alpha1beta2delta, a silent GABAA receptor: recruitment by tracazolate and neurosteroids

A therapeutic small molecule enhances γ-oscillations and improves cognition/memory in Alzheimer's disease model mice

Brain rhythms provide the timing for recruitment of brain activity required for linking together neuronal ensembles engaged in specific tasks. The γ-oscillations (30 to 120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here, we report on a potent brain-permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a class of therapeutics for AD. We employed anatomical, in vitro and in vivo electrophysiological, and behavioral methods to examine the effects of our lead therapeutic candidate small molecule. As a novel in central nervous system pharmacotherapy, our lead molecule acts as a potent, efficacious, and selective negative allosteric modulator of the γ-aminobutyric acid type A receptors most likely assembled from α1β2δ subunits. These receptors, identified through anatomical and pharmacological means, underlie the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. When orally administered twice daily for 2 wk, DDL-920 restored the cognitive/memory impairments of 3- to 4-mo-old AD model mice as measured by their performance in the Barnes maze. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

A therapeutic small molecule lead enhances γ-oscillations and improves cognition/memory in Alzheimer's disease model mice

Brain rhythms provide the timing and concurrence of brain activity required for linking together neuronal ensembles engaged in specific tasks. In particular, the γ-oscillations (30-120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here we report on a potent brain permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a new class of therapeutics for AD. As a first in CNS pharmacotherapy, our lead candidate acts as a potent, efficacious, and selective negative allosteric modulator (NAM) of the γ-aminobutyric acid type A receptors (GABA A Rs) assembled from α1β2δ subunits. We identified these receptors through anatomical and pharmacological means to mediate the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

Non-sedative cortical EEG signatures of allopregnanolone and functional comparators

Neurosteroids that positively modulate GABA_A receptors are among a growing list of rapidly acting antidepressants, including ketamine and psychedelics. To develop increasingly specific treatments with fewer side effects, we explored the possibility of EEG signatures in mice, which could serve as a cross-species screening tool. There are few studies of the impact of non-sedative doses of rapid antidepressants on EEG in either rodents or humans. Here we hypothesize that EEG features may separate a rapid antidepressant neurosteroid, allopregnanolone, from other GABA_A positive modulators, pentobarbital and diazepam. Further, we compared the actions GABA modulators with those of ketamine, an NMDA antagonist and prototype rapid antidepressant. We examined EEG spectra during active exploration at two cortical locations and examined cross-regional and cross-frequency interactions. We found that at comparable doses, the effects of allopregnanolone, despite purported selectivity for certain GABA_AR subtypes, was indistinguishable from pentobarbital during active waking exploration. The actions of diazepam had recognizable common features with allopregnanolone and pentobarbital but was also distinct, consistent with subunit selectivity of benzodiazepines. Finally, ketamine exhibited no distinguishing overlap with allopregnanolone in the parameters examined. Our results suggest that rapid antidepressants with different molecular substrates may remain separated at the level of large-scale ensemble activity, but the studies leave open the possibility of commonalities in more discrete circuits and/or in the context of a dysfunctional brain.

Barbiturates and pyrazolopyridines for the treatment of postpartum depression-repurposing of two drug classes

Zulresso (brexanolone) is an aqueous formulation of the neurosteroid, allopregnanolone, and the only FDA-approved medication for the treatment of postpartum depression (PPD). While brexanolone is effective for the treatment of PPD, lengthy infusion time and high cost can be prohibitive. Failure of GABA_A receptors to adapt to fluctuating neurosteroid levels is considered to predispose women to mood disorders in the postpartum period. Brexanolone is thought to act via stimulation of δ subunit-containing GABA_A receptors, which are extrasynaptic and localized to particular brain regions. Neurosteroid stimulation of δ subunit-containing GABA_A receptors leads to sustained inhibition (hyperpolarization) of GABAergic neurons, which makes δ subunit-containing GABA_A receptors a potentially important pharmacologic target. Barbiturates and pyrazolopyridines are potent stimulators of δ subunit-containing GABA_A receptors and therefore potentially cost-effective treatments for PPD. Barbiturates are often not prescribed, owing to risk of dependence and respiratory depression. The pyrazolopyridines were tested several decades ago for anxiety and depression but never developed commercially. Herein we use the FDA-approved dosing schedule of brexanolone and GABA_A receptor binding data from various animal models to examine the safety, efficacy, and potential clinical utility of barbiturates and pyrazolopyridines for the treatment of PPD. We suggest consideration of repurposing barbiturates and pyrazolopyridines as safe and readily available treatment alternatives for PPD.

Physiological markers of rapid antidepressant effects of allopregnanolone

The rise of ketamine and brexanolone as rapid antidepressant treatments raises the question of common mechanisms. Both drugs act without the long onset time of traditional antidepressants such as selective serotonin reuptake inhibitors. The drugs also share the interesting feature of benefit that persists beyond the initial drug lifetime. Here, we briefly review literature on functional changes that may mark the triggering mechanism of rapid antidepressant actions. Because ketamine has a longer history of study as a rapid antidepressant, we use this literature as a template to guide hypotheses about common action. Brexanolone has the complication of being a formulation of a naturally occurring neurosteroid; thus, endogenous levels need to be considered when studying the impact of exogenous administration. We conclude that network disinhibition and increased high-frequency oscillations are candidates to mediate acute triggering effects of rapid antidepressants.

Delta-containing GABAA receptors in pain management: Promising targets for novel analgesics

Communication between nerve cells depends on the balance between excitatory and inhibitory circuits. GABA, the major inhibitory neurotransmitter, regulates this balance and insufficient GABAergic activity is associated with numerous neuropathological disorders including pain. Of the various GABA_A receptor subtypes, the δ-containing receptors are particularly interesting drug targets in management of chronic pain. These receptors are pentameric ligand-gated ion channels composed of α, β and δ subunits and can be activated by ambient levels of GABA to generate tonic conductance. However, only a few ligands preferentially targeting δ-containing GABA_A receptors have so far been identified, limiting both pharmacological understanding and drug-discovery efforts, and more importantly, understanding of how they affect pain pathways. Here, we systemically review and discuss the known drugs and ligands with analgesic potential targeting δ-containing GABA_A receptors and further integrate the biochemical nature of the receptors with clinical perspectives in pain that might generate interest among researchers and clinical physicians to encourage analgesic discovery efforts leading to more efficient therapies.

Efficient expression of concatenated α1β2δ and α1β3δ GABAA receptors, their pharmacology and stoichiometry

BACKGROUND AND PURPOSE

GABA_A receptors containing δ-subunits are notorious for being difficult to study in vitro due to heterogeneity of expressed receptor populations and low GABA-evoked current amplitudes. Thus, there are some published misconceptions and contradictory conclusions made regarding the pharmacology and stoichiometry of δ-containing receptors. The aim of this study was to obtain robust homogenous expression of α1βδ receptors for in-depth investigation.

EXPERIMENTAL APPROACH

Novel δ-containing pentameric concatenated constructs were designed. The resulting α1β2δ and α1β3δ GABA_A receptor concatemers were investigated by two-electrode voltage-clamp electrophysiology using Xenopus laevis oocytes.

KEY RESULTS

First, while homogenous α1βδ GABA_A receptor pools could not be obtained by manipulating the ratio of injected cRNAs of free α1, β2/3, and δ subunits, concatenated pentameric α1β2δ and α1β3δ constructs resulted in robust expression levels of concatemers. Second, by using optimised constructs that give unidirectional assembly of concatemers, we found that the δ subunit cannot directly participate in GABA binding and receptor activation. Hence, functional δ-containing receptors are likely to all have a conventional 2α:2β:1δ stoichiometry arranged as βαβαδ when viewed counterclockwise from the extracellular side. Third, α1β2/3δ receptors were found to express efficiently in X. laevis oocytes but have a low estimated open probability of ~0.5% upon GABA activation. Because of this, these receptors are uniquely susceptible to positive allosteric modulation by, for example, neurosteroids.

