Release of gliotransmitters through astroglial connexin 43 hemichannels is necessary for fear memory consolidation in the basolateral amygdala

Recent in vitro evidence indicates that astrocytes can modulate synaptic plasticity by releasing neuroactive substances (gliotransmitters). However, whether gliotransmitter release from astrocytes is necessary for higher brain function in vivo, particularly for memory, as well as the contribution of connexin (Cx) hemichannels to gliotransmitter release, remain elusive. Here, we microinfused into the rat basolateral amygdala (BLA) TAT-Cx43L2, a peptide that selectively inhibits Cx43-hemichannel opening while maintaining synaptic transmission or interastrocyte gap junctional communication. In vivo blockade of Cx43 hemichannels during memory consolidation induced amnesia for auditory fear conditioning, as assessed 24 h after training, without affecting short-term memory, locomotion, or shock reactivity. The amnesic effect was transitory, specific for memory consolidation, and was confirmed after microinfusion of Gap27, another Cx43-hemichannel blocker. Learning capacity was recovered after coinfusion of TAT-Cx43L2 and a mixture of putative gliotransmitters (glutamate, glutamine, lactate, d-serine, glycine, and ATP). We propose that gliotransmitter release from astrocytes through Cx43 hemichannels is necessary for fear memory consolidation at the BLA. Thus, the present study is the first to demonstrate a physiological role for astroglial Cx43 hemichannels in brain function, making these channels a novel pharmacological target for the treatment of psychiatric disorders, including post-traumatic stress disorder.

Are extrasynaptic GABAA receptors important targets for sedative/hypnotic drugs?

High-affinity extrasynaptic GABA(A) receptors are persistently activated by the low ambient GABA levels that are known to be present in extracellular space. The resulting tonic conductance generates a form of shunting inhibition that is capable of altering cellular and network behavior. It has been suggested that this tonic inhibition will be enhanced by neurosteroids, antiepileptics, and sedative/hypnotic drugs. However, we show that the ability of sedative/hypnotic drugs to enhance tonic inhibition in the mouse cerebellum will critically depend on ambient GABA levels. For example, we show that the intravenous anesthetic propofol enhances tonic inhibition only when ambient GABA levels are <100 nm. More surprisingly, the actions of the sleep-promoting drug 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridin-3-ol (THIP) are attenuated at ambient GABA levels of just 20 nm. In contrast, our data suggest that neurosteroid enhancement of tonic inhibition will be greater at high ambient GABA concentrations. We present a model that takes into account realistic estimates of ambient GABA levels and predicted extrasynaptic GABA(A) receptor numbers when considering the ability of sedative/hypnotic drugs to enhance tonic inhibition. These issues will be important when considering drug strategies designed to target extrasynaptic GABA(A) receptors in the treatment of sleep disorders and other neurological conditions.

Exocytosis in astrocytes: transmitter release and membrane signal regulation

Astrocytes, a type of glial cells in the brain, are eukaryotic cells, and a hallmark of these are subcellular organelles, such as secretory vesicles. In neurons vesicles play a key role in signaling. Upon a stimulus-an increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](i))-the membrane of vesicle fuses with the presynaptic plasma membrane, allowing the exit of neurotransmitters into the extracellular space and their diffusion to the postsynaptic receptors. For decades it was thought that such vesicle-based mechanisms of gliotransmitter release were not present in astrocytes. However, in the last 30 years experimental evidence showed that astrocytes are endowed with mechanisms for vesicle- and non-vesicle-based gliotransmitter release mechanisms. The aim of this review is to focus on exocytosis, which may play a role in gliotransmission and also in other forms of cell-to-cell communication, such as the delivery of transporters, ion channels and antigen presenting molecules to the cell surface.

Tonic GABA(A) receptor-mediated signalling in temporal lobe epilepsy

The tonic activation of extrasynaptic GABAA receptors by extracellular GABA provides a powerful means of regulating neuronal excitability. A consistent finding from studies that have used various models of temporal lobe epilepsy is that tonic GABAA receptor-mediated conductances are largely preserved in epileptic brain (in contrast to synaptic inhibition which is often reduced). Tonic inhibition is therefore an attractive target for antiepileptic drugs. However, the network consequences of a commonly used approach to augment tonic GABAA receptor-mediated conductances by global manipulation of extracellular GABA are difficult to predict without understanding how epileptogenesis alters the pharmacology and GABA sensitivity of tonic inhibition, and how manipulation of tonic conductances modulates the output of individual neurons. Here we review the current literature on epilepsy-associated changes in tonic GABAA receptor-mediated signalling, and speculate about possible effects they have at the network level. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Neurotransmitters and integration in neuronal-astroglial networks

