The impact of tonic GABAA receptor-mediated inhibition on neuronal excitability varies across brain region and cell type

Chemogenetic Isolation Reveals Synaptic Contribution of δ GABAA Receptors in Mouse Dentate Granule Neurons

Two major GABA_A receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABA_A receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABA_A receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.

De novo variants in GABRA2 and GABRA5 alter receptor function and contribute to early-onset epilepsy

GABAA receptors are ligand-gated anion channels that are important regulators of neuronal inhibition. Mutations in several genes encoding receptor subunits have been identified in patients with various types of epilepsy, ranging from mild febrile seizures to severe epileptic encephalopathy. Using whole-genome sequencing, we identified a novel de novo missense variant in GABRA5 (c.880G > C, p.V294L) in a patient with severe early-onset epilepsy and developmental delay. Targeted resequencing of 279 additional epilepsy patients identified 19 rare variants from nine GABAA receptor genes, including a novel de novo missense variant in GABRA2 (c.875C > A, p.T292K) and a recurrent missense variant in GABRB3 (c.902C > T, p.P301L). Patients with the GABRA2 and GABRB3 variants also presented with severe epilepsy and developmental delay. We evaluated the effects of the GABRA5, GABRA2 and GABRB3 missense variants on receptor function using whole-cell patch-clamp recordings from human embryonic kidney 293T cells expressing appropriate α, β and γ subunits. The GABRA5 p.V294L variant produced receptors that were 10-times more sensitive to GABA but had reduced maximal GABA-evoked current due to increased receptor desensitization. The GABRA2 p.T292K variant reduced channel expression and produced mutant channels that were tonically open, even in the absence of GABA. Receptors containing the GABRB3 p.P301L variant were less sensitive to GABA and produced less GABA-evoked current. These results provide the first functional evidence that de novo variants in the GABRA5 and GABRA2 genes contribute to early-onset epilepsy and developmental delay, and demonstrate that epilepsy can result from reduced neuronal inhibition via a wide range of alterations in GABAA receptor function.

Calcium imaging and dynamic causal modelling reveal brain-wide changes in effective connectivity and synaptic dynamics during epileptic seizures

Pathophysiological explanations of epilepsy typically focus on either the micro/mesoscale (e.g. excitation-inhibition imbalance), or on the macroscale (e.g. network architecture). Linking abnormalities across spatial scales remains difficult, partly because of technical limitations in measuring neuronal signatures concurrently at the scales involved. Here we use light sheet imaging of the larval zebrafish brain during acute epileptic seizure induced with pentylenetetrazole. Spectral changes of spontaneous neuronal activity during the seizure are then modelled using neural mass models, allowing Bayesian inference on changes in effective network connectivity and their underlying synaptic dynamics. This dynamic causal modelling of seizures in the zebrafish brain reveals concurrent changes in synaptic coupling at macro- and mesoscale. Fluctuations of both synaptic connection strength and their temporal dynamics are required to explain observed seizure patterns. These findings highlight distinct changes in local (intrinsic) and long-range (extrinsic) synaptic transmission dynamics as a possible seizure pathomechanism and illustrate how our Bayesian model inversion approach can be used to link existing neural mass models of seizure activity and novel experimental methods.

Enhanced postsynaptic inhibitory strength in hippocampal principal cells in high-performing aged rats

Hyperactivity within the hippocampal formation, frequently observed in aged individuals, is thought to be a potential contributing mechanism to the memory decline often associated with aging. Consequently, we evaluated the postsynaptic strength of excitatory and inhibitory synapses in the granule cells of the dentate gyrus and CA1 pyramidal cells of a rat model of aging, in which each individual was behaviorally characterized as aged impaired (AI) or aged unimpaired (AU, with performance comparable to young (Y) individuals). In hippocampal slices of these 3 aged groups (Y, AI, AU), we found that compared to the young, the miniature excitatory and inhibitory currents (mEPSCs and mIPSCs) were larger in amplitude in the granule cells of the AU group and smaller in the AI group. In contrast, in CA1 cells, neither the mEPSCs nor the mIPSCs were affected by age, whereas the extrasynaptic conductance responsible for tonic inhibition was selectively enhanced in CA1 cells of AU individuals. Tonic inhibition conductance was not affected by age in the granule cells. These results support the notion that upregulation of synaptic inhibition could be a necessary condition for the maintenance of performance during aging. These findings also underscore the notions that successful aging requires adaptive upregulation, not merely the preservation of youthful functionality, and that age effects are not homogeneous across hippocampal subfields.

Common Ribs of Inhibitory Synaptic Dysfunction in the Umbrella of Neurodevelopmental Disorders

The term neurodevelopmental disorder (NDD) is an umbrella term used to group together a heterogeneous class of disorders characterized by disruption in cognition, emotion, and behavior, early in the developmental timescale. These disorders are heterogeneous, yet they share common behavioral symptomatology as well as overlapping genetic contributors, including proteins involved in the formation, specialization, and function of synaptic connections. Advances may arise from bridging the current knowledge on synapse related factors indicated from both human studies in NDD populations, and in animal models. Mounting evidence has shown a link to inhibitory synapse formation, specialization, and function among Autism, Angelman, Rett and Dravet syndromes. Inhibitory signaling is diverse, with numerous subtypes of inhibitory interneurons, phasic and tonic modes of inhibition, and the molecular and subcellular diversity of GABA_A receptors. We discuss common ribs of inhibitory synapse dysfunction in the umbrella of NDD, highlighting alterations in the developmental switch to inhibitory GABA, dysregulation of neuronal activity patterns by parvalbumin-positive interneurons, and impaired tonic inhibition. Increasing our basic understanding of inhibitory synapses, and their role in NDDs is likely to produce significant therapeutic advances in behavioral symptom alleviation for interrelated NDDs.

Control of motor coordination by astrocytic tonic GABA release through modulation of excitation/inhibition balance in cerebellum

Tonic inhibition in the brain is mediated through an activation of extrasynaptic GABA_A receptors by the tonically released GABA, resulting in a persistent GABAergic inhibitory action. It is one of the key regulators for neuronal excitability, exerting a powerful action on excitation/inhibition balance. We have previously reported that astrocytic GABA, synthesized by monoamine oxidase B (MAOB), mediates tonic inhibition via GABA-permeable bestrophin 1 (Best1) channel in the cerebellum. However, the role of astrocytic GABA in regulating neuronal excitability, synaptic transmission, and cerebellar brain function has remained elusive. Here, we report that a reduction of tonic GABA release by genetic removal or pharmacological inhibition of Best1 or MAOB caused an enhanced neuronal excitability in cerebellar granule cells (GCs), synaptic transmission at the parallel fiber-Purkinje cell (PF-PC) synapses, and motor performance on the rotarod test, whereas an augmentation of tonic GABA release by astrocyte-specific overexpression of MAOB resulted in a reduced neuronal excitability, synaptic transmission, and motor performance. The bidirectional modulation of astrocytic GABA by genetic alteration of Best1 or MAOB was confirmed by immunostaining and in vivo microdialysis. These findings indicate that astrocytes are the key player in motor coordination through tonic GABA release by modulating neuronal excitability and could be a good therapeutic target for various movement and psychiatric disorders, which show a disturbed excitation/inhibition balance.