CONCLUSION AND IMPLICATIONS

Our data answer important outstanding questions regarding the pharmacology and stoichiometry of α1δ-containing GABA_A receptors and pave the way for future analysis and drug discovery efforts.

Increased Dendritic Branching of and Reduced δ-GABAA Receptor Expression on Parvalbumin-Positive Interneurons Increase Inhibitory Currents and Reduce Synaptic Plasticity at Puberty in Female Mouse CA1 Hippocampus

Parvalbumin positive (PV+) interneurons play a pivotal role in cognition and are known to be regulated developmentally and by ovarian hormones. The onset of puberty represents the end of a period of optimal learning when impairments in synaptic plasticity are observed in the CA1 hippocampus of female mice. Therefore, we tested whether the synaptic inhibitory current generated by PV+ interneurons is increased at puberty and contributes to these deficits in synaptic plasticity. To this end, the spontaneous inhibitory postsynaptic current (sIPSC) was recorded using whole-cell patch-clamp techniques from CA1 pyramidal cells in the hippocampal slice before (PND 28–32) and after the onset of puberty in female mice (~PND 35–44, assessed by vaginal opening). sIPSC frequency and amplitude were significantly increased at puberty, but these measures were reduced by 1 μM DAMGO [1 μM, (D-Ala², N-MePhe⁴, Gly-ol)-enkephalin], which silences PV+ activity via μ-opioid receptor targets. At puberty, dendritic branching of PV+ interneurons in GAD67-GFP mice was increased, while expression of the δ subunit of the GABA_A receptor (GABAR) on these interneurons decreased. Both frequency and amplitude of sIPSCs were significantly increased in pre-pubertal mice with reduced δ expression, suggesting a possible mechanism. Theta burst induction of long-term potentiation (LTP), an in vitro model of learning, is impaired at puberty but was restored to optimal levels by DAMGO administration, implicating inhibition via PV+ interneurons as one cause. Administration of the neurosteroid/stress steroid THP (30 nM, 3α-OH, 5α-pregnan-20-one) had no effect on sIPSCs. These findings suggest that phasic inhibition generated by PV+ interneurons is increased at puberty when it contributes to impairments in synaptic plasticity. These results may have relevance for the changes in cognitive function reported during early adolescence.

GABAAR isoform and subunit structural motifs determine synaptic and extrasynaptic receptor localisation

GABA_A receptors (GABA_ARs) are the principal inhibitory neurotransmitter receptors in the central nervous system. They control neuronal excitability by synaptic and tonic forms of inhibition mostly mediated by different receptor subtypes located in specific cell membrane subdomains. A consensus suggests that α1-3βγ comprise synaptic GABA_ARs, whilst extrasynaptic α4βδ, α5βγ and αβ isoforms largely underlie tonic inhibition. Although some structural features that enable the spatial segregation of receptors are known, the mobility of key synaptic and extrasynaptic GABA_ARs are less understood, and yet this is a key determinant of the efficacy of GABA inhibition. To address this aspect, we have incorporated functionally silent α-bungarotoxin binding sites (BBS) into prominent hippocampal GABA_AR subunits which mediate synaptic and tonic inhibition. Using single particle tracking with quantum dots we demonstrate that GABA_ARs that are traditionally considered to mediate synaptic or tonic inhibition are all able to access inhibitory synapses. These isoforms have variable diffusion rates and are differentially retained upon entering the synaptic membrane subdomain. Interestingly, α2 and α4 subunits reside longer at synapses compared to α5 and δ subunits. Furthermore, a high proportion of extrasynaptic δ-containing receptors exhibited slower diffusion compared to δ subunits at synapses. A chimera formed from δ-subunits, with the intracellular domain of γ2L, reversed this behaviour. In addition, we observed that receptor activation affected the diffusion of extrasynaptic, but not of synaptic GABA_ARs. Overall, we conclude that the differential mobility profiles of key synaptic and extrasynaptic GABA_ARs are determined by receptor subunit composition and intracellular structural motifs. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

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.

Maternal care-related differences in males and females rats' sensitivity to ethanol and the associations between the GABAergic system and steroids in males

This study investigated the effect of maternal care on adolescent ethanol consumption, sensitivity to ethanol-induced hypnosis, as well as gonadal hormones and γ-aminobutyric acid type-A (GABA_A ) systems. Long Evans rat dams were categorized by maternal licking/grooming (LG) frequency into High- and Low-LG mothers. Both female and male offspring from Low-LG rats demonstrated a greater sensitivity to ethanol-induced hypnosis in the loss-of-righting-reflex test at ethanol doses of 3.0 and 3.5 g/kg during late-adolescence (postnatal Day 50) but not at mid-adolescence (postnatal Day 42). However, we found no effect of maternal care on consumption of a 5% ethanol solution in a two-bottle choice test. We further investigated the association between the observed variations in sensitivity to ethanol-induced hypnosis and baseline hormonal levels in males. In male offspring from Low-LG mothers compared to High-LG mothers, baseline plasma corticosterone and progesterone levels were higher. GABA_A α1 and δ subunit expressions were also higher in the cerebral cortex of Low-LG males but lower in the cerebellar synaptosomal fraction. Early environmental influences on adolescent sensitivity to ethanol-induced hypnosis, consumption, and preference may be mediated by gonadal hormones and possibly through GABAergic functions.

Comparison of αβδ and αβγ GABAA receptors: Allosteric modulation and identification of subunit arrangement by site-selective general anesthetics

GABA_A receptors play a dominant role in mediating inhibition in the mature mammalian brain, and defects of GABAergic neurotransmission contribute to the pathogenesis of a variety of neurological and psychiatric disorders. Two types of GABAergic inhibition have been described: αβγ receptors mediate phasic inhibition in response to transient high-concentrations of synaptic GABA release, and αβδ receptors produce tonic inhibitory currents activated by low-concentration extrasynaptic GABA. Both αβδ and αβγ receptors are important targets for general anesthetics, which induce apparently different changes both in GABA-dependent receptor activation and in desensitization in currents mediated by αβγ vs. αβδ receptors. Many of these differences are explained by correcting for the high agonist efficacy of GABA at most αβγ receptors vs. much lower efficacy at αβδ receptors. The stoichiometry and subunit arrangement of recombinant αβγ receptors are well established as β-α-γ-β-α, while those of αβδ receptors remain controversial. Importantly, some potent general anesthetics selectively bind in transmembrane inter-subunit pockets of αβγ receptors: etomidate acts at β⁺/α^- interfaces, and the barbiturate R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) acts at α⁺/β^- and γ⁺/β^- interfaces. Thus, these drugs are useful as structural probes in αβδ receptors formed from free subunits or concatenated subunit assemblies designed to constrain subunit arrangement. Although a definite conclusion cannot be drawn, studies using etomidate and R-mTFD-MPAB support the idea that recombinant α1β3δ receptors may share stoichiometry and subunit arrangement with α1β3γ2 receptors.

Toward Understanding Functional Properties and Subunit Arrangement of α4β2δ γ-Aminobutyric Acid, Type A (GABAA) Receptors

GABAA receptors are pentameric ligand-gated channels mediating inhibitory neurotransmission in the CNS. α4βxδ GABAA receptors are extrasynaptic receptors important for tonic inhibition. The functional properties and subunit arrangement of these receptors are controversial. We predefined subunit arrangement by using subunit concatenation. α4, β2, and δ subunits were concatenated to dimeric, trimeric, and, in some cases, pentameric subunits. We constructed in total nine different receptor pentamers in at least two different ways and expressed them in Xenopus oocytes. The δ subunit was substituted in any of the five positions in the α1β2 receptor. In addition, we investigated all receptors with the 2:2:1 subunit stoichiometry for α4, β2, and δ. Several functional receptors were obtained. Interestingly, all of these receptors had very similar EC50 values for GABA in the presence of the neurosteroid 3α, 21-dihydroxy-5α-pregnan-20-one (THDOC). All functional receptors containing δ subunits were sensitive to 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide (DS2). Moreover, none of the receptors was affected by ethanol up to 30 mm These properties recapitulate those of non-concatenated receptors expressed from a cRNA ratio of 1:1:5 coding for α4, β2, and δ subunits. We conclude that the subunit arrangement of α4β2δ GABAA receptors is not strongly predefined but is mostly satisfying the 2:2:1 subunit stoichiometry for α4, β2, and δ subunits and that several subunit arrangements result in receptors with similar functional properties tuned to physiological conditions.