Two major neural cell types, glia, astrocytes in particular, and neurones can release chemical transmitters that act as soluble signalling compounds for intercellular communication. Exocytosis, a process which depends on an increase in cytosolic Ca(2+) levels, represents a common denominator for release of neurotransmitters, stored in secretory vesicles, from these neural cells. While neurones rely predominately on the immediate entry of Ca(2+) from the extracellular space to the cytosol in this process, astrocytes support their cytosolic Ca(2+) increases by appropriating this ion from the intracellular endoplasmic reticulum store and extracellular space. Additionally, astrocytes can release neurotransmitters using a variety of non-vesicular pathways which are mediated by an assortment of plasmalemmal channels and transporters. Once a neuronal and/or astrocytic neurotransmitter is released into the extracellular space, it can activate plasma membrane neurotransmitter receptors on neural cells, causing autocrine and/or paracrine signalling. Moreover, chemical transmission is essential not only for homocellular, but also for heterocellular bi-directional communication in the brain. Further detailed understanding of chemical transmission will aid our comprehension of the brain (dys)function in heath and disease.

Modulation of glutamate release from parallel fibers by mGlu4 and pre-synaptic GABA(A) receptors

The regulation of pre-synaptic glutamate release is important in the maintenance and fidelity of excitatory transmission in the nervous system. In this study, we report a novel interaction between a ligand-gated ion channel and a G-protein coupled receptor which regulates glutamate release from parallel fiber axon terminals. Immunocytochemical analysis revealed that GABA(A) receptors and the high affinity group III metabotropic glutamate receptor subtype 4 (mGlu4) are co-localized on glutamatergic parallel fiber axon terminals in the cerebellum. GABA(A) and mGlu4 receptors were also found to co-immunoprecipitate from cerebellar membranes. Independently, these two receptors have opposing roles on glutamate release: pre-synaptic GABA(A) receptors promote, while mGlu4 receptors inhibit, glutamate release. However, coincident activation of GABA(A) receptors with muscimol and mGlu4 with the agonist (2S)-S-2-amino-4-phosphonobutanoic acid , increased glutamate release from [(3) H]glutamate-loaded cerebellar synaptosomes above that observed with muscimol alone. Further support for an interaction between GABA(A) and mGlu4 receptors was obtained in the mGlu4 knockout mouse which displayed reduced binding of the GABA(A) ligand [(35) S]tert-butylbicyclophosphorothionate, and decreased expression of the α1, α6, β2 GABA(A) receptor subunits in the cerebellum. Taken together, our data suggest a new role for mGlu4 whereby simultaneous activation with GABA(A) receptors acts to amplify glutamate release at parallel fiber-Purkinje cell synapses.

Raising cytosolic Cl- in cerebellar granule cells affects their excitability and vestibulo-ocular learning

Cerebellar cortical throughput involved in motor control comprises granule cells (GCs) and Purkinje cells (PCs), both of which receive inhibitory GABAergic input from interneurons. The GABAergic input to PCs is essential for learning and consolidation of the vestibulo-ocular reflex, but the role of GC excitability remains unclear. We now disrupted the Kcc2 K-Cl cotransporter specifically in either cell type to manipulate their excitability and inhibition by GABA(A)-receptor Cl(-) channels. Although Kcc2 may have a morphogenic role in synapse development, Kcc2 disruption neither changed synapse density nor spine morphology. In both GCs and PCs, disruption of Kcc2, but not Kcc3, increased [Cl(-)](i) roughly two-fold. The reduced Cl(-) gradient nearly abolished GABA-induced hyperpolarization in PCs, but in GCs it merely affected excitability by membrane depolarization. Ablation of Kcc2 from GCs impaired consolidation of long-term phase learning of the vestibulo-ocular reflex, whereas baseline performance, short-term gain-decrease learning and gain consolidation remained intact. These functions, however, were affected by disruption of Kcc2 in PCs. GC excitability plays a previously unknown, but specific role in consolidation of phase learning.

Extrasynaptic GABA(A) receptors: their function in the CNS and implications for disease

Over the past two decades, research has identified extrasynaptic GABA(A) receptor populations that enable neurons to sense the low ambient GABA concentrations present in the extracellular space in order to generate a form of tonic inhibition not previously considered in studies of neuronal excitability. The importance of this tonic inhibition in regulating states of consciousness is highlighted by the fact that extrasynaptic GABA(A) receptors (GABA(A)Rs) are believed to be key targets for anesthetics, sleep-promoting drugs, neurosteroids, and alcohol. The neurosteroid sensitivity of these extrasynaptic GABA(A)Rs may explain their importance in stress-, ovarian cycle-, and pregnancy-related mood disorders. Moreover, disruptions in network dynamics associated with schizophrenia, epilepsy, and Parkinson's disease may well involve alterations in the tonic GABA(A)R-mediated conductance. Extrasynaptic GABA(A)Rs may therefore present a therapeutic target for treatment of these diseases, with the potential to enhance cognition and aid poststroke functional recovery.

GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling

Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABA_ARs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl⁻) depending on its electro-chemical gradient (E_Cl). Since the flux of Cl⁻ is indissolubly associated to that of osmotically obliged water, GABA_ARs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABA_AR signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABA_AR-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.

GABA Potency at GABA(A) Receptors Found in Synaptic and Extrasynaptic Zones

The potency of GABA is vitally important for its primary role in activating GABA_A receptors and acting as an inhibitory neurotransmitter. Although numerous laboratories have presented information, directly or indirectly, on GABA potency, it is often difficult to compare across such studies given the inevitable variations in the methods used, the cell types studied, whether native or recombinant receptors are examined, and their relevance to native synaptic and extrasynaptic GABA_A receptors. In this review, we list the most relevant isoforms of synaptic and extrasynaptic GABA_A receptors that are thought to assemble in surface membranes of neurons in the central nervous system. Using consistent methodology in one cell type, the potencies of the endogenous neurotransmitter GABA are compared across a spectrum of GABA_A receptors. The highest potency for GABA is measured when activating extrasynaptic-type α6 subunit-containing receptors, whereas synaptic-type α2β3γ2 and α3β3γ2 receptors exhibited the lowest potency, and other GABA_A receptor subtypes that are found both in synaptic and extrasynaptic compartments, showed intermediate sensitivities to GABA. The relatively simple potency relationship between GABA and its target receptors is important as it serves as one of the major determinants of GABA_A receptor activation, with consequences for the development of inhibition, either by tonic or phasic mechanisms.

Bestrophin1 Channels are Insensitive to Ethanol and Do not Mediate Tonic GABAergic Currents in Cerebellar Granule Cells

The granule cell layer of the cerebellum functions in spatio-temporal encoding of information. Granule cells (GCs) are tonically inhibited by spillover of GABA released from Golgi cells and this tonic inhibition is facilitated by acute ethanol. Recently, it was demonstrated that a specialized Ca(2+)-activated anion-channel, bestrophin1 (Best1), found on glial cells, can release GABA that contributes up to 50-75% of the tonic GABAergic current. However, it is unknown if ethanol has any actions on Best1 function. Using whole-cell electrophysiology, we found that recombinant Best1 channels expressed in HEK-293 cells were insensitive to 40 and 80 mM ethanol. We attempted to measure the Best1-mediated component of the tonic current in slices using 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We confirmed that this agent blocks recombinant Best1 channels. Unexpectedly, we found that NPPB significantly potentiated the tonic current and the area and decay of GABA(A)-mediated spontaneous inhibitory post-synaptic currents (IPSCs) in GCs in rodent slices under two different recording conditions. To better isolate the Best1-dependent tonic current component, we blocked the Golgi cell component of the tonic current with tetrodotoxin and found that NPPB similarly and significantly potentiated the tonic current amplitude and decay time of miniature IPSCs. Two other Cl(-)-channel blockers were also tested: 4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate (DIDS) showed no effect on GABAergic transmission, while niflumic acid (NFA) significantly suppressed the tonic current noise, as well as the mIPSC frequency, amplitude, and area. These data suggest that acute ethanol exposure does not modulate Best1 channels and these findings serve to challenge recent data indicating that these channels participate in the generation of tonic GABAergic currents in cerebellar GCs.

Optogenetic reporters: Fluorescent protein-based genetically encoded indicators of signaling and metabolism in the brain

Fluorescent protein technology has evolved to include genetically encoded biosensors that can monitor levels of ions, metabolites, and enzyme activities as well as protein conformation and even membrane voltage. They are well suited to live-cell microscopy and quantitative analysis, and they can be used in multiple imaging modes, including one- or two-photon fluorescence intensity or lifetime microscopy. Although not nearly complete, there now exists a substantial set of genetically encoded reporters that can be used to monitor many aspects of neuronal and glial biology, and these biosensors can be used to visualize synaptic transmission and activity-dependent signaling in vitro and in vivo. In this review, we present an overview of design strategies for engineering biosensors, including sensor designs using circularly permuted fluorescent proteins and using fluorescence resonance energy transfer between fluorescent proteins. We also provide examples of indicators that sense small ions (e.g., pH, chloride, zinc), metabolites (e.g., glutamate, glucose, ATP, cAMP, lipid metabolites), signaling pathways (e.g., G protein-coupled receptors, Rho GTPases), enzyme activities (e.g., protein kinase A, caspases), and reactive species. We focus on examples where these genetically encoded indicators have been applied to brain-related studies and used with live-cell fluorescence microscopy.