Acid-base physiology, neurobiology and behaviour in relation to CO2-induced ocean acidification

Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma chloride and bicarbonate ion concentrations as a result of acid-base regulation, causing the reversal of ionic fluxes through GABA_A receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABA_A receptor antagonist gabazine on control animals and those exposed to elevated CO₂ However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABA_A receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid-base physiology, neurobiology, pharmacology and behaviour in relation to marine CO₂-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms.

Presynaptic Regulation of Tonic Inhibition by Neuromodulatory Transmitters in the Basal Amygdala

Tonic inhibition mediated by ambient levels of GABA that activate extrasynaptic GABA_A receptors emerges as an essential factor that tunes neuronal network excitability in vitro and shapes behavioral responses in vivo. To address the role of neuromodulatory transmitter systems on this type of inhibition, we employed patch clamp recordings in mouse amygdala slice preparations. Our results show that the current amplitude of tonic inhibition (I_tonic) in projection neurons of the basal amygdala (BA) is increased by preincubation with the neurosteroid THDOC, while the benzodiazepine diazepam is ineffective. This suggests involvement of THDOC sensitive δ subunit containing GABA_A receptors in mediating tonic inhibition. Moreover, we provide evidence that the neuromodulatory transmitters NE, 5HT, and ACh strongly enhance spontaneous IPSCs as well as I_tonic in the BA. As the increase in frequency, amplitude, and charge of sIPSCs by these neuromodulatory transmitters strongly correlated with the amplitude of I_tonic, we conclude that spill-over of synaptic GABA leads to activation of I_tonic and thereby to dampening of amygdala excitability. Since local injection of THDOC, as a positive modulator of tonic inhibition, into the BA interfered with the expression of contextual fear memory, our results point to a prominent role of I_tonic in fear learning.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Contribution of genes in the GABAergic pathway to bipolar disorder and its executive function deficit in the Chinese Han population

In this study, we investigated the association between bipolar I disorder (BDI) and between cognitive deficits therein and SNPs in GABAergic receptor genes. The sample comprised 477 patients with BDI and 438 healthy controls, with three neurocognitive tests being administered in 123 patients and 164 controls. For three SNPs, rs505474, rs1398175, and rs4868029 in the GABRA2, GABRA4, and GABRP genes, respectively, their allele frequencies were significantly different between patients and controls (Bonferroni-adjusted p = values 3.84 × 10^-4 , 9.92 × 10^-3 , and 1.22 × 10^-2 , respectively). Four haplotypes were significantly associated with BDI (TA and AG for rs3815762 and rs4868029 in GABRP, GG for rs11636988 and rs8024256 in GABRB3 and GAGG for rs2197414, rs4921195, rs13188991, and rs11956731 in GABRA6, with p values of 0.0038, 0.044, 0.0176, and 0.0267, respectively, on 10,000 permutations). Furthermore, the SNP (rs2912585) within 250 kb upstream of the GABRB3 gene displayed a strong association with the Tower of Hanoi (TOH) executive time in the patient group (p = 2.844 × 10^-6 ). One other SNP (rs754661), which is located at the intronic region of the same gene, was associated with the global trait of the executive function and post hoc analysis showed significant SNP by group effect (p = 0.0094). Our study supports previous findings that GABA_A receptor genes are associated with bipolar disorder; it also suggests that the GABA_A genes, especially the GABRB3 gene, might play a role in the executive function deficit in bipolar disorder, although future replication with a larger sample size is needed.

δ-Subunit Containing GABAA Receptors Modulate Respiratory Networks

Persistent and stable respiratory activity across behavioral states is key to homeostasis. Extrasynaptic δ-subunit containing GABA_A receptors (δGABA_ARs) mediate tonic inhibition and regulate network activity. However, the influence of δGABA_ARs on respiratory rhythm and motor outputs is unknown. We manipulated extra-synaptic GABA_A receptor function in the preBötzinger Complex (preBötC), a site central to the generation of inspiratory motor activity in mammals. Activation of preBötC δGABA_ARs in anesthetized rats and wild-type mice decreased breathing rate. In δGABA_AR knockout (Gabrd ^-/-) mice, however, δGABA_ARs activation had no effect on breathing rate. We then found that during active wakefulness associated with behaviors and movements, diaphragm activation was higher in the Gabrd ^-/- compared to wild-type mice, but not in other states. These findings identify that δGABA_ARs modulate the respiratory network, which is critical to understand how δGABA_ARs change breathing in pathological conditions affecting extra-synaptic GABA_A receptor function such as exposure to anesthetics and neurosteroids.

Depolarizing, inhibitory GABA type A receptor activity regulates GABAergic synapse plasticity via ERK and BDNF signaling

γ-aminobutyric acid (GABA) begins as the key excitatory neurotransmitter in newly forming circuits, with chloride efflux from GABA type A receptors (GABA_ARs) producing membrane depolarization, which promotes calcium entry, dendritic outgrowth and synaptogenesis. As development proceeds, GABAergic signaling switches to inhibitory hyperpolarizing neurotransmission. Despite the evidence of impaired GABAergic neurotransmission in neurodevelopmental disorders, little is understood on how agonist-dependent GABA_AR activation controls the formation and plasticity of GABAergic synapses. We have identified a weakly depolarizing and inhibitory GABA_AR response in cortical neurons that occurs during the transition period from GABA_AR depolarizing excitation to hyperpolarizing inhibitory activity. We show here that treatment with the GABA_AR agonist muscimol mediates structural changes that diminish GABAergic synapse strength through postsynaptic and presynaptic plasticity via intracellular Ca²⁺ stores, ERK and BDNF/TrkB signaling. Muscimol decreases synaptic localization of surface γ2 GABA_ARs and gephyrin postsynaptic scaffold while β2/3 non-γ2 GABA_ARs accumulate in the synapse. Concurrent with this structural plasticity, muscimol treatment decreases synaptic currents while enhancing the γ2 containing benzodiazepine sensitive GABA_AR tonic current in an ERK dependent manner. We further demonstrate that GABA_AR activation leads to a decrease in presynaptic GAD65 levels via BDNF/TrkB signaling. Together these data reveal a novel mechanism for agonist induced GABAergic synapse plasticity that can occur on the timescale of minutes, contributing to rapid modification of synaptic and circuit function.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

The Influence of Regional Distribution and Pharmacologic Specificity of GABAAR Subtype Expression on Anesthesia and Emergence

Anesthetics produce unconsciousness by modulating ion channels that control neuronal excitability. Research has shown that specific GABA_A receptor (GABA_AR) subtypes in particular regions of the central nervous system contribute to different hyperpolarizing conductances, and behaviorally to distinct components of the anesthetized state. The expression of these receptors on the neuron cell surface, and thus the strength of inhibitory neurotransmission, is dynamically regulated by intracellular trafficking mechanisms. Pharmacologic or activity-based perturbations to these regulatory systems have been implicated in pathology of several neurological conditions, and can alter the individual response to anesthesia. Furthermore, studies are beginning to uncover how anesthetic exposure itself elicits enduring changes in subcellular physiology, including the processes that regulate ion channel trafficking. Here, we review the mechanisms that determine GABA_AR surface expression, and elaborate on influences germane to anesthesia and emergence. We address known trafficking differences between the intrasynaptic receptors that mediate phasic current and the extra-synaptic receptors mediating tonic current. We also describe neurophysiologic consequences and network-level abnormalities in brain function that result from receptor trafficking aberrations. We hypothesize that the relationship between commonly used anesthetic agents and GABA_AR surface expression has direct consequences on mature functioning neural networks and by extension ultimately influence the outcome of patients that undergo general anesthesia. Rational design of new anesthetics, anesthetic techniques, EEG-based monitoring strategies, or emergence treatments will need to take these effects into consideration.