Neurosteroid Structure-Activity Relationships for Functional Activation of Extrasynaptic δGABA(A) Receptors

Synaptic GABAA receptors are primary mediators of rapid inhibition in the brain and play a key role in the pathophysiology of epilepsy and other neurologic disorders. The δ-subunit GABAA receptors are expressed extrasynaptically in the dentate gyrus and contribute to tonic inhibition, promoting network shunting as well as reducing seizure susceptibility. However, the neurosteroid structure-function relationship at δGABA(A) receptors within the native hippocampus neurons remains unclear. Here we report a structure-activity relationship for neurosteroid modulation of extrasynaptic GABAA receptor-mediated tonic inhibition in the murine dentate gyrus granule cells. We recorded neurosteroid allosteric potentiation of GABA as well as direct activation of tonic currents using a wide array of natural and synthetic neurosteroids. Our results shows that, for all neurosteroids, the C3α-OH group remains obligatory for extrasynaptic receptor functional activity, as C3β-OH epimers were inactive in activating tonic currents. Allopregnanolone and related pregnane analogs exhibited the highest potency and maximal efficacy in promoting tonic currents. Alterations at the C17 or C20 region of the neurosteroid molecule drastically altered the transduction kinetics of tonic current activation. The androstane analogs had the weakest modulatory response among the analogs tested. Neurosteroid potentiation of tonic currents was completely (approximately 95%) diminished in granule cells from δ-knockout mice, suggesting that δ-subunit receptors are essential for neurosteroid activity. The neurosteroid sensitivity of δGABA(A) receptors was confirmed at the systems level using a 6-Hz seizure test. A consensus neurosteroid pharmacophore model at extrasynaptic δGABA(A) receptors is proposed based on a structure-activity relationship for activation of tonic current and seizure protection.

Flumazenil decreases surface expression of α4β2δ GABAA receptors by increasing the rate of receptor internalization

Increases in expression of α4βδ GABAA receptors (GABARs), triggered by fluctuations in the neurosteroid THP (3α-OH-5α[β]-pregnan-20-one), are associated with changes in mood and cognition. We tested whether α4βδ trafficking and surface expression would be altered by in vitro exposure to flumazenil, a benzodiazepine ligand which reduces α4βδ expression in vivo. We first determined that flumazenil (100 nM-100 μM, IC50=∼1 μM) acted as a negative modulator, reducing GABA (10 μM)-gated current in the presence of 100 nM THP (to increase receptor efficacy), assessed with whole cell patch clamp recordings of recombinant α4β2δ expressed in HEK-293 cells. Surface expression of recombinant α4β2δ receptors was detected using a 3XFLAG reporter at the C-terminus of α4 (α4F) using confocal immunocytochemical techniques following 48 h exposure of cells to GABA (10 μM)+THP (100 nM). Flumazenil (10 μM) decreased surface expression of α4F by ∼60%, while increasing its intracellular accumulation, after 48 h. Reduced surface expression of α4β2δ after flumazenil treatment was confirmed by decreases in the current responses to 100 nM of the GABA agonist gaboxadol. Flumazenil-induced decreases in surface expression of α4β2δ were prevented by the dynamin blocker, dynasore, and by leupeptin, which blocks lysosomal enzymes, suggesting that flumazenil is acting to increase endocytosis and lysosomal degradation of the receptor. Flumazenil increased the rate of receptor removal from the cell surface by 2-fold, assessed using botulinum toxin B to block insertion of new receptors. These findings may suggest new therapeutic strategies for regulation of α4β2δ expression using flumazenil.

Allosteric ligands and their binding sites define γ-aminobutyric acid (GABA) type A receptor subtypes

GABAA receptors (GABA(A)Rs) mediate rapid inhibitory transmission in the brain. GABA(A)Rs are ligand-gated chloride ion channel proteins and exist in about a dozen or more heteropentameric subtypes exhibiting variable age and brain regional localization and thus participation in differing brain functions and diseases. GABA(A)Rs are also subject to modulation by several chemotypes of allosteric ligands that help define structure and function, including subtype definition. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABA(A)Rs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Two classes of pharmacologically important allosteric modulatory ligand binding sites reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site and the high-affinity, relevant to intoxication, ethanol site. The benzodiazepine site is specific for certain GABA(A)R subtypes, mainly synaptic, while the ethanol site is found at a modified benzodiazepine site on different, extrasynaptic, subtypes. In the transmembrane domain are allosteric modulatory ligand sites for diverse chemotypes of general anesthetics: the volatile and intravenous agents, barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are endogenous positive allosteric modulators. X-ray crystal structures of prokaryotic and invertebrate pentameric ligand-gated ion channels, and the mammalian GABA(A)R protein, allow homology modeling of GABA(A)R subtypes with the various ligand sites located to suggest the structure and function of these proteins and their pharmacological modulation.

Allosteric modulation of GABAA receptors via multiple drug-binding sites

GABAA receptors are ligand-gated ion channels composed of five subunits that can be opened by GABA and be modulated by multiple pharmacologically and clinically important drugs. Over the time, hundreds of compounds from different structural classes have been demonstrated to modulate, directly activate, or inhibit GABAA receptors, and most of these compounds interact with more than one binding site at these receptors. Crystal structures of proteins and receptors homologous to GABAA receptors as well as homology modeling studies have provided insights into the possible location of ligand interaction sites. Some of these sites have been identified by mutagenesis, photolabeling, and docking studies. For most of these ligands, however, binding sites are not known. Due to the high flexibility of GABAA receptors and the existence of multiple drug-binding sites, the unequivocal identification of interaction sites for individual drugs is extremely difficult. The existence of multiple GABAA receptor subtypes with distinct subunit composition, the contribution of distinct subunit sequences to binding sites of different receptor subtypes, as well as the observation that even subunits not directly contributing to a binding site are able to influence affinity and efficacy of drugs, contribute to a unique pharmacology of each GABAA receptor subtype. Thus, each receptor subtype has to be investigated to identify a possible subtype selectivity of a compound. Although multiple binding sites make GABAA receptor pharmacology even more complicated, the exploitation of ligand interaction with novel-binding sites also offers additional possibilities for a subtype-selective modulation of GABAA receptors.

Etomidate produces similar allosteric modulation in α1β3δ and α1β3γ2L GABA(A) receptors

BACKGROUND AND PURPOSE

Neuronal GABA(A) receptors are pentameric chloride ion channels, which include synaptic αβγ and extrasynaptic αβδ isoforms, mediating phasic and tonic inhibition respectively. Although the subunit arrangement of αβγ receptors is established as β-α-γ-β-α, that of αβδ receptors is uncertain and possibly variable. We compared receptors formed from free α1, β3 and δ or γ2L subunits and concatenated β3-α1-δ and β3-α1 subunit assemblies (placing δ in the established γ position) by investigating the effects of R-(+)-etomidate (ETO), an allosteric modulator that selectively binds to transmembrane interfacial sites between β3 and α1.

EXPERIMENTAL APPROACH

GABA-activated receptor-mediated currents in Xenopus oocytes were measured electrophysiologically, and ETO-induced allosteric shifts were quantified using an established model.

KEY RESULTS

ETO (3.2 μM) similarly enhanced maximal GABA (1 mM)-evoked currents in oocytes injected with 5 ng total mRNA and varying subunit ratios, for α1β3(1:1), α1β3δ(1:1:1) and α1β3δ(1:1:3), but this potentiation by ETO was significantly greater for β3-α1-δ/β3-α1(1:1) receptors. Reducing the amount of α1β3δ(1:1:3) mRNA mixture injected (0.5 ng) increased the modulatory effect of ETO, matching that seen with β3-α1-δ/β3-α1(1:1, 1 ng). ETO similarly reduced EC₅₀s and enhanced maxima of GABA concentration-response curves for both α1β3δ and β3-α1-δ/β3-α1 receptors. Allosteric shift parameters derived from these data depended on estimates of maximal GABA efficacy, and the calculated ranges overlap with allosteric shift values for α1β3γ2L receptors.