Heterogeneity of astrocytic form and function

Astrocytes participate in all essential CNS functions, including blood flow regulation, energy metabolism, ion and water homeostasis, immune defence, neurotransmission, and adult neurogenesis. It is thus not surprising that astrocytic morphology and function differ between regions, and that different subclasses of astrocytes exist within the same brain region. Recent lines of work also show that the complexity of protoplasmic astrocytes increases during evolution. Human astrocytes are structurally more complex, larger, and propagate calcium signals significantly faster than rodent astrocytes. In this chapter, we review the diversity of astrocytic form and function, while considering the markedly expanded roles of astrocytes with phylogenetic evolution. We also define major challenges for the future, which include determining how astrocytic functions are locally specified, defining the molecular controls upon astrocytic fate and physiology and establishing how evolutionary changes in astrocytes contribute to higher cognitive functions.

Neurosteroid influences on sensitivity to ethanol

This review will highlight a variety of mechanisms by which neurosteroids affect sensitivity to ethanol, including physiological states associated with activity of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, and the effects of chronic exposure to ethanol, in addition to behavioral implications. To date, γ-aminobutyric acid (GABA(A)) receptor mechanisms are a major focus of the modulation of ethanol effects by neuroactive steroids. While NMDA receptor mechanisms are gaining prominence in the literature, these complex data would be best discussed separately. Accordingly, GABA(A) receptor mechanisms are emphasized in this review with brief mention of some NMDA receptor mechanisms to point out contrasting neuroactive steroid pharmacology. Overall, the data suggest that neurosteroids are virtually ubiquitous modulators of inhibitory neurotransmission. Neurosteroids appear to affect sensitivity to ethanol in specific brain regions and, consequently, specific behavioral tests, possibly related to the efficacy and potency of ethanol to potentiate the release of GABA and increase neurosteroid concentrations. Although direct interaction of ethanol and neuroactive steroids at common receptor binding sites has been suggested in some studies, this proposition is still controversial. It is currently difficult to assign a specific mechanism by which neuroactive steroids could modulate the effects of ethanol in particular behavioral tasks.

TRPA1 channels regulate astrocyte resting calcium and inhibitory synapse efficacy through GAT-3

Astrocytes contribute to the formation and function of synapses and are found throughout the brain, where they show intracellular store-mediated Ca(2+) signals. Here, using a membrane-tethered, genetically encoded calcium indicator (Lck-GCaMP3), we report the serendipitous discovery of a new type of Ca(2+) signal in rat hippocampal astrocyte-neuron cocultures. We found that Ca(2+) fluxes mediated by transient receptor potential A1 (TRPA1) channels gave rise to frequent and highly localized 'spotty' Ca(2+) microdomains near the membrane that contributed appreciably to resting Ca(2+) in astrocytes. Mechanistic evaluations in brain slices showed that decreases in astrocyte resting Ca(2+) concentrations mediated by TRPA1 channels decreased interneuron inhibitory synapse efficacy by reducing GABA transport by GAT-3, thus elevating extracellular GABA. Our data show how a transmembrane Ca(2+) source (TRPA1) targets a transporter (GAT-3) in astrocytes to regulate inhibitory synapses.

Imaging synaptic inhibition with the genetically encoded chloride indicator Clomeleon

Several techniques are available to image excitatory processes in the brain, but synaptic inhibition has remained largely invisible. Most synaptic inhibition in the brain arises from transmembrane fluxes of chloride ions (Cl(-)), so imaging intracellular Cl(-) concentration ([Cl(-)](i)) is, in principle, a natural way to visualize the spatiotemporal dynamics of inhibition. This protocol describes the use of Clomeleon, a genetically encoded indicator of Cl(-), as a tool for monitoring synaptic inhibition. It outlines procedures that can be used to image neuronal [Cl(-)](i) in brain slices prepared from Clomeleon transgenic mice. With only minor adjustments, the same procedures should be suitable for imaging from cultured cells as well.

The amount of astrocytic GABA positively correlates with the degree of tonic inhibition in hippocampal CA1 and cerebellum

A tonic form of synaptic inhibition occurs in discrete regions of the central nervous system and has an important role in controlling neuronal excitability. Recently, we reported that GABA present in astrocyte is the major source of tonic inhibition in cerebellum and that GABA is released through Bestrophin-1 channel by direct permeation. In this study, we screened for the presence of astrocytic GABA in various brain regions such as hippocampus, thalamus, hypothalamus and cerebellum using immunohistochemistry. We found that astrocytic GABA was present in the regions that were reported to show tonic inhibition. Because the existence of tonic inhibition in hippocampal CA1 is somewhat controversial, we compared the amount of astrocytic GABA and tonic inhibition between the hippocampal CA1 pyramidal cell layer and the cerebellar granule cell layer. Unlike cerebellar glial cells, hippocampal astrocytes did not contain GABA. The tonic inhibition was also much lower in the pyramidal neurons of hippocampal CA1 compared to the granule cells of cerebellum. Nevertheless, most of the hippocampal astrocytes expressed Bestrophin-1 channel. These data indicate that the absence of astrocytic GABA results in a low level of tonic inhibition in hippocampal CA1 region.

