we used diverse biological models both in vitro and in vivo to explore the interplay between these processes. We found that removal of Glu by astrocytic transporters triggers an elevation in the extracellular level of GABA. This coupling between excitatory and inhibitory signaling was found to be independent of Glu receptor-mediated depolarization, external presence of Ca²⁺ and glutamate decarboxylase activity. It was abolished in the presence of non-transportable blockers of glial Glu or GABA transporters, suggesting that the concerted action of these transporters underlies the process.

Conclusions/Significance

Our results suggest that activation of Glu transporters results in GABA release through reversal of glial GABA transporters. This transporter-mediated interplay represents a direct link between inhibitory and excitatory neurotransmission and may function as a negative feedback combating intense excitation in pathological conditions such as epilepsy or ischemia.

Activation of the tonic GABAC receptor current in retinal bipolar cell terminals by nonvesicular GABA release

Within the second synaptic layer of the retina, bipolar cell (BC) output to ganglion cells is regulated by inhibitory input to BC axon terminals. GABA(A) receptors (GABA(A)Rs) mediate rapid synaptic currents in BC terminals, whereas GABA(C) receptors (GABA(C)Rs) mediate slow evoked currents and a tonic current, which is strongly regulated by GAT-1 GABA transporters. We have used voltage-clamp recordings from BC terminals in goldfish retinal slices to determine the source of GABA for activation of these currents. Inhibition of vesicular release with concanamycin A or tetanus toxin significantly inhibited GABA(A)R inhibitory postsynaptic currents and glutamate-evoked GABA(A)R and GABA(C)R currents but did not reduce the tonic GABA(C)R current, which was also not dependent on extracellular Ca(2+). The tonic current was strongly potentiated by inhibition of GABA transaminase, under both normal and Ca(2+)-free conditions, and was activated by exogenous taurine; however inhibition of taurine transport had little effect. The tonic current was unaffected by GAT-2/3 inhibition and was potentiated by GAT-1 inhibition even in the absence of vesicular release, indicating that it is unlikely to be evoked by reversal of GABA transporters or by ambient GABA. In addition, GABA release does not appear to occur via hemichannels or P2X(7) receptors. BC terminals therefore exhibit two forms of GABA(C)R-mediated inhibition, activated by vesicular and by nonvesicular GABA release, which are likely to have distinct functions in visual signal processing. The tonic GABA(C)R current in BC terminals exhibits similar properties to tonic GABA(A)R and glutamate receptor currents in the brain.

GABA transporter preserving ongoing spontaneous neuronal activity at firing subthreshold

There has been compelling evidence that the GABA transporter is crucial not only for removing gamma-aminobutyric acid (GABA) from but also releasing it into extracellular space, thereby clamping ambient GABA (GABA in extracellular space) at a certain level. The ambient GABA is known to activate extrasynaptic GABA receptors and provide tonic inhibitory current into neurons. We investigated how the transporter regulates the level of ambient GABA, mediates tonic neuronal inhibition, and influences ongoing spontaneous neuronal activity. A cortical neural network model is proposed in which GABA transporters on lateral (L) and feedback (F) inhibitory (GABAergic) interneurons are functionally made. Principal (P) cell assemblies participate in expressing information about elemental sensory features. At membrane potentials below the reversal potential, there is net influx of GABA, whereas at membrane potentials above the reversal potential, there is net efflux of GABA. Through this transport mechanism, ambient GABA concentration is kept within a submicromolar range during an ongoing spontaneous neuronal activity time period. Here we show that the GABA transporter on L cells regulates the overall level of ambient GABA across cell assemblies, and that on F cells it does so within individual cell assemblies. This combinatorial regulation of ambient GABA allows P cells to oscillate near firing threshold during the ongoing time period, thereby reducing their reaction time to externally applied stimuli. We suggest that the GABA transporter, with its forward and reverse transport mechanism, could regulate the ambient GABA. This transporter-mediated ambient GABA regulation may contribute to establishing an ongoing subthreshold neuronal state by which the network can respond rapidly to subsequent sensory input.