CONCLUSION AND IMPLICATIONS

Reducing total mRNA unexpectedly increased δ subunit incorporation into receptors on oocyte plasma membranes. Our results favour homologous locations for δ and γ2L subunits in α1β3γ2/δ GABA(A) receptors.

Characterization of neurosteroid effects on hyperpolarizing current at α4β2δ GABAA receptors

RATIONALE

The neurosteroid 3α,5β-THP (3α-OH-5β-pregnan-20-one, pregnanolone) is a modulator of the GABAA receptor (GABAR), with α4β2δ GABARs the most sensitive. However, the effects of 3α,5β-THP at α4β2δ are polarity-dependent: 3α,5β-THP potentiates depolarizing current, as has been widely reported, but decreases hyperpolarizing current by accelerating desensitization.

OBJECTIVES

The present study further characterized 3α,5β-THP inhibition of hyperpolarizing current at this receptor and compared effects of other related steroids at α4β2δ GABARs.

METHODS

α4β2δ GABARs were expressed in HEK-293 cells, and agonist-gated current recorded with whole cell voltage-clamp techniques using a theta tube to rapidly apply agonist before and after application of neurosteroids.

RESULTS

The GABA-modulatory steroids (30 nM) 3α,5α-THP (3α-OH-5α-pregnan-20-one, allopregnanolone) and THDOC (3α,21-dihydroxy-5α-pregnan-20-one) inhibited hyperpolarizing GABA (10 μM)-gated current at α4β2δ GABARs similar to 3α,5β-THP, while the inactive 3β,5β-THP isomer had no effect. Greater inhibition was seen for current gated by the high efficacy agonist gaboxadol (THIP, 100 μM) than for GABA (0.1-1000 μM), consistent with an effect of 3α,5β-THP on desensitization. Inhibitory effects of the steroid were not seen under low [Cl(-)] conditions or in the presence of calphostin C (500 nM), an inhibitor of protein kinase C. Chimeras swapping the IL (intracellular loop) of α4 with α1, when expressed with β2 and δ, produced receptors (α[414]β2δ) which were not inhibited by 3α,5β-THP when GABA-gated current was hyperpolarizing, while α[141]β2δ exhibited steroid-induced polarity-dependent modulation.

CONCLUSIONS

These findings suggest that numerous neurosteroids exhibit polarity-dependent effects at α4β2δ GABARs, which are dependent upon protein kinase C and the IL of α4.

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.

Analysis of γ-aminobutyric acid (GABA) type A receptor subtypes using isosteric and allosteric ligands

The GABAA receptors (GABAARs) play an important role in inhibitory transmission in the brain. The GABAARs could be identified using a medicinal chemistry approach to characterize with a series of chemical structural analogues, some identified in nature, some synthesized, to control the structural conformational rigidity/flexibility so as to define the 'receptor-specific' GABA agonist ligand structure. In addition to the isosteric site ligands, these ligand-gated chloride ion channel proteins exhibited modulation by several chemotypes of allosteric ligands, that help define structure and function. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABAARs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Also in the trans-membrane domain are allosteric modulatory ligand sites, mostly positive, for diverse chemotypes with general anesthetic efficacy, namely, the volatile and intravenous agents: barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are apparent endogenous positive allosteric modulators of GABAARs. These binding sites depend on the GABAAR heteropentameric subunit composition, i.e., subtypes. Two classes of pharmacologically very important allosteric modulatory ligand binding site reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site, and the low-dose ethanol site. The benzodiazepine site is specific for certain subunit combination subtypes, mainly synaptically localized. In contrast, the low-dose (high affinity) ethanol site(s) is found at a modified benzodiazepine site on different, extrasynaptic, subtypes.

Allopregnanolone in the brain: protecting pregnancy and birth outcomes

A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise foetal growth and development, protect the foetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and foetal brain. This review critically examines a role for allopregnanolone in both the maternal and foetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the foetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the foetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.

Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability

RATIONALE

Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids.

OBJECTIVE

This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior.

CONCLUSION

The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.

The influence of stress at puberty on mood and learning: role of the α4βδ GABAA receptor

It is well-known that the onset of puberty is associated with changes in mood as well as cognition. Stress can have an impact on these outcomes, which in many cases, can be more influential in females, suggesting that gender differences exist. The adolescent period is a vulnerable time for the onset of certain psychopathologies, including anxiety disorders, depression and eating disorders, which are also more prevalent in females. One factor which may contribute to stress-triggered anxiety at puberty is the GABAA receptor (GABAR), which is known to play a pivotal role in anxiety. Expression of α4βδ GABARs increases on the dendrites of CA1 pyramidal cells at the onset of puberty in the hippocampus, part of the limbic circuitry which governs emotion. This receptor is a sensitive target for the stress steroid 3α-OH-5[α]β-pregnan-20-one or [allo]pregnanolone, which paradoxically reduces inhibition and increases anxiety during the pubertal period (post-natal day ∼35-44) of female mice in contrast to its usual effect to enhance inhibition and reduce anxiety. Spatial learning and synaptic plasticity are also adversely impacted at puberty, likely a result of increased expression of α4βδ GABARs on the dendritic spines of CA1 hippocampal pyramidal cells, which are essential for consolidation of memory. This review will focus on the role of these receptors in mediating behavioral changes at puberty. Stress-mediated changes in mood and cognition in early adolescence may have relevance for the expression of psychopathologies in adulthood.

Unexpected Properties of δ-Containing GABAA Receptors in Response to Ligands Interacting with the α+ β- Site

GABAA receptors are the major inhibitory neurotransmitter receptors in the central nervous system and are the targets of many clinically important drugs, which modulate GABA induced chloride flux by interacting with separate and distinct allosteric binding sites. Recently, we described an allosteric modulation occurring upon binding of pyrazoloquinolinones to a novel binding site at the extracellular α+ β- interface. Here, we investigated the effect of 4-(8-methoxy-3-oxo-3,5-dihydro-2H-pyrazolo[4,3-c]quinolin-2-yl)benzonitrile (the pyrazoloquinolinone LAU 177) at several αβ, αβγ and αβδ receptor subtypes. LAU 177 enhanced GABA-induced currents at all receptors investigated, and the extent of modulation depended on the type of α and β subunits present within the receptors. Whereas the presence of a γ2 subunit within αβγ2 receptors did not dramatically change LAU 177 induced modulation of GABA currents compared to αβ receptors, we observed an unexpected threefold increase in modulatory efficacy of this compound at α1β2,3δ receptors. Steric hindrance experiments as well as inhibition by the functional α+ β- site antagonist LAU 157 indicated that the effects of LAU 177 at all receptors investigated were mediated via the α+ β- interface. The stronger enhancement of GABA-induced currents by LAU 177 at α1β3δ receptors was not observed at α4,6β3δ receptors. Other experiments indicated that this enhancement of modulatory efficacy at α1β3δ receptors was not observed with another α+ β- modulator, and that the efficacy of modulation by α+ β- ligands is influenced by all subunits present in the receptor complex and by structural details of the respective ligand.

The parvalbumin-positive interneurons in the mouse dentate gyrus express GABAA receptor subunits α1, β2, and δ along their extrasynaptic cell membrane

Neuronal circuitries in the hippocampus are involved in navigation and memory and are controlled by major networks of GABAergic interneurons. Parvalbumin (PV)-expressing interneurons in the dentate gyrus (DG) are identified as fast-spiking cells, playing a crucial role in network oscillation and synchrony. The inhibitory modulation of these interneurons is thought to be mediated mainly through GABAA receptors, the major inhibitory neurotransmitter receptors in the brain. Here we show that all PV-positive interneurons in the granular/subgranular layer (GL/SGL) of the mouse DG express high levels of the GABAA receptor δ subunit. PV-containing interneurons in the hilus and the molecular layer, however, express the δ subunit to a lower extent. Only 8% of the somatostatin-containing interneurons express the δ subunit, whereas calbindin- or calretinin-containing interneurons in the DG seem not to express the GABAA receptor δ subunit at all. Hence, these cells receive a GABAergic control different from that of PV-containing interneurons in the GL/SGL. Experiments investigating a possible co-expression of GABAA receptor α1, α2, α3, α4, α5, β1, β2, β3, or γ2 subunits with PV and δ subunits indicated that α1 and β2 subunits are co-expressed with δ subunits along the extrasynaptic membranes of PV-interneurons. These results suggest a robust tonic GABAergic control of PV-containing interneurons in the GL/SGL of the DG via δ subunit-containing receptors. Our data are important for better understanding of the neuronal circuitries in the DG and the role of specific cell types under pathological conditions.