Excitation of rat cerebellar Golgi cells by ethanol: further characterization of the mechanism

BACKGROUND

Studies with rodents suggest that acute ethanol exposure impairs information flow through the cerebellar cortex, in part, by increasing GABAergic input to granule cells. Experiments suggest that an increase in the excitability of specialized GABAergic interneurons that regulate granule cell activity (i.e., Golgi cells [GoCs]) contributes to this effect. In GoCs, ethanol increases spontaneous action potential firing frequency, decreases the afterhyperpolarization amplitude, and depolarizes the membrane potential. Studies suggest that these effects could be mediated by inhibition of the Na(+)/K(+) ATPase. The purpose of this study was to characterize the potential role of other GoC conductances in the mechanism of action of ethanol.

METHODS

Computer modeling techniques and patch-clamp electrophysiological recordings with acute slices from rat cerebella were used for these studies.

RESULTS

Computer modeling suggested that modulation of subthreshold Na(+) channels, hyperpolarization-activated currents, and several K(+) conductances could explain some but not all actions of ethanol on GoCs. Electrophysiological studies did not find evidence consistent with a contribution of these conductances. Quinidine, a nonselective blocker of several types of channels (including several K(+) channels) that also antagonizes the Na(+)/K(+) ATPase, reduced the effect of ethanol on GoC firing.

CONCLUSIONS

These findings further support that ethanol increases GoC excitability via modulation of the Na(+)/K(+) ATPase and suggest that a quinidine-sensitive K(+) channel may also play a role in the mechanism of action of ethanol.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Pharmacological analysis of the activation and receptor properties of the tonic GABA(C)R current in retinal bipolar cell terminals

GABAergic inhibition in the central nervous system (CNS) can occur via rapid, transient postsynaptic currents and via a tonic increase in membrane conductance, mediated by synaptic and extrasynaptic GABA_A receptors (GABA_ARs) respectively. Retinal bipolar cells (BCs) exhibit a tonic current mediated by GABA_CRs in their axon terminal, in addition to synaptic GABA_AR and GABA_CR currents, which strongly regulate BC output. The tonic GABA_CR current in BC terminals (BCTs) is not dependent on vesicular GABA release, but properties such as the alternative source of GABA and the identity of the GABA_CRs remain unknown. Following a recent report that tonic GABA release from cerebellar glial cells is mediated by Bestrophin 1 anion channels, we have investigated their role in non-vesicular GABA release in the retina. Using patch-clamp recordings from BCTs in goldfish retinal slices, we find that the tonic GABA_CR current is not reduced by the anion channel inhibitors NPPB or flufenamic acid but is reduced by DIDS, which decreases the tonic current without directly affecting GABA_CRs. All three drugs also exhibit non-specific effects including inhibition of GABA transporters. GABA_CR ρ subunits can form homomeric and heteromeric receptors that differ in their properties, but BC GABA_CRs are thought to be ρ1-ρ2 heteromers. To investigate whether GABA_CRs mediating tonic and synaptic currents may differ in their subunit composition, as is the case for GABA_ARs, we have examined the effects of two antagonists that show partial ρ subunit selectivity: picrotoxin and cyclothiazide. Tonic and synaptic GABA_CR currents were differentially affected by both drugs, suggesting that a population of homomeric ρ1 receptors contributes to the tonic current. These results extend our understanding of the multiple forms of GABAergic inhibition that exist in the CNS and contribute to visual signal processing in the retina.

Blockade of GABA(B) receptors facilitates evoked neurotransmitter release at spinal dorsal horn synapse