Gabapentin-induced delirium and dependence

Gabapentin (Neurontin) is approved by the US Food and Drug Administration for treatment of epilepsy and post-herpetic neuralgia. Despite lack of strong evidence, gabapentin is also often prescribed off-label for psychiatric conditions. The case described here involved a 38-year-old male physician with substance intoxication delirium and psychoactive substance dependence due to high self-administered doses of gabapentin, which had been prescribed at lower doses in combination with buspirone and bupropion for depression and anxiety. This unusual case of gabapentin dependence and abuse involved toxic delirium, intense cravings, and a prolonged post-withdrawal confusional state reminiscent of benzodiazepine withdrawal. Gabapentin is a central nervous system inhibitory agent with likely gamma-aminobutyric acid (GABA)-ergic and non-GABAergic mechanisms of action. The similarity between benzodiazepine withdrawal and what this patient experienced with gabapentin suggests a common role for GABA-related effects. The case reported here suggests the need for heightened concern regarding the off-label prescription of this drug to vulnerable individuals with psychiatric conditions.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

Plasmalemmal and vesicular gamma-aminobutyric acid transporter expression in the developing mouse retina

Plasmalemmal and vesicular gamma-aminobutyric acid (GABA) transporters influence neurotransmission by regulating high-affinity GABA uptake and GABA release into the synaptic cleft and extracellular space. Postnatal expression of the plasmalemmal GABA transporter-1 (GAT-1), GAT-3, and the vesicular GABA/glycine transporter (VGAT) were evaluated in the developing mouse retina by using immunohistochemistry with affinity-purified antibodies. Weak transporter immunoreactivity was observed in the inner retina at postnatal day 0 (P0). GAT-1 immunostaining at P0 and at older ages was in amacrine and displaced amacrine cells in the inner nuclear layer (INL) and ganglion cell layer (GCL), respectively, and in their processes in the inner plexiform layer (IPL). At P10, weak GAT-1 immunostaining was in Müller cell processes. GAT-3 immunostaining at P0 and older ages was in amacrine cells and their processes, as well as in Müller cells and their processes that extended radially across the retina. At P10, Müller cell somata were observed in the middle of the INL. VGAT immunostaining was present at P0 and older ages in amacrine cells in the INL as well as processes in the IPL. At P5, weak VGAT immunostaining was also observed in horizontal cell somata and processes. By P15, the GAT and VGAT immunostaining patterns appear similar to the adult immunostaining patterns; they reached adult levels by about P20. These findings demonstrate that GABA uptake and release are initially established in the inner retina during the first postnatal week and that these systems subsequently mature in the outer retina during the second postnatal week.

Hippocampal extracellular GABA correlates with metabolism in human epilepsy

As the major inhibitory neurotransmitter in human brain, GABA is an important modulator of hyperexcitability in epilepsy patients. Given the high energetic cost of neurotransmission and synaptic activity, GABA concentrations may be hypothesized to correlate with metabolic function. We studied human epilepsy patients undergoing intracranial EEG monitoring for seizure localization to examine microdialysis measures of extracellular GABA (ecGABA), pre-operative MR spectroscopic measures of neuronal mitochondrial function (NAA/Cr), and wherever possible, neuropathology and hippocampal volumetry. Two groups undergoing intracranial monitoring for seizure localization were studied: surgically treated hippocampal epilepsy (MTLE) and neocortical (non-hippocampal seizure onset) epilepsy. All data are hippocampal and thus these groups allow comparisons between the epileptogenic and non-epileptogenic regions. ecGABA was measured using in vivo microdialysis performed during intracranial monitoring. Pre-operative in vivo MR spectroscopic imaging was performed to measure the ratio of N-acetyl aspartate (NAA) to creatine. Standard methods for neuropathology and hippocampal volumetry were used. In the neocortical group, increased ecGABA correlated with greater NAA/Cr (R = +0.70, p < 0.015, n = 12) while in the MTLE group, increased ecGABA linked with decreased NAA/Cr (R = -0.94, p < 0.001, n = 8). In MTLE, ecGABA (increased) and NAA/Cr (decreased) correlated with increased glial cell numbers (R = +0.71, p < 0.01, n = 12, R = -0.76 p < 0.03 respectively). No relationship was seen between ecGABA and hippocampal volumes in either group. In epilepsy, ecGABA increases occur across a range of metabolic function. Outside the seizure focus, ecGABA and NAA/Cr increase together; in contrast, within the seizure focus, ecGABA increases with declining mitochondrial function.