Neurobiological correlates of state-dependent context fear

Retrieval of fear memories can be state-dependent, meaning that they are best retrieved if the brain states at encoding and retrieval are similar. Such states can be induced by activating extrasynaptic γ-aminobutyric acid type A receptors (GABA_AR) with the broad α-subunit activator gaboxadol. However, the circuit mechanisms and specific subunits underlying gaboxadol's effects are not well understood. Here we show that gaboxadol induces profound changes of local and network oscillatory activity, indicative of discoordinated hippocampal–cortical activity, that were accompanied by robust and long-lasting state-dependent conditioned fear. Episodic memories typically are hippocampus-dependent for a limited period after learning, but become cortex-dependent with the passage of time. In contrast, state-dependent memories continued to rely on hippocampal GABAergic mechanisms for memory retrieval. Pharmacological approaches with α-subunit-specific agonists targeting the hippocampus implicated the prototypic extrasynaptic subunits (α₄) as the mediator of state-dependent conditioned fear. Together, our findings suggest that continued dependence on hippocampal rather than cortical mechanisms could be an important feature of state-dependent memories that contributes to their conditional retrieval.

Physiological and pharmacological properties of inhibitory postsynaptic currents mediated by α5β1γ2, α5β2γ2 and α5β3γ2 GABAA receptors

α5-containing GABA_ARs are potential therapeutic targets for clinical conditions including age-related dementia, stroke, schizophrenia, Down syndrome, anaesthetic-induced amnesia, anxiety and pain. α5-containing GABA_ARs are expressed in layer 5 cortical neurons and hippocampal pyramidal neurons where they mediate both tonic currents and slow inhibitory postsynaptic currents (IPSCs). A range of drugs has been developed to specifically modulate these receptors. The main α5-containing GABA_ARs that are likely to exist in vivo are the α5β1γ2, α5β2γ2 and α5β3γ2 isoforms. We currently have few clues as to how these isoforms are distributed between synaptic and extrasynaptic compartments or their relative roles in controlling neuronal excitability. Accordingly, the aim of this study was to define the basic biophysical and pharmacological properties of IPSCs mediated by the three isoforms in a hippocampal neuron-HEK293 cell co-culture assay. The IPSC decay time constants were slow (α5β1γ2L: 45 ms; α5β1γ2L: 80 ms; α5β3γ2L: 184 ms) and were largely dominated by the intrinsic channel deactivation rates. By comparing IPSC rise times, we inferred that α5β1γ2L GABA_ARs are located postsynaptically whereas the other two are predominantly perisynaptic. α5β3γ2L GABA_ARs alone mediated tonic currents. We quantified the effects of four α5-specific inverse agonists (TB-21007, MRK-016, α5IA and L-655708) on IPSCs mediated by the three isoforms. All compounds selectively inhibited IPSC amplitudes and accelerated IPSC decay rates, albeit with distinct isoform specificities. MRK-016 also significantly accelerated IPSC rise times. These results provide a reference for future studies seeking to identify and characterize the properties of IPSCs mediated by α5-containing GABA_AR isoforms in neurons.

Delta Subunit-Containing Gamma-Aminobutyric Acid A Receptor Disinhibits Lateral Amygdala and Facilitates Fear Expression in Mice

BACKGROUND

Maintaining gamma-aminobutyric acidergic (GABAergic) inhibition in the amygdala within a physiological range is critical for the appropriate expression of emotions such as fear and anxiety. The synaptic GABA type A receptor (GABA_AR) is generally known to mediate the primary component of amygdala inhibition and prevent inappropriate expression of fear. However, little is known about the contribution of the extrasynaptic GABA_AR to amygdala inhibition and fear.

METHODS

By using mice expressing green fluorescent protein in interneurons (INs) and lacking the δ subunit-containing GABA_AR (GABA_A(δ)R), which is exclusively situated in the extrasynaptic membrane, we systematically investigated the role of GABA_A(δ)R in regulating inhibition in the lateral amygdala (LA) and fear learning using the combined approaches of immunohistochemistry, electrophysiology, and behavior.

RESULTS

In sharp contrast to the established role of synaptic GABA_AR in mediating LA inhibition, we found that either pharmacological or physiological recruitment of GABA_A(δ)R resulted in the weakening of GABAergic transmission onto projection neurons in LA while leaving the glutamatergic transmission unaltered, suggesting disinhibition by GABA_A(δ)R. The disinhibition arose from IN-specific expression of GABA_A(δ)R with its activation decreasing the input resistance of local INs and suppressing their activation. Genetic deletion of GABA_A(δ)R attenuated its role in suppressing LA INs and disinhibiting LA. Importantly, the GABA_A(δ)R facilitated long-term potentiation in sensory afferents to LA and permitted the expression of learned fear.

CONCLUSIONS

Our findings suggest that GABA_A(δ)R serves as a brake rather than a mediator of GABAergic inhibition in LA. The disinhibition by GABA_A(δ)R may help to prevent excessive suppression of amygdala activity and thus ensure the expression of emotion.

Development of a Robust Mammalian Cell-based Assay for Studying Recombinant α4 β1/3 δ GABAA Receptor Subtypes

δ-Containing GABA_A receptors are located extrasynaptically and mediate tonic inhibition. Their involvement in brain physiology positions them as interesting drug targets. There is thus a continued interest in establishing reliable recombinant expression systems for δ-containing GABA_A receptors. Inconveniently, the recombinant expression of especially α₄ β_1/3 δ receptors has been found to be notoriously difficult, resulting in mixed receptor populations and/or stoichiometries and differential pharmacology depending on the expression system used. With the aim of developing a facile and robust 96-well format cell-based assay for extrasynaptic α₄ β_1/3 δ receptors, we have engineered and validated a HEK293 Flp-In™ cell line stably expressing the human GABA_A δ-subunit. Upon co-transfection of α₄ and β_1/3 subunits, at optimized ratios, we have established a well-defined system for expressing α₄ β_1/3 δ receptors and used the fluorescence-based FLIPR Membrane Potential (FMP) assay to evaluate their pharmacology. Using the known reference compounds GABA and THIP, ternary α₄ β_1/3 δ and binary α₄ β_1/3 receptors could be distinguished based on potency and kinetic profiles but not efficacy. As expected, DS2 was able to potentiate only δ-containing receptors, whereas Zn²⁺ had an inhibitory effect only at binary receptors. By contrast, the hitherto reported δ-selective compounds, AA29504 and 3-OH-2'MeO6MF, were non-selective. The expression system was further validated using patch clamp electrophysiology, in which the superagonism of THIP was confirmed. The established FMP assay set-up, based on transient expression of human α₄ and β_1/3 subunits into a δ-subunit stable HEK293 Flp-In™ cell line, portrays a simple 96-well format assay as a useful supplement to electrophysiological recordings on δ-containing GABA_A receptors.