Metabotropic GABA type B (GABA(B)) receptors are abundantly expressed in the rat spinal dorsal horn. Activation of GABA(B) receptors by exogenous agonists inhibits synaptic transmission, which is believed to underlie the GABA(B) receptor-mediated analgesia. However, little effort has been made to test whether endogenous GABA might also mediate inhibition by acting on GABA(B) receptors. In this study, whole-cell recording techniques were employed to study the effect of endogenous GABA on GABA(B) receptors in substantia gelatinosa (SG) neurons in adult rat spinal cord slices. In current-clamp mode, blockade of GABA(B) receptors by their selective antagonist 3-[[[(3,4-dichlorophenyl)methyl]amino]propyl] (diethoxy-methyl) phosphinic acid (CGP 52432) facilitated presynaptic stimulation-induced action potential discharge and increased amplitude of postsynaptic potentials (PSPs), meaning a GABA(B) receptor-mediated inhibition of SG neuron excitability. In voltage-clamp mode, blockade of GABA(B) receptors increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) and decreased paired-pulse ratio, indicating a presynaptic CGP 52432 action. Primary afferent Aδ or C fiber-evoked EPSCs were also facilitated by CGP 52432 application. Amplitudes of evoked GABAergic and glycinergic inhibitory postsynaptic currents (eIPSCs) were enhanced by GABA(B) receptor blockade. The facilitation of amplitude persisted in the presence of a specific GABA transporter 1 (GAT-1) blocker, tiagabine, or GAT-2/3 blocker SNAP5114. However, blockade of GABA(B) receptors had no effect on action potential-independent miniature EPSCs (mEPSCs), miniature IPSCs (mIPSCs), or membrane conductance. Taken together, these results suggest that endogenous GABA modulates evoked synaptic transmission in SG neurons by acting on GABA(B) receptors. This GABA(B) receptor-mediated homeostatic regulation of neuronal excitability and neurotransmitter release might contribute to modulation of nociception in spinal dorsal horn.

GABAA receptors mediate motoneuron tonic inhibition in the turtle spinal cord

GABA(A) receptors mediating tonic inhibitory currents are present in neurons from hippocampus, cerebellum, sensory cortex and thalamus. These receptors located at peri- and extra-synaptic sites are constituted mainly by α(4/6) and α(5) subunits which confer them high affinity for GABA and low desensitization. Immunohistochemical and in vitro hybridization studies have shown the expression of these subunits, while functional studies have reported the presence of GABAergic tonic currents in spinal dorsal horn neurons. However, the presence of this inhibitory current has not been documented in motoneurons. In addition, we previously reported that the monosynaptic reflex is facilitated by furosemide, an antagonist of the α(4/6) GABA(A) receptors, without affecting the dorsal root potential, which suggests the presence of a GABAergic tonic inhibitory current in motoneurons. The aim of this work was to investigate the presence of high affinity GABA(A) receptors in motoneurons. By intracellular recordings made with sharp electrodes and the whole-cell patch clamp recording technique we show here that the membrane input resistance and the monosynaptic excitatory post-synaptic potential (EPSPs) are significantly increased by bicuculline. Likewise, the depression of the EPSPs and the input membrane resistance normally induced by muscimol was partially reverted by 20 μM bicuculline and abolished when the concentration of the antagonist was raised to 100 μM. Last, bicuculline at low concentration did not affect the holding current as occur with the high concentration that block the tonic inhibitory GABAergic current. Together these results suggest that the excitability in motoneurons may be tonically inhibited by high affinity GABA(A) receptors.

Ambient GABA-activated tonic inhibition sharpens auditory coincidence detection via a depolarizing shunting mechanism

Tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABA(A)Rs) has emerged as a novel form of neural inhibition in the CNS. However, little is known about its presence and function in the auditory system. Using whole-cell recordings in brain slices, we identified a tonic current mediated by GABA(A)Rs containing the δ subunit in middle/high-characteristic-frequency neurons of the chicken nucleus laminaris, the first interaural time difference encoder that computes information for sound localization. This tonic conductance was activated by ambient concentrations of GABA released from synaptic vesicles. Furthermore, pharmacological manipulations of the conductance demonstrated its essential role in coincidence detection. Remarkably, this depolarizing tonic conductance was strongly inhibitory primarily because of its shunting effect. These results demonstrate a novel role for tonic inhibition in central auditory information processing.

Serine proteases, serine protease inhibitors, and protease-activated receptors: roles in synaptic function and behavior

Serine proteases, serine protease inhibitors, and protease-activated receptors have been intensively investigated in the periphery and their roles in a wide range of processes-coagulation, inflammation, and digestion, for example-have been well characterized (see Coughlin, 2000; Macfarlane et al., 2001; Molinari et al., 2003; Wang et al., 2008; Di Cera, 2009 for reviews). A growing number of studies demonstrate that these protein systems are widely expressed in many cell types and regions in mammalian brains. Accumulating lines of evidence suggest that the brain has co-opted the activities of these interesting proteins to regulate various processes underlying synaptic activity and behavior. In this review, we discuss emerging roles for serine proteases in the regulation of mechanisms underlying synaptic plasticity and memory formation.