GABA transporter 1 tunes GABAergic synaptic transmission at output neurons of the mouse neostriatum

GABAergic medium-sized striatal output neurons (SONs) provide the principal output for the neostriatum. In vitro and in vivo data indicate that spike discharge of SONs is tightly controlled by effective synaptic inhibition. Although phasic GABAergic transmission critically depends on ambient GABA levels, the role of GABA transporters (GATs) in neostriatal GABAergic synaptic transmission is largely unknown. In the present study we aimed at elucidating the role of GAT-1 in the developing mouse neostriatum (postnatal day (P) 7-34). We recorded GABAergic postsynaptic currents (PSCs) using the whole-cell patch-clamp technique. Based on the effects of NO-711, a specific GAT-1 blocker, we demonstrate that GAT-1 is operative at this age and influences GABAergic synaptic transmission by presynaptic and postsynaptic mechanisms. Presynaptic GABA(B)R-mediated suppression of GABA release was found to be functional at all ages tested; however, there was no evidence for persistent GABA(B)R activity under control conditions, unless GAT-1 was blocked (P12-34). In addition, whereas no tonic GABA(A)R-mediated conductances were detected in SONs until P14, application of a specific GABA(A)R antagonist caused distinct tonic outward currents later in development (P19-34). In the presence of NO-711, tonic GABA(A)R-mediated currents were also observed at P7-14 and were dramatically increased at more mature stages. Furthermore, GAT-1 block reduced the median amplitude of GABAergic miniature PSCs indicating a decrease in quantal size. We conclude that in the murine neostriatum GAT-1 operates in a net uptake mode. It prevents the persistent activation of presynaptic GABA(B)Rs (P12-34) and prevents (P7-14) or reduces (P19-34) tonic postsynaptic GABA(A)R activity.

GABA, a forgotten gliotransmitter

The amino acid gamma-aminobutiric acid (GABA) is a major inhibitory transmitter in the vertebrate central nervous system (CNS) where it can be released by neurons and by glial cells. Neuronal GABAergic signaling is well characterized: the mechanisms of GABA release, the receptors it targets and the functional consequences of their activation have been extensively studied. In contrast, the corresponding features of glial GABAergic signaling have attracted less attention. In this review, we first discuss evidence from the literature for GABA accumulation, production and release by glial cells. We then review the results of recent experiments that point toward functional roles of GABA as a "gliotransmitter".

Imaging synaptic inhibition throughout the brain via genetically targeted Clomeleon

Here we survey a molecular genetic approach for imaging synaptic inhibition. This approach is based on measuring intracellular chloride concentration ([Cl(-)](i)) with the fluorescent chloride indicator protein, Clomeleon. We first describe several different ways to express Clomeleon in selected populations of neurons in the mouse brain. These methods include targeted viral gene transfer, conditional expression controlled by Cre recombination, and transgenesis based on the neuron-specific promoter, thy1. Next, we evaluate the feasibility of using different lines of thy1::Clomeleon transgenic mice to image synaptic inhibition in several different brain regions: the hippocampus, the deep cerebellar nuclei (DCN), the basolateral nucleus of the amygdala, and the superior colliculus (SC). Activation of hippocampal interneurons caused [Cl(-)](i) to rise transiently in individual postsynaptic CA1 pyramidal neurons. [Cl(-)](i) increased linearly with the number of electrical stimuli in a train, with peak changes as large as 4 mM. These responses were largely mediated by GABA receptors because they were blocked by antagonists of GABA receptors, such as GABAzine and bicuculline. Similar responses to synaptic activity were observed in DCN neurons, amygdalar principal cells, and collicular premotor neurons. However, in contrast to the hippocampus, the responses in these three regions were largely insensitive to antagonists of inhibitory neurotransmitter receptors. This indicates that synaptic activity can also cause Cl(-) influx through alternate pathways that remain to be identified. We conclude that Clomeleon imaging permits non-invasive, spatiotemporally precise recordings of [Cl(-)](i) in a large variety of neurons, and provides new opportunities for imaging synaptic inhibition and other forms of neuronal chloride signaling.

