Extrasynaptic GABAA Research Articles

Haploinsufficiency of GABAA Receptor-Associated Clptm1 Enhances Phasic and Tonic Inhibitory Neurotransmission, Suppresses Excitatory Synaptic Plasticity, and Impairs Memory.

The mechanisms utilized by neurons to regulate the efficacy of phasic and tonic inhibition and their impacts on synaptic plasticity and behavior are incompletely understood. Cleft lip and palate transmembrane protein 1 (Clptm1) is a membrane-spanning protein that interacts with multiple γ-aminobutyric acid type A receptor (GABAAR) subunits, trapping them in the endoplasmic reticulum and Golgi network. Overexpression and knock-down studies suggest that Clptm1 modulates GABAAR-mediated phasic inhibition and tonic inhibition as well as activity-induced inhibitory synaptic homeostasis in cultured hippocampal neurons. To investigate the role of Clptm1 in the modulation of GABAARs in vivo, we generated Clptm1 knock-out (KO) mice. Here, we show that genetic KO of Clptm1 elevated phasic and tonic inhibitory transmission in both male and female heterozygous mice. Although basal excitatory synaptic transmission was not affected, Clptm1 haploinsufficiency significantly blocked high-frequency stimulation-induced long-term potentiation (LTP) in hippocampal CA3→CA1 synapses. In the hippocampus-dependent contextual fear-conditioning behavior task, both male and female Clptm1 heterozygous KO mice exhibited impairment in contextual fear memory. In addition, LTP and contextual fear memory were rescued by application of L-655,708, a negative allosteric modulator of the extrasynaptic GABAAR α5 subunit. These results suggest that haploinsufficiency of Clptm1 contributes to cognitive deficits through altered synaptic transmission and plasticity by elevation of inhibitory neurotransmission, with tonic inhibition playing a major role.

Neurosteroid Modulation of Synaptic and Extrasynaptic GABAA Receptors of the Mouse Nucleus Accumbens.

The recent approval of formulations of the endogenous neurosteroid allopregnanolone (brexanolone) and the synthetic neuroactive steroid SAGE-217 (zuranolone) to treat postpartum depression (PPD) has encouraged further research to elucidate why these potent enhancers of GABAAR function are clinically effective in this condition. Dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens are associated with reward/motivation and brain imaging studies report that individuals with PPD show reduced activity of this pathway in response to reward and infant engagement. However, the influence of neurosteroids on GABA-ergic transmission in the nucleus accumbens has received limited attention. Here, we investigate, in the medium spiny neurons (MSNs) of the mouse nucleus accumbens core, the effect of allopregnanolone, SAGE-217 and other endogenous and synthetic steroids of interest on fast phasic and tonic inhibition mediated by synaptic (α1/2βγ2) and extrasynaptic (α4βδ) GABAARs, respectively. We present evidence suggesting the resident tonic current results from the spontaneous opening of δ-GABAARs, where the steroid-enhanced tonic current is GABA-dependent. Furthermore, we demonstrate local neurosteroid synthesis in the accumbal slice preparation and reveal that GABA-ergic neurotransmission of MSNs is influenced by an endogenous neurosteroid tone. Given the dramatic fluctuations in allopregnanolone levels during pregnancy and postpartum, this neurosteroid-mediated local fine-tuning of GABAergic transmission in the MSNs will probably be perturbed.

Enhancing GABAergic Tonic Inhibition Reduces Seizure-Like Activity in the Neonatal Mouse Hippocampus and Neocortex.

Approximately one-third of neonatal seizures do not respond to first-line anticonvulsants, including phenobarbital, which enhances phasic inhibition. Whether enhancing tonic inhibition decreases seizure-like activity in the neonate when GABA is mainly depolarizing at this age is unknown. We evaluated if increasing tonic inhibition using THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, gaboxadol], a δ-subunit-selective GABAA receptor agonist, decreases seizure-like activity in neonatal C57BL/6J mice (postnatal day P5-8, both sexes) using acute brain slices. Whole-cell patch-clamp recordings showed that THIP enhanced GABAergic tonic inhibitory conductances in layer V neocortical and CA1 pyramidal neurons and increased their rheobase without altering sEPSC characteristics. Two-photon calcium imaging demonstrated that enhancing the activity of extrasynaptic GABAARs decreased neuronal firing in both brain regions. In the 4-aminopyridine and the low-Mg2+ model of pharmacoresistant seizures, THIP reduced epileptiform activity in the neocortex and CA1 hippocampal region of neonatal and adult brain slices in a dose-dependent manner. We conclude that neocortical layer V and CA1 pyramidal neurons have tonic inhibitory conductances, and when enhanced, they reduce neuronal firing and decrease seizure-like activity. Therefore, augmenting tonic inhibition could be a viable approach for treating neonatal seizures.