α4βδ GABAA receptors and tonic inhibitory current during adolescence: effects on mood and synaptic plasticity

The onset of puberty is associated with alterations in mood as well as changes in cognitive function, which can be more pronounced in females. Puberty onset in female mice is associated with increased expression of α4βδ γ-amino-butyric acid-A (GABAA) receptors (GABARs) in CA1 hippocampus. These receptors, which normally have low expression in this central nervous system (CNS) site, emerge along the apical dendrites as well as on the dendritic spines of pyramidal neurons, adjacent to excitatory synapses where they underlie a tonic inhibition that shunts excitatory current and impairs activation of N-methyl-D-aspartate (NMDA) receptors, the trigger for synaptic plasticity. As would be expected, α4βδ expression at puberty also prevents long-term potentiation (LTP), an in vitro model of learning which is a function of network activity, induced by theta burst stimulation of the Schaffer collaterals to the CA1 hippocampus. The expression of these receptors also impairs spatial learning in a hippocampal-dependent task. These impairments are not seen in δ knock-out (-/-) mice, implicating α4βδ GABARs. α4βδ GABARs are also a sensitive target for steroids such as THP ([allo]pregnanolone or 3α-OH-5α[β]-pregnan-20-one), which are dependent upon the polarity of GABAergic current. It is well-known that THP can increase depolarizing current gated by α4βδ GABARs, but more recent data suggest that THP can reduce hyperpolarizing current by accelerating receptor desensitization. At puberty, THP reduces the hyperpolarizing GABAergic current, which removes the shunting inhibition that impairs synaptic plasticity and learning at this time. However, THP, a stress steroid, also increases anxiety, via its action at α4βδ GABARs because it is not seen in δ(-/-) mice. These findings will be discussed as well as their relevance to changes in mood and cognition at puberty, which can be a critical period for certain types of learning and when anxiety disorders and mood swings can emerge.

A study of subunit selectivity, mechanism and site of action of the delta selective compound 2 (DS2) at human recombinant and rodent native GABA(A) receptors

BACKGROUND AND PURPOSE

Most GABA(A) receptor subtypes comprise 2α, 2β and 1γ subunit, although for some isoforms, a δ replaces a γ-subunit. Extrasynaptic δ-GABA(A) receptors are important therapeutic targets, but there are few suitable pharmacological tools. We profiled DS2, the purported positive allosteric modulator (PAM) of δ-GABA(A) receptors to better understand subtype selectivity, mechanism/site of action and activity at native δ-GABA(A) receptors.

EXPERIMENTAL APPROACH

Subunit specificity of DS2 was determined using electrophysiological recordings of Xenopus laevis oocytes expressing human recombinant GABA(A) receptor isoforms. Effects of DS2 on GABA concentration-response curves were assessed to define mechanisms of action. Radioligand binding and electrophysiology utilising mutant receptors and pharmacology were used to define site of action. Using brain-slice electrophysiology, we assessed the influence of DS2 on thalamic inhibition in wild-type and δ(0/0) mice.

KEY RESULTS

Actions of DS2 were primarily determined by the δ-subunit but were additionally influenced by the α, but not the β, subunit (α4/6βxδ > α1βxδ >> γ2-GABA(A) receptors > α4β3). For δ-GABA(A) receptors, DS2 enhanced maximum responses to GABA, with minimal influence on GABA potency. (iii) DS2 did not act via the orthosteric, or known modulatory sites on GABA(A) receptors. (iv) DS2 enhanced tonic currents of thalamocortical neurones from wild-type but not δ(0/0) mice.

CONCLUSIONS AND IMPLICATIONS

DS2 is the first PAM selective for α4/6βxδ receptors, providing a novel tool to investigate extrasynaptic δ-GABA(A) receptors. The effects of DS2 are mediated by an unknown site leading to GABA(A) receptor isoform selectivity.

Estrous cycle regulation of extrasynaptic δ-containing GABA(A) receptor-mediated tonic inhibition and limbic epileptogenesis

The ovarian cycle affects susceptibility to behavioral and neurologic conditions. The molecular mechanisms underlying these changes are poorly understood. Deficits in cyclical fluctuations in steroid hormones and receptor plasticity play a central role in physiologic and pathophysiologic menstrual conditions. It has been suggested that synaptic GABA(A) receptors mediate phasic inhibition in the hippocampus and extrasynaptic receptors mediate tonic inhibition in the dentate gyrus. Here we report a novel role of extrasynaptic δ-containing GABA(A) receptors as crucial mediators of the estrous cycle-related changes in neuronal excitability in mice, with hippocampus subfield specificity. In molecular and immunofluorescence studies, a significant increase occurred in δ-subunit, but not α4- and γ2-subunits, in the dentate gyrus during diestrus. However, δ-subunit upregulation was not evident in the CA1 region. The δ-subunit expression was undiminished by age and ovariectomy and in mice lacking progesterone receptors, but it was significantly reduced by finasteride, a neurosteroid synthesis inhibitor. Electrophysiologic studies confirmed greater potentiation of GABA currents by progesterone-derived neurosteroid allopregnanolone in dissociated dentate gyrus granule cells in diestrus than in CA1 pyramidal cells. The baseline conductance and allopregnanolone potentiation of tonic currents in dentate granule cells from hippocampal slices were higher than in CA1 pyramidal cells. In behavioral studies, susceptibility to hippocampus kindling epileptogenesis was lower in mice during diestrus. These results demonstrate the estrous cycle-related plasticity of neurosteroid-sensitive, δ-containing GABA(A) receptors that mediate tonic inhibition and seizure susceptibility. These findings may provide novel insight on molecular cascades of menstrual disorders like catamenial epilepsy, premenstrual syndrome, and migraine.

Allopregnanolone and puberty: modulatory effect on glutamate and GABA release and expression of 3α-hydroxysteroid oxidoreductase in the hypothalamus of female rats

The hypothalamic release of glutamate and GABA regulates neurosecretory functions that may control the onset of puberty. This release may be influenced by neurosteroids such as allopregnanolone. Using superfusion experiments we examined the role of allopregnanolone on the K(+)-evoked and basal [(3)H]-glutamate and [(3)H]-GABA release from mediobasal hypothalamus and anterior preoptic area in prepubertal, vaginal opening and pubertal (P) rats and evaluated its modulatory effect on GABAA and NMDA (N-methyl-d-aspartic acid) receptors. Also, we examined the hypothalamic activity and mRNA expression of 3α-hydroxysteroid oxidoreductase (3α-HSOR) - enzyme that synthesizes allopregnanolone - using a spectrophotometric method and RT-PCR, respectively. Allopregnanolone increased both the K(+)-evoked [(3)H]-glutamate and [(3)H]-GABA release in P rats, being the former effect mediated by the modulation of NMDA receptors - as was reverted by Mg(2+) and by the NMDA receptor antagonist AP-7 and the latter by the modulation of NMDA and GABAA receptors - as was reverted by Mg(2+) and the GABAA receptor antagonist bicuculline. The neurosteroid also increased the basal release of [(3)H]-glutamate in VO rats in an effect that was dependent on the modulation of NMDA receptors as was reverted by Mg(2+). On the other hand we show that allopregnanolone reduced the basal release of [(3)H]-GABA in P rats although we cannot elucidate the precise mechanism by which the neurosteroid exerted this latter effect. The enzymatic activity and the mRNA expression of 3α-HSOR were both increased in P rats regarding the other two studied stages of sexual development. These results suggest an important physiological function of allopregnanolone in the hypothalamus of the P rat where it might be involved in the 'fine tuning' of neurosecretory functions related to the biology of reproduction of the female rats.