Anticonvulsant profile of the neuroactive steroid, SGE-516, in animal models

Despite the availability of multiple antiepileptic drugs (AED), failure to adequately control seizures is a challenge for approximately one third of epilepsy patients, and new therapies with a differentiated mechanism of action are needed. The neuroactive steroid, SGE-516, is a positive allosteric modulator of both gamma- and delta-containing GABA_A receptors. This broad GABA_A receptor activity differentiates neuroactive steroids like SGE-516 from benzodiazepines, a class of anticonvulsants which have been shown in vitro to selectively target gamma-subunit containing GABA_A receptors. As a neuroactive steroid, SGE-516 has pharmacokinetic properties that are intended to allow for chronic oral dosing. We investigated the anticonvulsant activity of SGE-516 across numerous in vitro and in vivo models of seizure activity. SGE-516 dose-dependently reduced neuronal firing rates and epileptiform activity in vitro. In mice, SGE-516 protected against acute seizures in the PTZ-induced chemo-convulsant seizure model and the 6Hz psychomotor seizure model. In addition, SGE-516 demonstrated anticonvulsant activity in the mouse corneal kindling model. These data suggest that SGE-516 may have potential for development as a novel oral AED for the treatment of refractory seizures.

Astrocyte dysfunction in Alzheimer disease

Astrocytes are glial cells that are distributed throughout the central nervous system in an arrangement optimal for chemical and physical interaction with neuronal synapses and brain blood supply vessels. Neurotransmission modulates astrocytic excitability by activating an array of cell surface receptors and transporter proteins, resulting in dynamic changes in intracellular Ca²⁺ or Na⁺ . Ionic and electrogenic astrocytic changes, in turn, drive vital cell nonautonomous effects supporting brain function, including regulation of synaptic activity, neuronal metabolism, and regional blood supply. Alzheimer disease (AD) is associated with aberrant oligomeric amyloid β generation, which leads to extensive proliferation of astrocytes with a reactive phenotype and abnormal regulation of these processes. Astrocytic morphology, Ca²⁺ responses, extracellular K⁺ removal, glutamate transport, amyloid clearance, and energy metabolism are all affected in AD, resulting in a deleterious set of effects that includes glutamate excitotoxicity, impaired synaptic plasticity, reduced carbon delivery to neurons for oxidative phosphorylation, and dysregulated linkages between neuronal energy demand and regional blood supply. This review summarizes how astrocytes are affected in AD and describes how these changes are likely to influence brain function. © 2017 Wiley Periodicals, Inc.

Angelman syndrome: Current and emerging therapies in 2016

Angelman syndrome (AS) is a severe neurodevelopmental disorder caused by a loss of the maternally-inherited UBE3A; the paternal UBE3A is silenced in neurons by a mechanism involving an antisense transcript (UBE3A-AS) at the unmethylated paternal locus. We reviewed all published information on the clinical trials that have been completed as well as the publicly available information on ongoing trials of therapies in AS. To date, all clinical trials that strove to improve neurodevelopment in AS have been unsuccessful. Attempts at hypermethylating the maternal locus through dietary compounds were ineffective. The results of an 8-week open-label trial using minocycline as a matrix metalloproteinase-9 inhibitor were inconclusive, while a subsequent randomized placebo-controlled trial suggested that treatment with minocycline for 8 weeks did not result in any neurodevelopmental gains. A 1-year randomized placebo-controlled trial using levodopa to alter the phosphorylation of calcium/calmodulin-dependent kinase II did not lead to any improvement in neurodevelopment. Topoisomerase inhibitors and antisense oligonucleotides are being developed to directly inhibit UBE3A-AS. Artificial transcription factors are being developed to "super activate" UBE3A or inhibit UBE3A-AS. Other strategies targeting specific pathways are briefly discussed. We also reviewed the medications that are currently used to treat seizures and sleep disturbances, which are two of the more common complications of AS. © 2016 Wiley Periodicals, Inc.

Methylmercury induces an initial increase in GABA-evoked currents in Xenopus oocytes expressing α1 and α6 subunit-containing GABAA receptors

Early onset effects of methylmercury (MeHg) on recombinant α₁β₂γ_2S or α₆β₂γ_2S subunit-containing GABA_A receptors were examined. These are two of the most prevalent receptor types found in cerebellum-a consistent target of MeHg-induced neurotoxicity. Heterologously expressed receptors were used in order to: (1) isolate receptor-mediated events from extraneous effects of MeHg due to stimulation of the receptor secondary to increased release of GABA seen with MeHg in neurons in situ and (2) limit the phenotypes of GABA_A receptors present at one time. Initial changes in I_GABA in Xenopus laevis oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors were compared during continuous bath application of MeHg. A time-dependent increase in I_GABA mediated by both receptor subtypes occurred following the first 25-30min of MeHg (5μM) exposure. In α₆β₂γ_2S containing receptors, the MeHg-induced increase in I_GABA was less pronounced compared to that mediated by α₁β₂γ_2S containing receptors, although the pattern of effects was generally similar. Washing with MeHg-free solution reversed the increase in current amplitude. Application of bicuculline at the time of peak potentiation of I_GABA rapidly and completely reversed the MeHg-induced currents. Therefore these MeHg-increased inward currents are mediated specifically by the two subtypes of GABA_A receptors and appear to entail direct actions of MeHg on the receptor. However bicuculline did not affect stimulation by MeHg of oocyte endogenous Cl^- -mediated current, which presumably results from increased [Ca²⁺]_i. Thus, MeHg initially potentiates I_GABA in oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors prior to its more defined later effects, suggesting that MeHg may initially interact directly with GABA_A receptors in a reversible manner to cause this potentiation.

Reduced expression of α5GABAA receptors elicits autism-like alterations in EEG patterns and sleep-wake behavior

A reduction in the activity of GABA_A receptors, particularly α5 subunit-containing GABA_A receptors (α5GABA_ARs), has been implicated in the etiology of Autism Spectrum Disorders (ASD). Genetically modified mice that lack α5GABA_ARs (Gabra5^-/-) exhibit autism-like behaviors and both enhanced and impaired learning and memory, depending on the behavioral task. The aim of this study was to examine the electroencephalogram (EEG) activity and sleep-wake behaviors in Gabra5^-/- mice and wild-type mice. In addition, since some individuals with ASD can exhibit elevated innate immune response, mice were treated with lipopolysaccharide (LPS; 125mg/kg intraperitoneal injection) or vehicle and EEG and sleep-wake patterns were assessed. The results showed that Gabra5^-/- mice (n=3) exhibited elevated 0-2Hz EEG activity during all sleep-wake states (all p<0.04), decreased 8-12Hz EEG activity during REM sleep (p=0.04), and decreased sleep spindles under baseline conditions compared to wild-type controls (n=4) (all p≤0.03). Alterations in EEG activity and sleep-wake behavior were identified in Gabra5^-/- mice following treatment with LPS, however these changes were similar to those in wild-type mice. Our findings support the hypothesis that reduced α5GABA_AR activity contributes to an ASD phenotype. The results also suggest that Gabra5^-/- mice may serve as an animal model for ASD, as assessed through EEG activity and sleep-wake behaviors.