The serine hydrolase ABHD6 controls survival and thermally induced seizures in a mouse model of Dravet syndrome

Evidence suggests that inhibition of α/β hydrolase-domain containing 6 (ABHD6) reduces seizures; however, the molecular mechanism of this therapeutic response remains unknown. We discovered that heterozygous expression of Abhd6 (Abhd6+/−) significantly reduced the premature lethality of Scn1a+/− mouse pups, a genetic mouse model of Dravet Syndrome (DS). Both Abhd6+/− mutation and pharmacological inhibition of ABHD6 reduced the duration and incidence of thermally induced seizures in Scn1a+/− pups. Mechanistically, the in vivo anti-seizure response resulting from ABHD6 inhibition is mediated by potentiation of gamma-aminobutyric acid receptors Type-A (GABAAR). Brain slice electrophysiology showed that blocking ABHD6 potentiates extrasynaptic (tonic) GABAAR currents that reduce dentate granule cell excitatory output without affecting synaptic (phasic) GABAAR currents. Our results unravel an unexpected mechanistic link between ABHD6 activity and extrasynaptic GABAAR currents that controls hippocampal hyperexcitability in a genetic mouse model of DS. Brief summaryThis study provides the first evidence for a mechanistic link between ABHD6 activity and the control of extrasynaptic GABAAR currents that controls hippocampal hyperexcitability in a genetic mouse model of Dravet Syndrome and can be targeted to dampened seizures.

The fate of interneurons, GABAA receptor sub-types and perineuronal nets in Alzheimer's disease.

Alzheimer's disease (AD) is the most common neurological disease, which is associated with gradual memory loss and correlated with synaptic hyperactivity and abnormal oscillatory rhythmic brain activity that precedes phenotypic alterations and is partly responsible for the spread of the disease pathology. Synaptic hyperactivity is thought to be because of alteration in the homeostasis of phasic and tonic synaptic inhibition, which is orchestrated by the GABAA inhibitory system, encompassing subclasses of interneurons and GABAA receptors, which play a vital role in cognitive functions, including learning and memory. Furthermore, the extracellular matrix, the perineuronal nets (PNNs) which often go unnoticed in considerations of AD pathology, encapsulate the inhibitory cells and neurites in critical brain regions and have recently come under the light for their crucial role in synaptic stabilisation and excitatory-inhibitory balance and when disrupted, serve as a potential trigger for AD-associated synaptic imbalance. Therefore, in this review, we summarise the current understanding of the selective vulnerability of distinct interneuron subtypes, their synaptic and extrasynaptic GABAA R subtypes as well as the changes in PNNs in AD, detailing their contribution to the mechanisms of disease development. We aim to highlight how seemingly unique malfunction in each component of the interneuronal GABA inhibitory system can be tied together to result in critical circuit dysfunction, leading to the irreversible symptomatic damage observed in AD.

Analyzing the mechanisms that facilitate the subtype-specific assembly of γ-aminobutyric acid type A receptors.