Monoterpene α-thujone exerts a differential inhibitory action on GABA(A) receptors implicated in phasic and tonic GABAergic inhibition

A monoterpene ketone, α-thujone originally attracted attention as a major natural ingredient of absinthe and was suspected to cause adverse effects such as hallucinations and seizures in persons excessively consuming this beverage. Although subsequent studies ruled out any major role of α-thujone in the "absynthism", it was found that at high doses it may induce epileptic activity pointing to an interaction with GABAergic inhibition. Indeed, subsequent studies, including those from this laboratory, showed that α-thujone inhibits GABAergic currents. However, GABAA receptors are extremely heterogeneous and in the present study we have investigated the effect of α-thujone on different recombinant GABAA receptors (α1β2γ2L, α1β2, α1β2δ and α4β2δ) relevant to phasic or tonic forms of inhibition. We report that α-thujone inhibits all considered receptor types by a qualitatively similar mechanism but the strongest effect is observed for α1β2δ receptors, suggesting that tonic currents might be more sensitive to α-thujone than the phasic ones. Moreover, we demonstrate that tonic currents, mimicked by response to a submicromollar GABA concentration (0.3 μM) in cultured neurons, showed a substantially larger sensitivity to α-thujone than responses elicited by higher [GABA] (more similar to phasic currents) or Inhibitory Postsynaptic Currents in the same preparation. Importantly, the extent of tonic current inhibition by α-thujone was as strong as in the case of currents mediated by α1β2δ receptors. Altogether, these data provide evidence that different GABAA receptor subtypes show distinct sensitivities to α-thujone and suggest that this compound may differentially affect tonic and phasic components of GABAergic inhibition.

Structure, function, and modulation of GABA(A) receptors

The GABA(A) receptors are the major inhibitory neurotransmitter receptors in mammalian brain. Each isoform consists of five homologous or identical subunits surrounding a central chloride ion-selective channel gated by GABA. How many isoforms of the receptor exist is far from clear. GABA(A) receptors located in the postsynaptic membrane mediate neuronal inhibition that occurs in the millisecond time range; those located in the extrasynaptic membrane respond to ambient GABA and confer long-term inhibition. GABA(A) receptors are responsive to a wide variety of drugs, e.g. benzodiazepines, which are often used for their sedative/hypnotic and anxiolytic effects.

Regulation of the surface expression of α4β2δ GABAA receptors by high efficacy states

α4βδ GABA(A) receptors (GABARs) have low CNS expression, but their expression is increased by 48h exposure to the neurosteroid THP (3α-OH-5α[β]-pregnan-20-one). THP also increases the efficacy of δ-containing GABARs acutely, where GABA is a partial agonist. Thus, we examined effects of THP (100 nM) and full GABA agonists at α4β2δ (gaboxadol, 10 μM, and β-alanine, 10 μM-1mM), on surface expression of α4β2δ. To this end, we used an α4 construct tagged with a 3XFLAG (F) epitope or measured expression of native α4 and δ. HEK-293 cells or cultured hippocampal neurons were transfected with α4Fβ2δ and treated 24h later with GABA agonists, THP, GABA plus THP or vehicle (0.01% DMSO) for 0.5 h-48 h. Immunocytochemistry was performed under both non-permeabilized and permeabilized conditions to detect surface and intracellular labeling, respectively, using confocal microscopy. The high efficacy agonists and GABA (1 or 10 μM) plus THP increased α4β2δ surface expression up to 3-fold after 48h, an effect first seen by 0.5h. This effect was not dependent upon the polarity of GABAergic current, although expression was increased by KCC2. Intracellular labeling was decreased while functional expression was confirmed by whole cell patch clamp recordings of responses to GABA agonists. GABA plus THP treatment did not alter the rate of receptor removal from the surface membrane, suggesting that THP-induced α4β2δ expression is likely via receptor insertion. Surface expression of α4β2δ was decreased by rottlerin (10 μM), suggesting a role for PKC-δ. These results suggest that trafficking of α4β2δ GABARs is regulated by high efficacy states.

The major central endocannabinoid directly acts at GABA(A) receptors

GABA(A) receptors are the major ionotropic inhibitory neurotransmitter receptors. The endocannabinoid system is a lipid signaling network that modulates different brain functions. Here we show a direct molecular interaction between the two systems. The endocannabinoid 2-arachidonoyl glycerol (2-AG) potentiates GABA(A) receptors at low concentrations of GABA. Two residues of the receptor located in the transmembrane segment M4 of β(2) confer 2-AG binding. 2-AG acts in a superadditive fashion with the neurosteroid 3α, 21-dihydroxy-5α-pregnan-20-one (THDOC) and modulates δ-subunit-containing receptors, known to be located extrasynaptically and to respond to neurosteroids. 2-AG inhibits motility in CB(1)/CB(2) cannabinoid receptor double-KO, whereas β(2)-KO mice show hypermotility. The identification of a functional binding site for 2-AG in the GABA(A) receptor may have far-reaching consequences for the study of locomotion and sedation.

Allosteric modulators of the α4β2 subtype of neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are ligand-gated ion conducting transmembrane channels from the Cys-loop receptor super-family. The α4β2 subtype is the predominant heteromeric subtype of nicotinic receptors found in the brain. Allosteric modulators for α4β2 receptors interact at a site other than the orthosteric site where acetylcholine binds. Many compounds which act as allosteric modulators of the α4β2 receptors have been identified, with both positive and negative effects. Such allosteric modulators either increase or decrease the response induced by agonist on the α4β2 receptors. Here we discuss the concept of allosterism as it pertains to the α4β2 receptors and summarize the important features of allosteric modulators for this nicotinic receptor subtype.

Kinetic and structural determinants for GABA-A receptor potentiation by neuroactive steroids

Endogenous neurosteroids and synthetic neuroactive steroid analogs are among the most potent and efficacious potentiators of the mammalian GABA-A receptor. The compounds interact with one or more sites on the receptor leading to an increase in the channel open probability through a set of changes in the open and closed time distributions. The endogenous neurosteroid allopregnanolone potentiates the α1β2γ2L GABA-A receptor by enhancing the mean duration and prevalence of the longest-lived open time component and by reducing the prevalence of the longest-lived intracluster closed time component. Thus the channel mean open time is increased and the mean closed time duration is decreased, resulting in potentiation of channel function. Some of the other previously characterized neurosteroids and steroid analogs act through similar mechanisms while others affect a subset of these parameters. The steroids modulate the GABA-A receptor through interactions with the membrane-spanning region of the receptor. However, the number of binding sites that mediate the actions of steroids is unclear. We discuss data supporting the notions of a single site vs. multiple sites mediating the potentiating actions of steroids.

Allosteric Modulation of αβδ GABAA Receptors

GABA_A receptors mediate the majority of the fast inhibition in the mature brain and play an important role in the pathogenesis of many neurological and psychiatric disorders. The αβδ GABA_A receptor localizes extra- or perisynaptically and mediates GABAergic tonic inhibition. Compared with synaptically localized αβγ receptors, αβδ receptors are more sensitive to GABA, display relatively slower desensitization and exhibit lower efficacy to GABA agonism. Interestingly, αβδ receptors can be positively modulated by a variety of structurally different compounds, even at saturating GABA concentrations. This review focuses on allosteric modulation of recombinant αβδ receptor currents and αβδ receptor-mediated tonic currents by anesthetics and ethanol. The possible mechanisms for the positive modulation of αβδ receptors by these compounds will also be discussed.