Localization and pharmacological modulation of GABA-B receptors in the globus pallidus of parkinsonian monkeys

Changes in GABAergic transmission in the external and internal segments of the globus pallidus (GPe and GPi) contribute to the pathophysiology of the basal ganglia network in Parkinson's disease. Because GABA-B receptors are involved in the modulation of GABAergic transmission in GPe and GPi, it is possible that changes in the functions or localization of these receptors contribute to the changes in GABAergic transmission. To further examine this question, we investigated the anatomical localization of GABA-B receptors and the electrophysiologic effects of microinjections of GABA-B receptor ligands in GPe and GPi of MPTP-treated (parkinsonian) monkeys. We found that the pattern of cellular and ultrastructural localization of the GABA-BR1 subunit of the GABA-B receptor in GPe and GPi was not significantly altered in parkinsonian monkeys. However, the magnitude of reduction in firing rate of GPe and GPi neurons produced by microinjections of the GABA-B receptor agonist baclofen was larger in MPTP-treated animals than in normal monkeys. Injections of the GABA-B receptor antagonist CGP55845A were more effective in reducing the firing rate of GPi neurons in parkinsonian monkeys than in normal animals. In addition, the injections of baclofen in GPe and GPi, or of CGP55845A in GPi lead to a significant increase in the proportion of spikes in rebound bursts in parkinsonian animals, but not in normal monkeys. Thus, despite the lack of changes in the localization of GABA-BR1 subunits in the pallidum, GABA-B receptor-mediated effects are altered in the GPe and GPi of parkinsonian monkeys. These changes in GABA-B receptor function may contribute to bursting activities in the parkinsonian state.

Sustained neuronal activity generated by glial plasticity

Astrocytes release gliotransmitters, notably glutamate, that can affect neuronal and synaptic activity. In particular, astrocytic glutamate release results in the generation of NMDA receptor (NMDA-R)-mediated slow inward currents (SICs) in neurons. However, factors underlying the emergence of SICs and their physiological roles are essentially unknown. Here we show that, in acute slices of rat somatosensory thalamus, stimulation of lemniscal or cortical afferents results in a sustained increase of SICs in thalamocortical (TC) neurons that outlasts the duration of the stimulus by 1 h. This long-term enhancement of astrocytic glutamate release is induced by group I metabotropic glutamate receptors and is dependent on astrocytic intracellular calcium. Neuronal SICs are mediated by extrasynaptic NR2B subunit-containing NMDA-Rs and are capable of eliciting bursts. These are distinct from T-type Ca(2+) channel-dependent bursts of action potentials and are synchronized in neighboring TC neurons. These findings describe a previously unrecognized form of excitatory, nonsynaptic plasticity in the CNS that feeds forward to generate local neuronal firing long after stimulus termination.

Central Role of GABA in Neuron–Glia Interactions

The major types of glial cells—astrocytes, microglia, and cells of the oligodendroglial lineage—are known to express functional metabotropic and ionotropic GABA receptors. Neuronal signaling mechanisms allowing for the activation of these receptors in glia are probably as complex as those described among neurons and involve synaptic and extrasynaptic transmission modes. In addition, astrocytes can signal back to neurons by releasing GABA, probably through unconventional nonvesicular mechanisms. The decryption of the roles played by GABAergic signaling in neuron–glia interactions is only beginning, but it has been suggested that activation of glial cells by GABA influences important functions of the brain such as neuronal activity, differentiation, myelination, and neuroprotection. This review discusses the cellular mechanisms allowing the major types of glial cells to sense and transmit GABAergic signals and gives an overview of potential roles of this signaling pathway in developing and mature brains.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.

Activity-dependent targeting of TRPV1 with a pore-permeating capsaicin analog

The capsaicin receptor TRPV1 is the principal transduction channel for nociception. Excessive TRPV1 activation causes pathological pain. Ideal pain mangement requires selective inhibition of hyperactive pain-sensing neurons, but sparing normal nociception. We sought to determine whether it is possible to use activity-dependent TRPV1 agonists to identify nerves with excessive TRPV1 activity, as well as exploit the TRPV1 pore to deliver charged anesthetics for neuronal silencing. We synthesized a series of permanently charged capsaicinoids and found that one, cap-ET, efficaciously evoked TRPV1-dependent entry of Ca(2+) or the large cationic dye YO-PRO-1 comparably to capsaicin, but far smaller electrical currents. Cap-ET-induced YO-PRO-1 transport required permeation of both the agonist and the dye through the TRPV1 pore and could be enhanced by kinase activation or oxidative covalent modification. Moreover, cap-ET reduced capsaicin-induced currents by a voltage-dependent block of the pore. A low dose of cap-ET elicited entry of permanently charged Na(+) channel blockers to effectively suppress Na(+) currents in sensory neurons presensitized with oxidative chemicals. These results implicate therapeutic potential of these unique TRPV1 agonists exhibiting activity-dependent ion transport but of minimal pain-producing risks.