Impaired inhibitory signaling underlies the pathophysiology of many neuropsychiatric and neurodevelopmental disorders including autism spectrum disorders and epilepsy. Neuronal inhibition is regulated by synaptic and extrasynaptic γ-aminobutyric acid type A receptors (GABAARs), which mediate phasic and tonic inhibition, respectively. These two GABAAR subtypes differ in their function, ligand sensitivity, and physiological properties. Importantly, they contain different α subunit isoforms: synaptic GABAARs contain the α1–3 subunits whereas extrasynaptic GABAARs contain the α4–6 subunits. While the subunit composition is critical for the distinct roles of synaptic and extrasynaptic GABAAR subtypes in inhibition, the molecular mechanism of the subtype-specific assembly has not been elucidated. To address this issue, we purified endogenous α1- and α4-containing GABAARs from adult murine forebrains and examined their subunit composition and interacting proteins using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and quantitative analysis. We found that the α1 and α4 subunits form separate populations of GABAARs and interact with distinct sets of binding proteins. We also discovered that the β3 subunit, which co-purifies with both the α1 and α4 subunits, has different levels of phosphorylation on serines 408 and 409 (S408/9) between the two receptor subtypes. To understand the role S408/9 plays in the assembly of α1- and α4-containing GABAARs, we examined the effects of S408/9A (alanine) knock-in mutation on the subunit composition of the two receptor subtypes using LC-MS/MS and quantitative analysis. We discovered that the S408/9A mutation results in the formation of novel α1α4-containing GABAARs. Moreover, in S408/9A mutants, the plasma membrane expression of the α4 subunit is increased whereas its retention in the endoplasmic reticulum is reduced. These findings suggest that S408/9 play a critical role in determining the subtype-specific assembly of GABAARs, and thus the efficacy of neuronal inhibition.

P140. Preference for Extrasynaptic GABAA Receptors Conveys a Wider Therapeutic Window Between Anxiolytic and Sedative-Like Effects in Rats for the Positive Allosteric Modulator, PRAX-114, Compared With Zuranolone

The antidepressant effects of neuroactive steroid positive allosteric modulators of GABAA receptors (GABAAR PAMs) likely involve extrasynaptic GABAAR. PRAX-114 is a novel GABAAR PAM with preference for extrasynaptic GABAAR, which is hypothesized to improve tolerability. Here we compared the therapeutic window of PRAX-114 and zuranolone, using EEG β-power as a biomarker of GABAAR potentiation.

Differential assembly diversifies GABAA receptor structures and signalling.

Type A γ-aminobutyric acid receptors (GABAARs) are pentameric ligand-gated chloride channels that mediate fast inhibitory signalling in neural circuits1,2 and can be modulated by essential medicines including general anaesthetics and benzodiazepines3. Human GABAAR subunits are encoded by 19 paralogous genes that can, in theory, give rise to 495,235 receptor types. However, the principles that govern the formation of pentamers, the permutational landscape of receptors that may emerge from a subunit set and theeffect that this has on GABAergic signalling remain largely unknown. Here we use cryogenic electron microscopy to determine the structures of extrasynaptic GABAARs assembled from α4, β3 and δ subunits, and their counterparts incorporating γ2 instead of δ subunits. In each case, we identified two receptor subtypes with distinct stoichiometries and arrangements, all four differing from those previously observed for synaptic, α1-containing receptors4-7. This, in turn, affects receptor responses to physiological and synthetic modulators by creating or eliminating ligand-binding sites at subunit interfaces. We provide structural and functional evidence that selected GABAAR arrangements can act as coincidence detectors, simultaneously responding to two neurotransmitters: GABA and histamine. Using assembly simulations and single-cell RNA sequencing data8,9, we calculated the upper bounds for receptor diversity in recombinant systems and in vivo. We propose that differential assembly is a pervasive mechanism for regulating the physiology and pharmacology of GABAARs.

Distinct regulation of tonic GABAergic inhibition by NMDA receptor subtypes.

SUMMARYTonic inhibition mediated by extrasynaptic GABAARs regulates various brain functions. However, the mechanisms that regulate tonic inhibition remain largely unclear. Here, we report distinct actions of GluN2A- and GluN2B-NMDA receptors (NMDARs) on tonic inhibition in hippocampal neurons under basal and high activity conditions. Specifically, overexpression of GluN2B, but not GluN2A, reduces α5-GABAAR surface expression and tonic currents. Additionally, knockout of GluN2A and GluN2B decreases and increases tonic currents, respectively. Mechanistically, GluN2A-NMDARs inhibit and GluN2B-NMDARs promote α5-GABAAR internalization, resulting in increased and decreased surface α5-GABAAR expression, respectively. Furthermore, GluN2A-NMDARs, but not GluN2B-NMDARs, are required for homeostatic potentiation of tonic inhibition induced by prolonged increase of neuronal activity. Last, tonic inhibition decreases during acute seizures, whereas it increases 24 h later, involving GluN2-NMDAR-dependent signaling. Collectively, these data reveal an NMDAR subunit-specific regulation of tonic inhibition in physiological and pathological conditions and provide mechanistic insight into activity-dependent modulation of tonic inhibition.