Dihydropyrimidinone positive modulation of delta-subunit-containing gamma-aminobutyric acid type A receptors, including an epilepsy-linked mutant variant

Gamma-aminobutyric acid type A receptors (GABA(A) receptors) are ligand-gated chloride channels that play a central role in signal transmission within the mammalian central nervous system. Compounds that modulate specific GABA(A) receptor subtypes containing the delta-subunit are scarce but would be valuable research tools and starting points for potential therapeutic agents. Here we report a class of dihydropyrimidinone (DHPM) heterocycles that preferentially potentiate peak currents of recombinant GABA(A) receptor subtypes containing the delta-subunit expressed in HEK293T cells. Using the three-component Biginelli reaction, 13 DHPMs with structural features similar to those of the barbiturate phenobarbital were synthesized; one DHPM used (monastrol) is commercially available. An up to approximately 3-fold increase in the current from recombinant alpha1beta2delta receptors was observed with the DHPM compound JM-II-43A or monastrol when co-applied with saturating GABA concentrations, similar to the current potentiation observed with the nonselective potentiating compounds phenobarbital and tracazolate. No agonist activity was observed for the DHPMs at the concentrations tested. A kinetic model was used in conjunction with dose-dependent measurements to calculate apparent dissociation constant values for JM-II-43A (400 muM) and monastrol (200 microM) at saturating GABA concentrations. We examined recombinant receptors composed of combinations of subunits alpha1, alpha4, alpha5, alpha6, beta2, beta3, gamma2L, and delta with JM-II-43A to demonstrate the preference for potentiation of delta-subunit-containing receptors. Lastly, reduced currents from receptors containing the mutated delta(E177A) subunit, described by Dibbens et al. [(2004) Hum. Mol. Genet. 13, 1315-1319] as a heritable susceptibility allele for generalized epilepsy with febrile seizures plus, are also potentiated by these DHPMs.

The sticky issue of neurosteroids and GABA(A) receptors

Endogenous neurosteroids and their synthetic analogs (neuroactive steroids) are potent modulators of GABA(A) receptors. Thus, they are of physiological and clinical relevance for their ability to modulate inhibitory function in the CNS. Despite their importance, fundamental issues of neurosteroid actions remain unresolved. Recent evidence suggests that glutamatergic principal neurons, rather than glia, are the major sources of neurosteroid synthesis. Other recent studies have identified putative neurosteroid binding sites on GABA(A) receptors. In this Opinion, we argue that neurosteroids require a membranous route of access to transmembrane-domain binding sites within GABA(A) receptors. This has implications for the design of future neuroactive steroids because the lipid solubility and related accessibility properties of the ligand are likely to be key determinants of receptor modulation.

Diversity of structure and function of alpha1alpha6beta3delta GABAA receptors: comparison with alpha1beta3delta and alpha6beta3delta receptors

delta subunit-containing gamma-aminobutyric acid, type A (GABA(A))receptors are expressed extrasynaptically and mediate tonic inhibition. In cerebellar granule cells, they often form receptors together with alpha(1) and/or alpha(6) subunits. We were interested in determining the architecture of receptors containing both subunits. We predefined the subunit arrangement of several different GABA(A) receptor pentamers by concatenation. These receptors composed of alpha(1), alpha(6), beta(3), and delta subunits were expressed in Xenopus oocytes. Currents elicited in response to GABA were determined in the presence and absence of 3alpha,21-dihydroxy-5alpha-pregnan-20-one (THDOC) or ethanol, or currents were elicited by 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP). Several subunit configurations formed active channels. We therefore conclude that delta can assume multiple positions in a receptor pentamer made up of alpha(1), alpha(6), beta(3), and delta subunits. The different receptors differ in their functional properties. Functional expression of one receptor type was only evident in the combined presence of the neurosteroid THDOC with the channel agonist GABA. Most, but not all, receptors active with GABA/THDOC responded to THIP. None of the receptors was modulated by ethanol concentrations up to 30 mm. Several observations point to a preferred position of delta subunits between two alpha subunits in alpha(1)alpha(6)beta(3)delta receptors. This property is shared by alpha(1)beta(3)delta and alpha(6)beta(3)delta receptors, but there are differences in the additionally expressed isoforms.

Agonist function of the recombinant alpha 4 beta 3 delta GABAA receptor is dependent on the human and rat variants of the alpha 4-subunit

1. It is known that the alpha(4)-subunit is likely to occur in the brain predominantly in alpha(4)beta(3)delta receptors at extrasynaptic sites. Recent studies have revealed that the alpha(1)-, alpha(4)-, gamma(2)- and delta-subunits may colocalize extrasynaptically in dentate granule cells of the hippocampus. In the present study, we characterized a series of recombinant GABA(A) receptors containing human (H) and rat (R) alpha(1)/alpha(4)-, beta(2)/beta(3)- and gamma(2S)/delta-subunits in Xenopus oocytes using the two-electrode voltage-clamp technique. 2. Both H alpha(1)beta(3)delta and H alpha(4)beta(3)gamma(2S) receptors were sensitive to activation by GABA and pentobarbital. Contrary to earlier findings that the alpha(4)beta(3)delta combination was more sensitive to agonist action than the alpha(4)beta(3)gamma(2S) receptor, we observed extremely small GABA- and pentobarbital-activated currents at the wild-type H alpha(4)beta(3)delta receptor. However, GABA and pentobarbital activated the wild-type R alpha(4)beta(3)delta receptor with high potency (EC(50) = 0.5 +/- 0.7 and 294 +/- 5 micromol/L, respectively). 3. Substituting the H alpha(4) subunit with R alpha(4) conferred a significant increase in activation on the GABA and pentobarbital site in terms of reduced EC(50) and increased I(max). When the H alpha(4) subunit was combined with the R beta(3) and R delta subunit in a heteropentameric form, the amplitude of GABA- and pentobarbital-activated currents increased significantly compared with the wild-type H alpha(4)beta(3)delta receptor. 4. Thus, the results indicate that the R alpha(4)beta(3)delta, H alpha(1)beta(3)delta and H alpha(4)beta(3)gamma(2S) combinations may contribute to functions of extrasynaptic GABA(A) receptors. The presence of the R alpha(4) subunit at recombinant GABA(A) receptors containing the delta-subunit is a strong determinant of agonist action. The recombinant H alpha(4)beta(3)delta receptor is a less sensitive subunit composition in terms of agonist activation.

Use of concatamers to study GABAA receptor architecture and function: application to delta-subunit-containing receptors and possible pitfalls

Many membrane proteins, including the GABA(A) [GABA (gamma-aminobutyric acid) type A] receptors, are oligomers often built from different subunits. As an example, the major adult isoform of the GABA(A) receptor is a pentamer built from three different subunits. Theoretically, co-expression of three subunits may result in many different receptor pentamers. Subunit concatenation allows us to pre-define the relative arrangement of the subunits. This method may thus be used to study receptor architecture, but also the nature of binding sites. Indeed, it made possible the discovery of a novel benzodiazepine site. We use here subunit concatenation to study delta-subunit-containing GABA(A) receptors. We provide evidence for the formation of different functional subunit arrangements in recombinant alpha(1)beta(3)delta and alpha(6)beta(3)delta receptors. As with all valuable techniques, subunit concatenation has also some pitfalls. Most of these can be avoided by carefully titrating and minimizing the length of the linker sequences joining the two linked subunits and avoiding inclusion of the signal sequence of all but the N-terminal subunit of a multi-subunit construct. Maybe the most common error found in the literature is that low expression can be overcome by simply overloading the expression system with genetic information. As some concatenated constructs result by themselves in a low level of expression, this erroneous assembly leading to receptor function may be promoted by overloading the expression system and leads to wrong conclusions.

Function and modulation of delta-containing GABA(A) receptors

alphabetadelta-Containing GABA(A) receptors are (1) localized to extra- and perisynaptic membranes, (2) exhibit a high sensitivity to GABA, (3) show little desensitization, and (4) are believed to be one of the primary mediators of tonic inhibition in the central nervous system. This type of signaling appears to play a key role in controlling cell excitability. This review article briefly summarizes recent knowledge on tonic GABA-mediated inhibition. We will also consider the mechanism of action of many clinically important drugs such as anxiolytics, anticonvulsants, and sedative/hypnotics and their effects on delta-containing GABA receptor activation. We will conclude that alphabetadelta-containing GABA(A) receptors exhibit a relatively low efficacy that can be potentiated by endogenous modulators and anxiolytic agents. This scenario enables these particular GABA receptor combinations, upon neurosteroid exposure for example, to impart a profound effect on excitability in the central nervous system.