Effects of early-life exposure to THIP on phenotype development in a mouse model of Rett syndrome

Background

Rett syndrome (RTT) is a neurodevelopmental disorder caused mostly by disruptions in the MECP2 gene. MECP2-null mice show imbalances in neuronal excitability and synaptic communications. Several previous studies indicate that augmenting synaptic GABA receptors (GABA_ARs) can alleviate RTT-like symptoms in mice. In addition to the synaptic GABA_ARs, there is a group of GABA_ARs found outside the synaptic cleft with the capability to produce sustained inhibition, which may be potential therapeutic targets for the control of neuronal excitability in RTT.

Methods

Wild-type and MECP2-null mice were randomly divided into four groups, receiving the extrasynaptic GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP) and vehicle control, respectively. Low-dose THIP was administered to neonatal mice through lactation. RTT-like symptoms including lifespan, breathing, motor function, and social behaviors were studied when mice became mature. Changes in neuronal excitability and norepinephrine biosynthesis enzyme expression were studied in electrophysiology and molecular biology.

Results

With no evident sedation and other adverse side effects, early-life exposure to THIP extended the lifespan, alleviated breathing abnormalities, enhanced motor function, and improved social behaviors of MECP2-null mice. Such beneficial effects were associated with stabilization of locus coeruleus neuronal excitability and improvement of norepinephrine biosynthesis enzyme expression.

Conclusions

THIP treatment in early lives might be a therapeutic approach to RTT-like symptoms in MECP2-null mice and perhaps in people with RTT as well.

What's in a Name? Roles of RFamide-Related Peptides Beyond Gonadotrophin Inhibition

RFamide-related peptides (RFRPs) have been heavily implicated in the control of reproductive function subsequent to their discovery more than 16 years ago. However, recent studies using genetic and pharmacological tools have challenged their importance in regulating the hypothalamic-pituitary-gonadal axis. It is generally accepted that RFRPs act as part of a wider RFamide system, which involves two receptors, called the neuropeptide FF receptors (NPFFR1 and R2), and includes the closely-related neuropeptide NPFF. NPFF has been studied ever since the 1980s and many of the functions of NPFF are also shared by RFRPs. The current review questions whether these functions of NPFF are indeed specific to just NPFF alone and presents evidence from both neuroendocrine and pharmacological perspectives. Furthermore, recently emerging new functions of RFRPs are discussed with the overall goal of clarifying the functions of RFRPs beyond the hypothalamic-pituitary-gonadal axis.

Segregation of Acetylcholine and GABA in the Rat Superior Cervical Ganglia: Functional Correlation

Sympathetic neurons have the capability to segregate their neurotransmitters (NTs) and co-transmitters to separate varicosities of single axons; furthermore, in culture, these neurons can even segregate classical transmitters. In vivo sympathetic neurons employ acetylcholine (ACh) and other classical NTs such as gamma aminobutyric acid (GABA). Herein, we explore whether these neurons in vivo segregate these classical NTs in the superior cervical ganglia of the rat. We determined the topographical distribution of GABAergic varicosities, somatic GABA_A receptor, as well as the regional distribution of the segregation of ACh and GABA. We evaluated possible regional differences in efficacy of ganglionic synaptic transmission, in the sensitivity of GABA_A receptor to GABA and to the competitive antagonist picrotoxin (PTX). We found that sympathetic preganglionic neurons in vivo do segregate ACh and GABA. GABAergic varicosities and GABA_A receptor expression showed a rostro-caudal gradient along ganglia; in contrast, segregation exhibited a caudo-rostral gradient. These uneven regional distributions in expression of GABA, GABA_A receptors, and level of segregation correlate with stronger synaptic transmission found in the caudal region. Accordingly, GABA_A receptors of rostral region showed larger sensitivity to GABA and PTX. These results suggest the presence of different types of GABA_A receptors in each region that result in a different regional levels of endogenous GABA inhibition. Finally, we discuss a possible correlation of these different levels of GABA modulation and the function of the target organs innervated by rostral and caudal ganglionic neurons.

Brief Dark Exposure Reduces Tonic Inhibition in Visual Cortex

Tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABARs) sensing ambient levels of GABA can profoundly alter the membrane input resistance to affect cellular excitability. Therefore, regulation of tonic inhibition is an attractive mechanism to control the levels of cortical firing. In cortical pyramidal cells, tonic inhibition is regulated by age and several neurotransmitters and is affected by stroke and epilepsy. However, the possible role of sensory experience has not been examined. Here, we report that a brief 2-day exposure to dark reduces by 1/3 the inhibitory tonic conductance recorded in layer II/III pyramidal cells of the mouse juvenile (postnatal day 12-27) visual cortex. In these cells, tonic inhibition is carried primarily by GABARs containing the δ subunit. Consistently, the dark exposure reduction in conductance was associated with a reduction in δ subunit levels, which were not affected in control frontal cortex. We propose that a deprivation-induced reduction in tonic inhibition might serve a homeostatic function by increasing the firing levels of cells in deprived cortical circuits.

GABAergic Neuron-Specific Loss of Ube3a Causes Angelman Syndrome-Like EEG Abnormalities and Enhances Seizure Susceptibility

Loss of maternal UBE3A causes Angelman syndrome (AS), a neurodevelopmental disorder associated with severe epilepsy. We previously implicated GABAergic deficits onto layer (L) 2/3 pyramidal neurons in the pathogenesis of neocortical hyperexcitability, and perhaps epilepsy, in AS model mice. Here we investigate consequences of selective Ube3a loss from either GABAergic or glutamatergic neurons, focusing on the development of hyperexcitability within L2/3 neocortex and in broader circuit and behavioral contexts. We find that GABAergic Ube3a loss causes AS-like increases in neocortical EEG delta power, enhances seizure susceptibility, and leads to presynaptic accumulation of clathrin-coated vesicles (CCVs)-all without decreasing GABAergic inhibition onto L2/3 pyramidal neurons. Conversely, glutamatergic Ube3a loss fails to yield EEG abnormalities, seizures, or associated CCV phenotypes, despite impairing tonic inhibition onto L2/3 pyramidal neurons. These results substantiate GABAergic Ube3a loss as the principal cause of circuit hyperexcitability in AS mice, lending insight into ictogenic mechanisms in AS.

GABAergic Neurotransmission in the Premammillary Nucleus of the Turkey Hypothalamus Regulates Reproductive Seasonality and the Onset of Photorefractoriness

BACKGROUND/AIMS

Photoperiod is a major environmental cue in temperate-zone birds which synchronizes breeding with the time of year that offers the optimal environment for offspring survival. Despite continued long photoperiods, these birds eventually become refractory to the stimulating photoperiod and their reproductive systems regress. In this study, we characterized the role of γ-aminobutyric acid (GABA)ergic neurotransmission in modulating the response of the premammillary nucleus (PMM) to a gonad stimulatory photoperiod and the onset of photorefractoriness.