Hypoalgesia in mice lacking GABA transporter subtype 1

gamma-Aminobutyric acid (GABA) transporters play a key role in the regulation of GABA neurotransmission. We reported previously that overexpression of the GABA transporter subtype 1 (GAT1), the major form of the GABA transporter in the CNS, led to hyperalgesia in mice. In the present study, nociceptive responses of GAT1-knockout mice (GAT1(-/-)) were compared with those of heterozygous (GAT(+/-)) and wild-type (GAT(+/+)) mice by four conventional pain models (tail-immersion test, hot-plate test, acetic acid-induced abdominal constriction test, and formalin test). In addition, the analgesic effects of two GAT1-selective inhibitors, NO-711 and tiagabine, were examined in all three genotypes using the same four models. Our data demonstrated that GAT1 deficiency because of genetic knockout or acute blockade by selective inhibitors leads to hypoalgesia in mice. These results confirmed the crucial role of GAT1 in the regulation of nociceptive threshold and suggested that GAT1 inhibitors have the potential for clinical use in pain therapy.

Physiology and pharmacology of alcohol: the imidazobenzodiazepine alcohol antagonist site on subtypes of GABAA receptors as an opportunity for drug development?

Alcohol (ethanol, EtOH) has pleiotropic actions and induces a number of acute and long-term effects due to direct actions on alcohol targets, and effects of alcohol metabolites and metabolism. Many detrimental health consequences are due to EtOH metabolism and metabolites, in particular acetaldehyde, whose high reactivity leads to nonspecific chemical modifications of proteins and nucleic acids. Like acetaldehyde, alcohol has been widely considered a nonspecific drug, despite rather persuasive evidence implicating inhibitory GABA(A) receptors (GABA(A)Rs) in acute alcohol actions, for example, a GABA(A)R ligand, the imidazobenzodiazepine Ro15-4513 antagonizes many low-to-moderate dose alcohol actions in mammals. It was therefore rather surprising that abundant types of synaptic GABA(A)Rs are generally not responsive to relevant low concentrations of EtOH. In contrast, delta-subunit-containing GABA(A)Rs and extrasynaptic tonic GABA currents mediated by these receptors are sensitive to alcohol concentrations that are reached in blood and tissues during low-to-moderate alcohol consumption. We recently showed that low-dose alcohol enhancement on highly alcohol-sensitive GABA(A)R subtypes is antagonized by Ro15-4513 in an apparently competitive manner, providing a molecular explanation for behavioural Ro15-4513 alcohol antagonism. The identification of a Ro15-4513/EtOH binding site on unique GABA(A)R subtypes opens the possibility to characterize this alcohol site(s) and screen for compounds that modulate the function of EtOH/Ro15-4513-sensitive GABA(A)Rs. The utility of such drugs might range from novel alcohol antagonists that might be useful in the emergency room, to drugs for the treatment of alcoholism, as well as alcohol-mimetic drugs to harness acute positive effects of alcohol.

Regulation of excitability by extrasynaptic GABA(A) receptors

Not only are GABA(A) receptors activated transiently by GABA released at synapses, but high affinity, extrasynaptic GABA(A) receptors are also activated by ambient, extracellular GABA as a more persistent form of signalling (often termed tonic inhibition). Over the last decade tonic GABA(A) receptor-mediated inhibition and the properties of GABA(A) receptors mediating this signalling have received increasing attention. Tonic inhibition is present throughout the central nervous system, but is expressed in a cell-type specific manner (e.g. in interneurons more so than in pyramidal cells in the hippocampus, and in thalamocortical neurons more so than in reticular thalamic neurons in the thalamus). As a consequence, tonic inhibition can have a complex effect on network activity. Tonic inhibition is not fixed but can be modulated by endogenous and exogenous modulators, such as neurosteroids, and by developmental, physiological and pathological regulation of GABA uptake and GABA(A) receptor expression. There is also growing evidence that tonic currents play an important role in epilepsy, sleep (also actions of anaesthetics and sedatives), memory and cognition. Therefore, drugs specifically aimed at targeting the extrasynaptic receptors involved in tonic inhibition could be a novel approach to regulating both physiological and pathological processes.