Puberty, steroids and GABA(A) receptor plasticity

GABA(A) receptors (GABAR) mediate most inhibition in the CNS and are also a target for neuroactive steroids such as 3alpha,5[alpha]beta-THP (3alphaOH-5[alpha]beta-OH-pregnan-20-one or [allo]pregnanolone). Although these steroids robustly enhance current gated by alpha1beta2delta GABAR, we have shown that 3alpha,5[alpha]beta-THP effects at recombinant alpha4beta2delta GABAR depend on the direction of Cl(-) flux, where the steroid increases outward flux, but decreases inward flux through the receptor. This polarity-dependent inhibition of alpha4beta2delta GABAR resulted from an increase in the rate and extent of rapid desensitization of the receptor, recorded from recombinant receptors expressed in HEK-293 cells with whole cell voltage clamp techniques. This inhibitory effect of 3alpha,5[alpha]beta-THP was not observed at other receptor subtypes, suggesting it was selective for alpha4beta2delta GABAR. Furthermore, it was prevented by a selective mutation of basic residue arginine 353 in the intracellular loop of the receptor, suggesting that this might be a putative chloride modulatory site. Expression of alpha4betadelta GABAR increases markedly at extrasynaptic sites at the onset of puberty in female mice. At this time, 3alpha,5[alpha]beta-THP decreased the inhibitory tonic current, recorded with perforated patch techniques to maintain the physiological Cl(-) gradient. By decreasing this shunting inhibition, 3alpha,5[alpha]beta-THP increased the excitability of CA1 hippocampal pyramidal cells at puberty. These effects of the steroid were opposite to those observed before puberty when 3alpha,5[alpha]beta-THP reduced neuronal excitability as a pre-synaptic effect. Behaviorally, the excitatory effect of 3alpha,5[alpha]beta-THP was reflected as an increase in anxiety at the onset of puberty in female mice. Taken together, these findings suggest that the emergence of alpha4beta2delta GABAR at the onset of puberty reverses the effect of a stress steroid. These findings may be relevant for the mood swings and increased response to stressful events reported in adolescence.

Neurosteroids: endogenous allosteric modulators of GABA(A) receptors

In the mammalian central nervous system activation of the ionotropic GABA(A) receptor by the neurotransmitter GABA plays a crucial role in controlling neuronal excitability. This essential form of neuronal regulation may be subject to "fine tuning" by particular metabolites of progesterone and deoxycorticosterone, which bind directly to the GABA(A) receptor to enhance the actions of GABA. Originally such steroids were considered to act as endocrine messengers, being synthesised in peripheral glands such as the adrenals and ovaries and crossing the blood brain barrier to influence neuronal signalling. However, it is now evident that certain neurons and glia may produce such "neurosteroids" and that these locally synthesised modulators may act in a paracrine, or indeed an autocrine manner to influence neuronal activity. Neurosteroid synthesis may change dynamically in a variety of physiological situations (e.g. stress, pregnancy) and perturbations in their levels are implicated in a variety of neurological and psychiatric disorders. Here we will consider (1) evidence supporting the concept that neurosteroids act as local regulators of neuronal inhibition, (2) that extrasynaptic GABA(A) receptors appear to be a particularly important neurosteroid target and (3) recent advances in defining the neurosteroid binding site(s) on the GABA(A) receptor.

Modulation of spontaneous and GABA-evoked tonic alpha4beta3delta and alpha4beta3gamma2L GABAA receptor currents by protein kinase A

Protein kinase A (PKA) has been reported to regulate synaptic alphabetagamma gamma-aminobutyric acid type A (GABA(A)) receptor currents, but whether PKA regulates GABA(A) receptor peri- and extrasynaptic tonic currents is unknown. GABA(A) receptors containing alpha4 subunits are important in mediating tonic inhibition and exist as both alpha4betadelta and alpha4betagamma receptors in the brain. To mimic GABA-independent and GABA-dependent tonic currents, we transfected HEK 293T cells with alpha4beta3delta or alpha4beta3gamma2L subunits and recorded spontaneous currents in the absence of applied GABA and steady-state currents in the presence of 1 muM GABA. Both alpha4beta3delta and alpha4beta3gamma2L receptors displayed spontaneous currents, but PKA activation increased spontaneous alpha4beta3delta currents substantially more than spontaneous alpha4beta3gamma2L currents. The increase in spontaneous alpha4beta3delta currents was due to an increase in single-channel open frequency. In contrast, PKA activation did not alter steady-state tonic currents recorded in the presence of 1 muM GABA. We concluded that PKA had a GABA concentration-dependent effect on alpha4beta3delta and alpha4beta3gamma2L currents. In the absence of GABA, spontaneous alpha4beta3delta and, to a lesser extent, alpha4beta3gamma2L currents could provide a basal, tonic current that could be regulated by PKA. With increasing concentrations of extracellular GABA, however, tonic alpha4beta3delta and alpha4beta3gamma2L currents would become more GABA dependent and less PKA sensitive. Thus in brain regions with fluctuating extracellular GABA levels, the dynamic range of GABA-activated tonic currents would be set by PKA and the increase in tonic current produced by increasing GABA would be reduced by PKA-mediated phosphorylation. When ambient GABA reaches micromolar concentrations, PKA would have no effect on steady-state tonic currents.

Extrasynaptic GABAA receptors: form, pharmacology, and function

GABA is the principal inhibitory neurotransmitter in the CNS and acts via GABA(A) and GABA(B) receptors. Recently, a novel form of GABA(A) receptor-mediated inhibition, termed "tonic" inhibition, has been described. Whereas synaptic GABA(A) receptors underlie classical "phasic" GABA(A) receptor-mediated inhibition (inhibitory postsynaptic currents), tonic GABA(A) receptor-mediated inhibition results from the activation of extrasynaptic receptors by low concentrations of ambient GABA. Extrasynaptic GABA(A) receptors are composed of receptor subunits that convey biophysical properties ideally suited to the generation of persistent inhibition and are pharmacologically and functionally distinct from their synaptic counterparts. This mini-symposium review highlights ongoing work examining the properties of recombinant and native extrasynaptic GABA(A) receptors and their preferential targeting by endogenous and clinically relevant agents. In addition, it emphasizes the important role of extrasynaptic GABA(A) receptors in GABAergic inhibition throughout the CNS and identifies them as a major player in both physiological and pathophysiological processes.

Structure of alpha6 beta3 delta GABA(A) receptors and their lack of ethanol sensitivity

Delta (delta) subunit containing GABA(A) receptors are expressed extra-synaptically and mediate tonic inhibition. In cerebellar granule cells, they often form a receptor together with alpha(6) subunits. We were interested to determine the architecture of these receptors. We predefined the subunit arrangement of 24 different GABA(A) receptor pentamers by subunit concatenation. These receptors (composed of alpha(6), beta(3) and delta subunits) were expressed in Xenopus oocytes and their electrophysiological properties analyzed. Currents elicited in response to GABA were determined in presence and absence of 3alpha, 21-dihydroxy-5alpha-pregnan-20-one and to 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. alpha(6)-beta(3)-alpha(6)/delta receptors showed a substantial response to GABA alone. Three receptors, beta(3)-alpha(6)-delta/alpha(6)-beta(3), alpha(6)-beta(3)-alpha(6)/beta(3)-delta and beta(3)-delta-beta(3)/alpha(6)-beta(3), were only uncovered in the combined presence of the neurosteroid 3alpha, 21-dihydroxy-5alpha-pregnan-20-one with GABA. All four receptors were activated by 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. None of the functional receptors was modulated by physiological concentrations (up to 30 mM) of ethanol. GABA concentration response curves indicated that the delta subunit can contribute to the formation of an agonist site. We conclude from the investigated receptors that the delta subunit can assume multiple positions in a receptor pentamer composed of alpha(6), beta(3) and delta subunits.