Modelling the functional roles of synaptic and extra-synaptic γ-aminobutyric acid receptor dynamics in circadian timekeeping.

In the suprachiasmatic nucleus (SCN), γ-aminobutyric acid (GABA) is a primary neurotransmitter. GABA can signal through two types of GABAA receptor subunits, often referred to as synaptic GABAA (gamma subunit) and extra-synaptic GABAA (delta subunit). To test the functional roles of these distinct GABAA in regulating circadian rhythms, we developed a multicellular SCN model where we could separately compare the effects of manipulating GABA neurotransmitter or receptor dynamics. Our model predicted that blocking GABA signalling modestly increased synchrony among circadian cells, consistent with published SCN pharmacology. Conversely, the model predicted that lowering GABAA receptor density reduced firing rate, circadian cell fraction, amplitude and synchrony among individual neurons. When we tested these predictions, we found that the knockdown of delta GABAA reduced the amplitude and synchrony of clock gene expression among cells in SCN explants. The model further predicted that increasing gamma GABAA densities could enhance synchrony, as opposed to increasing delta GABAA densities. Overall, our model reveals how blocking GABAA receptors can modestly increase synchrony, while increasing the relative density of gamma over delta subunits can dramatically increase synchrony. We hypothesize that increased gamma GABAA density in the winter could underlie the tighter phase relationships among SCN cells.

PRAX-114 Produces Robust Increases in EEG Beta Frequency Power, a Translational Biomarker of GABA-A Receptor Modulation, Without Dose-Limiting Sedation

PRAX-114 is a GABA-A receptor positive allosteric modulator (GABAA-R-PAM) neuroactive steroid which achieves preferential potentiation of extrasynaptic GABAAR compared to synaptic GABAAR. GABAAR-PAMs cause increases in EEG β-frequency band power. The effects of PRAX-114 on quantitative EEG (qEEG) were measured in rodents and healthy human volunteers.

Tonic GABAergic Inhibition Is Essential for Nerve Injury-Induced Afferent Remodeling in the Somatosensory Thalamus and Ectopic Sensations

Peripheral nerve injury induces functional and structural remodeling of neural circuits along the somatosensory pathways, forming the basis for somatotopic reorganization and ectopic sensations, such as referred phantom pain. However, the mechanisms underlying that remodeling remain largely unknown. Whisker sensory nerve injury drives functional remodeling in the somatosensory thalamus: the number of afferent inputs to each thalamic neuron increases from one to many. Here, we report that extrasynaptic γ-aminobutyric acid-type A receptor (GABAAR)-mediated tonic inhibition is necessary for that remodeling. Extrasynaptic GABAAR currents were potentiated rapidly after nerve injury in advance of remodeling. Pharmacological activation of the thalamic extrasynaptic GABAARs in intact mice induced similar remodeling. Notably, conditional deletion of extrasynaptic GABAARs in the thalamus rescued both the injury-induced remodeling and the ectopic mechanical hypersensitivity. Together, our results reveal a molecular basis for injury-induced remodeling of neural circuits and may provide a new pharmacological target for referred phantom sensations after peripheral nerve injury.

Hijacking of GABAA Receptors by Mutant Glycine Receptors

Startle disease results from mutations in genes encoding inhibitory GlyR α1 and β subunits or the presynaptic glycine transporter GlyT2. However, the most effective therapies are benzodiazepines that potentiate inhibitory GABAAR function. A recent publication by Zou etal. adds further complexity by suggesting that dominant GlyR α1 mutants assemble into pre- and extrasynaptic GABAARs.

Human Hyperekplexic Mutations in Glycine Receptors Disinhibit the Brainstem by Hijacking GABAA Receptors.