METHODS AND RESULTS

Bilateral ablation of the PMM blocked the light-induced neuroendocrine response from occurring in photosensitive turkeys. Microarray analyses revealed an increase in GABAergic activity in the PMM of photorefractory birds as opposed to photosensitive ones, and this enhanced GABAergic activity appeared to inhibit the photoperiodic signal. Additionally, GABAA and GABAB receptors were expressed by dopamine-melatonin neurons in the PMM, and the administration of the GABA receptor agonist baclofen blocked the photoperiodic reproductive neuroendocrine responses.

CONCLUSIONS

Consistent with the present findings, we propose that the long-sought-after mechanism underlying photorefractoriness is linked to the inhibitory actions of GABA. We suggest that (1) GABAergic interference with photoperiodic entrainment in the PMM initiates the photorefractory state and terminates the annual breeding season in temperate-zone birds, and (2) the PMM is a site of photoreception and photorefractoriness that controls the initiation and termination of avian reproductive seasonality.

How sympathetic are your spinal cord circuits?

NEW FINDINGS

What is the topic of this review? This review focuses on the role of gap junctions and interneurones in sympathetic control at the spinal cord level. What advances does it highlight? The review considers the importance of these local spinal circuits in contributing to rhythmic autonomic activity and enabling appropriate responses to homeostatic perturbations. Sympathetic control of end organs relies on the activity of sympathetic preganglionic neurones (SPNs) within the spinal cord. These SPNs exhibit heterogeneity with respect to function, neurochemistry, location, descending inputs and patterns of activity. Part of this heterogeneity is bestowed by local spinal circuitry. Our understanding of the role of these local circuits, including the significance of connections between the SPNs themselves through specialized gap junctions, is patchy. This report focuses on interneurones and gap junctions within these circuits. Gap junctions play a role in sympathetic control; they are located on SPNs in the intermediolateral cell column. Mefloquine, a chemical that blocks these gap junctions, reduces local rhythmic activity in the spinal cord slice and disrupts autonomic control in the working heart-brainstem preparation. The role that these gap junctions may play in health and disease in adult animals remains to be elucidated fully. Presympathetic interneurones are located in laminae V, VII and X and the intermediolateral cell column; those in lamina X are GABAergic and directly inhibit SPNs. The GABAergic inputs onto SPNs exert their effects through activation of synaptic and extrasynaptic receptors, which stabilize the membrane at negative potentials. The GABAergic interneurones contribute to rhythmic patterns of activity that can be generated in the spinal cord, because bicuculline reduces network oscillatory activity. These studies indicate that local spinal cord circuitry is critical in enabling appropriate levels and patterning of activity in sympathetic outflow. We need to understand how these circuits may be harnessed in the situation of spinal cord injury.

Astrocytic Actions on Extrasynaptic Neuronal Currents

In the last few decades, knowledge about astrocytic functions has significantly increased. It was demonstrated that astrocytes are not passive elements of the central nervous system (CNS), but active partners of neurons. There is a growing body of knowledge about the calcium excitability of astrocytes, the actions of different gliotransmitters and their release mechanisms, as well as the participation of astrocytes in the regulation of synaptic functions and their contribution to synaptic plasticity. However, astrocytic functions are even more complex than being a partner of the “tripartite synapse,” as they can influence extrasynaptic neuronal currents either by releasing substances or regulating ambient neurotransmitter levels. Several types of currents or changes of membrane potential with different kinetics and via different mechanisms can be elicited by astrocytic activity. Astrocyte-dependent phasic or tonic, inward or outward currents were described in several brain areas. Such currents, together with the synaptic actions of astrocytes, can contribute to neuromodulatory mechanisms, neurosensory and -secretory processes, cortical oscillatory activity, memory, and learning or overall neuronal excitability. This mini-review is an attempt to give a brief summary of astrocyte-dependent extrasynaptic neuronal currents and their possible functional significance.

Ectopic Expression of α6 and δ GABAA Receptor Subunits in Hilar Somatostatin Neurons Increases Tonic Inhibition and Alters Network Activity in the Dentate Gyrus

The role of GABAA receptor (GABAAR)-mediated tonic inhibition in interneurons remains unclear and may vary among subgroups. Somatostatin (SOM) interneurons in the hilus of the dentate gyrus show negligible expression of nonsynaptic GABAAR subunits and very low tonic inhibition. To determine the effects of ectopic expression of tonic GABAAR subtypes in these neurons, Cre-dependent viral vectors were used to express GFP-tagged GABAAR subunits (α6 and δ) selectively in hilar SOM neurons in SOM-Cre mice. In single-transfected animals, immunohistochemistry demonstrated strong expression of either the α6 or δ subunit; in cotransfected animals, both subunits were consistently expressed in the same neurons. Electrophysiology revealed a robust increase of tonic current, with progressively larger increases following transfection of δ, α6, and α6/δ subunits, respectively, indicating formation of functional receptors in all conditions and likely coassembly of the subunits in the same receptor following cotransfection. An in vitro model of repetitive bursting was used to determine the effects of increased tonic inhibition in hilar SOM interneurons on circuit activity in the dentate gyrus. Upon cotransfection, the frequency of GABAAR-mediated bursting in granule cells was reduced, consistent with a reduction in synchronous firing among hilar SOM interneurons. Moreover, in vivo studies of Fos expression demonstrated reduced activation of α6/δ-cotransfected neurons following acute seizure induction by pentylenetetrazole. The findings demonstrate that increasing tonic inhibition in hilar SOM interneurons can alter dentate gyrus circuit activity during strong stimulation and suggest that tonic inhibition of interneurons could play a role in regulating excessive synchrony within the network.

SIGNIFICANCE STATEMENT

In contrast to many hippocampal interneurons, somatostatin (SOM) neurons in the hilus of the dentate gyrus have very low levels of nonsynaptic GABAARs and exhibit very little tonic inhibition. In an effort to increase tonic inhibition selectively in these interneurons, we used Cre-dependent viral vectors in SOM-Cre mice to achieve interneuron-specific expression of the nonsynaptic GABAAR subunits (α6 and δ) in vivo. We show, for the first time, that such recombinant GFP-tagged GABAAR subunits are expressed robustly, assemble to form functional receptors, substantially increase tonic inhibition in SOM interneurons, and alter circuit activity within the dentate gyrus.

Tonic GABAA Receptors as Potential Target for the Treatment of Temporal Lobe Epilepsy

Tonic GABA_A receptors are a subpopulation of receptors that generate long-lasting inhibition and thereby control network excitability. In recent years, these receptors have been implicated in various neurological and psychiatric disorders, including Parkinson’s disease, schizophrenia, and epilepsy. Their distinct subunit composition and function, compared to phasic GABA_A receptors, opens the possibility to specifically modulate network properties. In this review, the role of tonic GABA_A receptors in epilepsy and as potential antiepileptic target will be discussed.