Dynamics of binaural processing in the mammalian sound localization pathway--the role of GABA(B) receptors

The initial binaural processing in the superior olive represents the fastest computation known in the entire mammalian brain. Although the binaural system has to perform under very different and often highly dynamic acoustic conditions, the integration of binaural information in the superior olivary complex (SOC) has not been considered to be adaptive or dynamic itself. Recent evidence, however, shows that the initial processing of interaural level and interaural time differences relies on well-adjusted interactions of both the excitatory and the inhibitory projections, respectively. Under static conditions, these inputs seem to be tightly balanced, but may also require dynamic adjustment for proper function when the acoustic environment changes. GABA(B) receptors are at least one mechanism rendering the system more dynamic than considered so far. A comprehensive description of how binaural processing in the SOC is dynamically regulated by GABA(B) receptors in adults and in early development is important for understanding how spatial auditory processing changes with acoustic context.

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

Cell cycle restriction by histone H2AX limits proliferation of adult neural stem cells

Adult neural stem cell proliferation is dynamic and has the potential for massive self-renewal yet undergoes limited cell division in vivo. Here, we report an epigenetic mechanism regulating proliferation and self-renewal. The recruitment of the PI3K-related kinase signaling pathway and histone H2AX phosphorylation following GABA(A) receptor activation limits subventricular zone proliferation. As a result, NSC self-renewal and niche size is dynamic and can be directly modulated in both directions pharmacologically or by genetically targeting H2AX activation. Surprisingly, changes in proliferation have long-lasting consequences on stem cell numbers, niche size, and neuronal output. These results establish a mechanism that continuously limits proliferation and demonstrates its impact on adult neurogenesis. Such homeostatic suppression of NSC proliferation may contribute to the limited self-repair capacity of the damaged brain.

Non-neuronal, slow GABA signalling in the ventrobasal thalamus targets δ-subunit-containing GABA(A) receptors

The rodent ventrobasal (VB) thalamus contains a relatively uniform population of thalamocortical (TC) neurons that receive glutamatergic input from the vibrissae and the somatosensory cortex, and inhibitory input from the nucleus reticularis thalami (nRT). In this study we describe γ-aminobutyric acid (GABA)(A) receptor-dependent slow outward currents (SOCs) in TC neurons that are distinct from fast inhibitory postsynaptic currents (IPSCs) and tonic currents. SOCs occurred spontaneously or could be evoked by hypo-osmotic stimulus, and were not blocked by tetrodotoxin, removal of extracellular Ca(2+) or bafilomycin A1, indicating a non-synaptic, non-vesicular GABA origin. SOCs were more common in TC neurons of the VB compared with the dorsal lateral geniculate nucleus, and were rarely observed in nRT neurons, whilst SOC frequency in the VB increased with age. Application of THIP, a selective agonist at δ-subunit-containing GABA(A) receptors, occluded SOCs, whereas the benzodiazepine site inverse agonist β-CCB had no effect, but did inhibit spontaneous and evoked IPSCs. In addition, the occurrence of SOCs was reduced in mice lacking the δ-subunit, and their kinetics were also altered. The anti-epileptic drug vigabatrin increased SOC frequency in a time-dependent manner, but this effect was not due to reversal of GABA transporters. Together, these data indicate that SOCs in TC neurons arise from astrocytic GABA release, and are mediated by δ-subunit-containing GABA(A) receptors. Furthermore, these findings suggest that the therapeutic action of vigabatrin may occur through the augmentation of this astrocyte-neuron interaction, and highlight the importance of glial cells in CNS (patho) physiology.

An historical perspective on GABAergic drugs

In 1950, γ-aminobutyric acid (GABA) was discovered in the brain and in 1967 it was recognized as an inhibitory neurotransmitter. The discovery of the benzodiazepines Librium® (launched in 1960) and Valium® by Sternbach initiated huge research activities resulting in 50 marketed drugs. In 1975, Haefely found that GABA is involved in the actions of benzodiazepines. The baclofen-sensitive, bicuculline-insensitive GABAB receptor was discovered by Bowery in 1980, and the baclofen-insensitive, bicuculline-insensitive GABAC receptor by Johnston in 1984. Barnard & Seeburg reported the cloning of the GABAA receptor in 1987, Cutting the GABAC receptor in 1991 and Bettler the GABAB1a and GABAB1b receptors in 1997. Six groups cloned the GABAB2 receptor in 1998/1999 showing that the GABAB receptor functions as a heterodimer with GABAB1b/GABAB2 mediating postsynaptic inhibition and GABAB1a/GABAB2 mediating presynaptic inhibition. Möhler and McKernan dissected the pharmacology of the benzodiazepine-receptor subtypes. Antagonists and positive allosteric modulators of GABAB receptors were discovered in 1987 and 2001, respectively. GABA transporter inhibitor, tiagabine, was launched in 1996, a GABA aminotransferase inhibitor, vigabatrin, in 1998 and a glutamic acid decarboxylase activator, pregabalin, in 2004. Most recently, brain-penetrating GABAC-receptor antagonists were reported in 2009.

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.