SummaryHyperekplexia disease is usually caused by naturally occurring point mutations in glycine receptors (GlyRs). However, the γ-aminobutyric acid type A receptor (GABAAR) seems to be also involved regarding the therapeutic basis for hyperekplexia using benzodiazepines, which target GABAARs but not GlyRs. Here, we show that the function of GABAARs was significantly impaired in the hypoglossal nucleus of hyperekplexic transgenic mice. Such impairment appeared to be mediated by interaction between GABAAR and mutant GlyR. The GABAAR dysfunction was caused only by mutant GlyR consisting of homomeric α1 subunits, which locate primarily at pre- and extra-synaptic sites. In addition, the rescue effects of diazepam were attenuated by Xli-093, which specifically blocked diazepam-induced potentiation on α5-containing GABAAR, a major form of pre- and extra-synaptic GABAAR in the brainstem. Thus, our results suggest that the pre- and extra-synaptic GABAARs could be a potential therapeutic target for hyperekplexia disease caused by GlyR mutations.

Enaminone Modulators of Extrasynaptic α4β3δ γ-Aminobutyric AcidA Receptors Reverse Electrographic Status Epilepticus in the Rat After Acute Organophosphorus Poisoning.

Seizures induced by organophosphorus nerve agent exposure become refractory to treatment with benzodiazepines because these drugs engage synaptic γ-aminobutyric acid-A receptors (GABAARs) that rapidly internalize during status epilepticus (SE). Extrasynaptic GABAARs, such as those containing α4β3δ subunits, are a putative pharmacological target to comprehensively manage nerve agent-induced seizures since they do not internalize during SE and are continuously available for activation. Neurosteroids related to allopregnanolone have been tested as a possible replacement for benzodiazepines because they target both synaptic and extrasynaptic GABAARs receptors. A longer effective treatment window, extended treatment efficacy, and enhanced neuroprotection represent significant advantages of neurosteroids over benzodiazepines. However, neurosteroid use is limited by poor physicochemical properties arising from the intrinsic requirement of the pregnane steroid core structure for efficacy rendering drug formulation problematic. We tested a non-steroidal enaminone GABAAR modulator that interacts with both synaptic and extrasynaptic GABAARs on a binding site distinct from neurosteroids or benzodiazepines for efficacy to control electrographic SE induced by diisopropyl fluorophosphate or soman intoxication in rats. Animals were treated with standard antidotes, and experimental therapeutic treatment was given following 1 h (diisopropyl fluorophosphate model) or 20 min (soman model) after SE onset. We found that the enaminone 2-261 had an extended duration of seizure termination (>10 h) in the diisopropyl fluorophosphate intoxication model in the presence or absence of midazolam (MDZ). 2-261 also moderately potentiated MDZ in the soman-induced seizure model but had limited efficacy as a stand-alone anticonvulsant treatment due to slow onset of action. 2-261 significantly reduced neuronal death in brain areas associated with either diisopropyl fluorophosphate- or soman-induced SE. 2-261 represents an alternate chemical template from neurosteroids for enhancing extrasynaptic α4β3δ GABAAR activity to reverse SE from organophosphorous intoxication.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission.

Ionotropic type of γ-aminobutyric acid receptors (GABAARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABAARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABAARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABAARs (s-GABAARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABAARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABAARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABAARs functional properties and experimental approaches that allow isolation of s-GABAARs effects from those of conventional (GABA-dependent) GABAARs.

GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit.

The lateral superior olive (LSO), a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABAB Rs modulate Ca2+ influx into medial nucleus of the trapezoid body axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABAA Rs. RNA sequencing revealed highly abundant δ subunits, which are characteristic of extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter. GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole-cell patch-clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABAA receptors (GABAA Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABAA Rs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca2+ influx into MNTB axon terminals following activation of presynaptic GABAB Rs. GABAB R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABAA Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABAA R agonist. RNA sequencing showed high mRNA levels for the δ-subunit in the LSO. Moreover, GABA transporters GAT-1 and GAT-3 appear to control extracellular GABA. Finally, we show an age-dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABAA Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