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

Wakefulness Is Governed by GABA and Histamine Cotransmission

Histaminergic neurons in the tuberomammilary nucleus (TMN) of the hypothalamus form a widely projecting, wake-active network that sustains arousal. Yet most histaminergic neurons contain GABA. Selective siRNA knockdown of the vesicular GABA transporter (vgat, SLC32A1) in histaminergic neurons produced hyperactive mice with an exceptional amount of sustained wakefulness. Ablation of the vgat gene throughout the TMN further sharpened this phenotype. Optogenetic stimulation in the caudate-putamen and neocortex of “histaminergic” axonal projections from the TMN evoked tonic (extrasynaptic) GABA_A receptor Cl⁻ currents onto medium spiny neurons and pyramidal neurons. These currents were abolished following vgat gene removal from the TMN area. Thus wake-active histaminergic neurons generate a paracrine GABAergic signal that serves to provide a brake on overactivation from histamine, but could also increase the precision of neocortical processing. The long range of histamine-GABA axonal projections suggests that extrasynaptic inhibition will be coordinated over large neocortical and striatal areas.

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Histaminergic axons corelease GABA into the neocortex and striatum

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The released GABA produces slow tonic inhibition

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Reducing vgat expression in histaminergic neurons increases wakefulness

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Histamine-GABA axons will coordinate tonic inhibition over large cortical areas

Potentiation of tonic GABAergic inhibition by activation of postsynaptic kainate receptors

Presynaptic kainate-type glutamate ionotropic receptors (KARs) that mediate either the depression or the facilitation of GABA release have been intensively studied. Little attention has been given to the modulation of GABAA receptors (GABAARs) by postsynaptic KARs. Recent studies suggest that two GABAAR populations, synaptic (sGABAAR) and extrasynaptic (eGABAAR) GABAARs, mediate phasic and tonic forms of inhibition, respectively. Tonic inhibition plays an important role in the excitability of neuronal circuits and the occurrence of epileptic seizures. For this study, we are the first to report that the activation of postsynaptic KARs by the KAR agonist, Kainic acid (KA, 5 μM), enhanced tonic inhibition by potentiating eGABAARs. KA enhanced THIP-induced eGABAAR currents and prolonged the rise and decay time of muscimol-induced sGABAAR/eGABAAR currents, but also depressed the amplitude of evoked inhibitory postsynaptic currents (IPSCs), unitary IPSCs (uIPSCs), and muscimol-induced sGABAAR/eGABAAR currents. The PKC inhibitor, staurosporine (1 μM), in the patch pipette solution fully blocked the KA-induced potentiation of tonic inhibition, suggesting the involvement of an intracellular PKC pathway. Our study suggests that the activation of postsynaptic KARs potentiates eGABAARs but depresses sGABAARs. By activating postsynaptic KARs, synaptically released glutamate depresses phasic inhibition to facilitate neuronal plasticity, but potentiates tonic inhibition to protect neurons from over-excitation.

Blocking the GABA transporter GAT-1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life-saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel-induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT-1 but not GAT-3. In the spinal dorsal horn, GAT-1 was expressed at presynaptic terminals and astrocytes while GAT-3 was only expressed in astrocytes. In rats with paclitaxel-induced neuropathic pain, the protein expression of GAT-1 was increased while GAT-3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel-induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT-1 but not GAT-3 transporters. Paclitaxel-induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT-1 inhibitor. These findings suggest that targeting GAT-1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel-induced neuropathic pain. Patients receiving paclitaxel for cancer therapy often develop neuropathic pain and have a reduced quality of life. In this study, we demonstrated that animals treated with paclitaxel develop neuropathic pain, have enhancements of GABA transporter-1 protein expression and global GABA uptake, as well as suppression of GABAergic tonic inhibition in the spinal dorsal horn. Pharmacological inhibition of GABA transporter-1 ameliorates the paclitaxel-induced suppression of GABAergic tonic inhibition and neuropathic pain. Thus, targeting GAT-1 transporters for reversing GABAergic disinhibition in the spinal dorsal horn could be a useful approach for treating paclitaxel-induced neuropathic pain.

Reduction in focal ictal activity following transplantation of MGE interneurons requires expression of the GABAA receptor α4 subunit

Despite numerous advances, treatment-resistant seizures remain an important problem. Loss of neuronal inhibition is present in a variety of epilepsy models and is suggested as a mechanism for increased excitability, leading to the proposal that grafting inhibitory interneurons into seizure foci might relieve refractory seizures. Indeed, transplanted medial ganglionic eminence interneuron progenitors (MGE-IPs) mature into GABAergic interneurons that increase GABA release onto cortical pyramidal neurons, and this inhibition is associated with reduced seizure activity. An obvious conclusion is that inhibitory coupling between the new interneurons and pyramidal cells underlies this effect. We hypothesized that the primary mechanism for the seizure-limiting effects following MGE-IP transplantation is the tonic conductance that results from activation of extrasynaptic GABA_A receptors (GABA_A-Rs) expressed on cortical pyramidal cells. Using in vitro and in vivo recording techniques, we demonstrate that GABA_A-R α4 subunit deletion abolishes tonic currents (I_tonic) in cortical pyramidal cells and leads to a failure of MGE-IP transplantation to attenuate cortical seizure propagation. These observations should influence how the field proceeds with respect to the further development of therapeutic neuronal transplants (and possibly pharmacological treatments).

GABA receptors in brain development, function, and injury

This review presents a brief overview of the γ-aminobutyric acid (GABA) system in the developing and mature central nervous system (CNS) and its potential connections to pathologies of the CNS. γ-aminobutyric acid (GABA) is a major neurotransmitter expressed from the embryonic stage and throughout life. At an early developmental stage, GABA acts in an excitatory manner and is implicated in many processes of neurogenesis, including neuronal proliferation, migration, differentiation, and preliminary circuit-building, as well as the development of critical periods. In the mature CNS, GABA acts in an inhibitory manner, a switch mediated by chloride/cation transporter expression and summarized in this review. GABA also plays a role in the development of interstitial neurons of the white matter, as well as in oligodendrocyte development. Although the underlying cellular mechanisms are not yet well understood, we present current findings for the role of GABA in neurological diseases with characteristic white matter abnormalities, including anoxic-ischemic injury, periventricular leukomalacia, and schizophrenia. Development abnormalities of the GABAergic system appear particularly relevant in the etiology of schizophrenia. This review also covers the potential role of GABA in mature brain injury, namely transient ischemia, stroke, and traumatic brain injury/post-traumatic epilepsy.

Zolpidem reduces hippocampal neuronal activity in freely behaving mice: a large scale calcium imaging study with miniaturized fluorescence microscope

Therapeutic drugs for cognitive and psychiatric disorders are often characterized by their molecular mechanism of action. Here we demonstrate a new approach to elucidate drug action on large-scale neuronal activity by tracking somatic calcium dynamics in hundreds of CA1 hippocampal neurons of pharmacologically manipulated behaving mice. We used an adeno-associated viral vector to express the calcium sensor GCaMP3 in CA1 pyramidal cells under control of the CaMKII promoter and a miniaturized microscope to observe cellular dynamics. We visualized these dynamics with and without a systemic administration of Zolpidem, a GABAA agonist that is the most commonly prescribed drug for the treatment of insomnia in the United States. Despite growing concerns about the potential adverse effects of Zolpidem on memory and cognition, it remained unclear whether Zolpidem alters neuronal activity in the hippocampus, a brain area critical for cognition and memory. Zolpidem, when delivered at a dose known to induce and prolong sleep, strongly suppressed CA1 calcium signaling. The rate of calcium transients after Zolpidem administration was significantly lower compared to vehicle treatment. To factor out the contribution of changes in locomotor or physiological conditions following Zolpidem treatment, we compared the cellular activity across comparable epochs matched by locomotor and physiological assessments. This analysis revealed significantly depressive effects of Zolpidem regardless of the animal's state. Individual hippocampal CA1 pyramidal cells differed in their responses to Zolpidem with the majority (∼ 65%) significantly decreasing the rate of calcium transients, and a small subset (3%) showing an unexpected and significant increase. By linking molecular mechanisms with the dynamics of neural circuitry and behavioral states, this approach has the potential to contribute substantially to the development of new therapeutics for the treatment of CNS disorders.

Sensitization of neurons in the central nucleus of the amygdala via the decreased GABAergic inhibition contributes to the development of neuropathic pain-related anxiety-like behaviors in rats

BACKGROUND

Despite high prevalence of anxiety accompanying with chronic pain, the mechanisms underlying pain-related anxiety are largely unknown. With its well-documented role in pain and emotion processing, the amygdala may act as a key player in pathogenesis of neuropathic pain-related anxiety. Pain-related plasticity and sensitization of CeA (central nucleus of the amygdala) neurons have been shown in several models of chronic pain. In addition, firing pattern of neurons with spike output can powerfully affect functional output of the brain nucleus, and GABAergic neurons are crucial in the modulation of neuronal excitability. In this study, we first investigated whether pain-related plasticity (e.g. alteration of neuronal firing patterns) and sensitization of CeA neurons contribute to nerve injury-evoked anxiety in neuropathic rats. Furthermore, we explored whether GABAergic disinhibition is responsible for regulating firing patterns and intrinsic excitabilities of CeA neurons as well as for pain-related anxiety in neuropathic rats.

RESULTS

We discovered that spinal nerve ligation (SNL) produced neuropathic pain-related anxiety-like behaviors in rats, which could be specifically inhibited by intra-CeA administration of anti-anxiety drug diazepam. Moreover, we found potentiated plasticity and sensitization of CeA neurons in SNL-induced anxiety rats, of which including: 1) increased burst firing pattern and early-adapting firing pattern; 2) increased spike frequency and intrinsic excitability; 3) increased amplitude of both after-depolarized-potential (ADP) and sub-threshold membrane potential oscillation. In addition, we observed a remarkable reduction of GABAergic inhibition in CeA neurons in SNL-induced anxiety rats, which was proved to be important for altered firing patterns and hyperexcitability of CeA neurons, thereby greatly contributing to the development of neuropathic pain-related anxiety. Accordantly, activation of GABAergic inhibition by intra-CeA administration of muscimol, a selective GABAA receptors agonist, could inhibit SNL-induced anxiety-like behaviors in neuropathic rats. By contrast, suppression of GABAergic inhibition by intra-CeA administration of bicuculline, a selective GABAA receptors antagonist, produced anxiety-like behavior in normal rats.

CONCLUSIONS

This study suggests that reduction of GABAergic inhibition may be responsible for potentiated plasticity and sensitization of CeA neurons, which likely underlie the enhanced output of amygdala and neuropathic pain-related anxiety in SNL rats.

Hydrogen peroxide increases GABAA receptor-mediated tonic current in hippocampal neurons

Hydrogen peroxide (H2O2), a key reactive oxygen species, is produced at low levels during normal cellular metabolism and at higher concentrations under pathological conditions such as ischemia-reperfusion injury. The mechanisms by which H2O2 contributes to physiological and pathological processes in the brain remain poorly understood. Inhibitory GABA type A (GABAA) receptors critically regulate brain function by generating tonic and synaptic currents; however, it remains unknown whether H2O2 directly modulates GABAA receptor function. Here, we performed patch-clamp recordings, together with pharmacological and genetic approaches, to investigate the effects of H2O2 on GABAA receptor-mediated tonic and synaptic currents recorded in cultured mouse hippocampal neurons and CA1 pyramidal neurons in hippocampal slices. We found that H2O2 caused a dramatic increase in tonic current, whereas synaptic currents were unaffected. This increase in tonic current resulted from an extracellular oxidative reaction, which increased the potency of GABA, but only when GABAA receptors were activated by low concentrations of GABA. Oxygen-glucose deprivation, which produces high endogenous levels of H2O2, similarly increased the tonic current. These results suggest that GABAA receptor-mediated tonic current, which is potentiated by H2O2, might contribute to H2O2-induced brain dysfunction.

Altered expression of δGABAA receptors in health and disease

γ-Aminobutyric acid type A receptors that contain the δ subunit (δGABAA receptors) are expressed in multiple types of neurons throughout the central nervous system, where they generate a tonic conductance that shapes neuronal excitability and synaptic plasticity. These receptors regulate a variety of important behavioral functions, including memory, nociception and anxiety, and may also modulate neurogenesis. Given their functional significance, δGABAA receptors are considered to be novel therapeutic targets for the treatment of memory dysfunction, pain, insomnia and mood disorders. These receptors are highly responsive to sedative-hypnotic drugs, general anesthetics and neuroactive steroids. A further remarkable feature of δGABAA receptors is that their expression levels are highly dynamic and fluctuate substantially during development and in response to physiological changes including stress and the reproductive cycle. Furthermore, the expression of these receptors varies in pathological conditions such as alcoholism, fragile X syndrome, epilepsy, depression, schizophrenia, mood disorders and traumatic brain injury. Such fluctuations in receptor expression have significant consequences for behavior and may alter responsiveness to therapeutic drugs. This review considers the alterations in the expression of δGABAA receptors associated with various states of health and disease and the implications of these changes.

Honeybee Kenyon cells are regulated by a tonic GABA receptor conductance

The higher cognitive functions of insects are dependent on their mushroom bodies (MBs), which are particularly large in social insects such as honeybees. MB Kenyon cells (KCs) receive multisensory input and are involved in associative learning and memory. In addition to receiving sensory input via excitatory nicotinic synapses, KCs receive inhibitory GABAergic input from MB feedback neurons. Cultured honeybee KCs exhibit ionotropic GABA receptor currents, but the properties of GABA-mediated inhibition in intact MBs are currently unknown. Here, using whole cell recordings from KCs in acutely isolated honeybee brain, we show that KCs exhibit a tonic current that is inhibited by picrotoxin but not by bicuculline. Bath application of GABA (5 μM) and taurine (1 mM) activate a tonic current in KCs, but l-glutamate (0.1-0.5 mM) has no effect. The tonic current is strongly potentiated by the allosteric GABAA receptor modulator pentobarbital and is reduced by inhibition of Ca(2+) channels with Cd(2+) or nifedipine. Noise analysis of the GABA-evoked current gives a single-channel conductance value for the underlying receptors of 27 ± 3 pS, similar to that of resistant to dieldrin (RDL) receptors. The amount of injected current required to evoke action potential firing in KCs is significantly lower in the presence of picrotoxin. KCs recorded in an intact honeybee head preparation similarly exhibit a tonic GABA receptor conductance that reduces neuronal excitability, a property that is likely to contribute to the sparse coding of sensory information in insect MBs.