HIV gp120 upregulates tonic inhibition through α5-containing GABAARs

HIV-Associated Neurocognitive disorder (HAND) affects nearly half of infected patients. The HIV envelope protein gp120 is shed by infected cells and is a potent neurotoxin in vitro that reproduces many aspects of HAND when expressed in vivo. Here, we show that HIV gp120 increases the amplitude of a tonic current mediated by γ-aminobutyric acid type-A receptors (GABAARs).Treating rat hippocampal cultures with 600 pM gp120IIIB for 4 h increased a tonic bicuculline-sensitive current, which remained elevated for 24 h. The increased current resulted from upregulation of extrasynaptic α5-containing GABAARs, as indicated by inhibition with the selective inverse agonist basmisanil. Treatment with gp120 increased α5-GABAAR immunoreactivity on the cell surface without new protein synthesis. The increase in tonic inhibition was prevented by a C-X-C chemokine receptor type 4 (CXCR4) antagonist or elimination of microglia from the culture. Treatment with interleukin-1β (IL-1β) increased the tonic current and an IL-1 receptor antagonist blocked the gp120-evoked response. Pharmacological or genetic inhibition of p38 mitogen-activated protein kinase (MAPK) prevented the gp120-evoked increase in tonic current and direct activation of a mutant form of p38 MAPK expressed in neurons increased the current.Collectively, these data show that gp120 activates CXCR4 to stimulate microglia to release IL-1β. Subsequent stimulation of IL-1 receptors activates p38 MAPK in neurons leading to the upregulation of α5-containing GABAARs. Increased tonic inhibition impairs neuroplasticity and inhibition of α5-containing GABAARs improves cognitive function in disease models. Thus, gp120-induced upregulation of α5-containing GABAARs presents a novel therapeutic target for HAND.

Positive allosteric modulators of nonbenzodiazepine γ-aminobutyric acidA receptor subtypes for the treatment of chronic pain.

Chronic neuropathic pain may be caused, in part, by loss of inhibition in spinal pain processing pathways due to attenuation of local GABAergic tone. Nociception and nocifensive behaviors are reduced after enhancement of tonically activated extrasynaptic GABAAR-mediated currents by agonist ligands for δ subunit-containing GABAARs. However, typical ligands that target δ subunit-containing GABAARs are limited due to sedative effects at higher doses. We used the spinal nerve ligation (SNL) and gp120 models of experimental neuropathic pain to evaluate compound 2-261, a nonbenzodiazepine site positive allosteric modulator of α4β3δ GABAARs optimized to be nonsedative by selective activation of β2/3-subunit-containing GABAARs over receptor subtypes incorporating β1 subunits. Similar levels of 2-261 were detected in the brain and plasma after intraperitoneal administration. Although systemic 2-261 did not alter sensory thresholds in sham-operated animals, it significantly reversed SNL-induced thermal and tactile hypersensitivity in a GABAAR-dependent fashion. Intrathecal 2-261 produced conditioned place preference and elevated dopamine levels in the nucleus accumbens of nerve-injured, but not sham-operated, rats. In addition, systemic pretreatment with 2-261 blocked conditioned place preference from spinal clonidine in SNL rats. Moreover, 2-261 reversed thermal hyperalgesia and partially reversed tactile allodynia in the gp120 model of HIV-related neuropathic pain. The effects of 2-261 likely required interaction with the α4β3δ GABAAR because 2-301, a close structural analog of 2-261 with limited extrasynaptic receptor efficacy, was not active. Thus, 2-261 may produce pain relief with diminished side effects through selective modulation of β2/3-subunit-containing extrasynaptic GABAARs.

Feature Article: Selective modulation of tonically active GABAA receptor functional subgroups by G-proteins and protein kinase C.

Here we study intracellular mechanisms which regulate inhibitory signaling delivered through continuously (tonically) open ionotropic receptors of γ-aminobutyric acid (GABA) of dentate gyrus granule cells (DGCs). We found that, apart of classical GABA-A receptors (GABAARs) which can be activated by GABA binding, a significant part of tonic inhibitory current is delivered by newly discovered spontaneously opening GABAARs (s-GABAARs), which enter active state without binding of GABA. We have also found that conventional GABAARs and s-GABAARs are regulated by different intracellular mechanisms, which may overlap and thus induce various signaling repercussions. Our results demonstrate that s-GABAARs play a key role in the mechanism that implements DGCs functional role in the brain. On top of that, since regulatory mechanisms under study are affected in a number of pathological states, our results may have broad implications for treatment of neurological disorders.