The relative amount of cRNA coding for gamma2 subunits affects stimulation by benzodiazepines in GABA(A) receptors expressed in Xenopus oocytes

GABAA receptor subunit M2-M3 linkers have asymmetric roles in pore gating and diazepam modulation

GABAA receptors (GABAARs) are neurotransmitter-gated ion channels critical for inhibitory synaptic transmission as well as the molecular target for benzodiazepines (BZDs), one of the most widely prescribed class of psychotropic drugs today. Despite structural insight into the conformations underlying functional channel states, the detailed molecular interactions involved in conformational transitions and the physical basis for their modulation by BZDs are not fully understood. We previously identified that alanine substitution at the central residue in the α1 subunit M2-M3 linker (V279A) enhances the efficiency of linkage between the BZD site and the pore gate. Here, we expand on this work by investigating the effect of alanine substitutions at the analogous positions in the M2-M3 linkers of β2 (I275A) and γ2 (V290A) subunits, which together with α1 comprise typical heteromeric α1β2γ2 synaptic GABAARs. We find that these mutations confer subunit-specific effects on the intrinsic pore closed-open equilibrium and its modulation by the BZD diazepam (DZ). The mutations α1(V279A) or γ2(V290A) bias the channel toward a closed conformation, whereas β2(I275A) biases the channel toward an open conformation to the extent that the channel becomes leaky and opens spontaneously in the absence of agonist. In contrast, only α1(V279A) enhances the efficiency of DZ-to-pore linkage, whereas mutations in the other two subunits have no effect. These observations show that the central residue in the M2-M3 linkers of distinct subunits in synaptic α1β2γ2 GABAARs contribute asymmetrically to the intrinsic closed-open equilibrium and its modulation by DZ.

A neolignan from Connarus tuberosus as an allosteric GABAA receptor modulator at the neurosteroid binding site

In a screening of a small library of extracts from plants of the Amazonian and Cerrado biomes, a hexane extract of Connarus tuberosus roots was found to significantly potentiate the GABA induced fluorescence in a fluorescence (FLIPR) assay in CHO cells stably expressing the α₁β₂γ₂ subtype of human GABA_A receptors. With the aid of HPLC-based activity profiling the activity was linked to the neolignan connarin. In CHO cells the activity of connarin was not abolished by increasing concentrations of flumazenil, while the effect of diazepam was increased by increasing concentrations of connarin. The effect of connarin was abolished by pregnenolone sulfate (PREGS) in a concentration-dependent manner, and the effect of allopregnanolone was further increased by increasing concentrations of connarin. In a two-microelectrode voltage clamp assay with Xenopus laevis oocytes transiently expressing GABA_A receptors composed of human α₁β₂γ₂S and α1β2 subunits connarin potentiated the GABA-induced currents, with EC₅₀ values of 1.2 ± 0.3 μM (α₁β₂γ₂S) and 1.3 ± 0.4 μM (α₁β₂), and with a maximum enhancement of currents E_max of 1959 ± 70% (α₁β₂γ₂S) and 185 ± 48% (α₁β₂). The activation induced by connarin was abolished by increasing concentrations of PREGS.

Apo2ph4: A Versatile Workflow for the Generation of Receptor-based Pharmacophore Models for Virtual Screening

Pharmacophore models are widely used as efficient virtual screening (VS) filters for the target-directed enrichment of large compound libraries. However, the generation of pharmacophore models that have the power to discriminate between active and inactive molecules traditionally requires structural information about ligand-target complexes or at the very least knowledge of one active ligand. The fact that the discovery of the first known active ligand of a newly investigated target represents a major hurdle at the beginning of every drug discovery project underscores the need for methods that are able to derive high-quality pharmacophore models even without the prior knowledge of any active ligand structures. In this work, we introduce a novel workflow, called apo2ph4, that enables the rapid derivation of pharmacophore models solely from the three-dimensional structure of the target receptor. The utility of this workflow is demonstrated retrospectively for the generation of a pharmacophore model for the M2 muscarinic acetylcholine receptor. Furthermore, in order to show the general applicability of apo2ph4, the workflow was employed for all 15 targets of the recently published LIT-PCBA dataset. Pharmacophore-based VS runs using the apo2ph4-derived models achieved a significant enrichment of actives for 13 targets. In the last presented example, a pharmacophore model derived from the etomidate site of the α1β2γ2 GABA_A receptor was used in VS campaigns. Subsequent in vitro testing of selected hits revealed that 19 out of 20 (95%) tested compounds were able to significantly enhance GABA currents, which impressively demonstrates the applicability of apo2ph4 for real-world drug design projects.

β subunits of GABAA receptors form proton-gated chloride channels: Insights into the molecular basis

Gamma-aminobutyric acid type A receptors (GABA_ARs) are ligand gated channels mediating inhibition in the central nervous system. Here, we identify a so far undescribed function of β-subunit homomers as proton-gated anion channels. Mutation of a single H267A in β3 subunits completely abolishes channel activation by protons. In molecular dynamic simulations of the β3 crystal structure protonation of H267 increased the formation of hydrogen bonds between H267 and E270 of the adjacent subunit leading to a pore stabilising ring formation and accumulation of Cl^- within the transmembrane pore. Conversion of these residues in proton insensitive ρ1 subunits transfers proton-dependent gating, thus highlighting the role of this interaction in proton sensitivity. Activation of chloride and bicarbonate currents at physiological pH changes (pH₅₀ is in the range 6- 6.3) and kinetic studies suggest a physiological role in neuronal and non-neuronal tissues that express beta subunits, and thus as potential novel drug target.

Beta subunits of GABAA receptors are unexpectedly shown to form homomeric proton gated ion channels attributable to a single histidine residue.

Novel Three-Finger Neurotoxins from Naja melanoleuca Cobra Venom Interact with GABAA and Nicotinic Acetylcholine Receptors

Cobra venoms contain three-finger toxins (TFT) including α-neurotoxins efficiently binding nicotinic acetylcholine receptors (nAChRs). As shown recently, several TFTs block GABA_A receptors (GABA_ARs) with different efficacy, an important role of the TFTs central loop in binding to these receptors being demonstrated. We supposed that the positive charge (Arg36) in this loop of α-cobratoxin may explain its high affinity to GABA_AR and here studied α-neurotoxins from African cobra N. melanoleuca venom for their ability to interact with GABAARs and nAChRs. Three α-neurotoxins, close homologues of the known N. melanoleuca long neurotoxins 1 and 2, were isolated and sequenced. Their analysis on Torpedocalifornica and α7 nAChRs, as well as on acetylcholine binding proteins and on several subtypes of GABA_ARs, showed that all toxins interacted with the GABA_AR much weaker than with the nAChR: one neurotoxin was almost as active as α-cobratoxin, while others manifested lower activity. The earlier hypothesis about the essential role of Arg36 as the determinant of high affinity to GABA_AR was not confirmed, but the results obtained suggest that the toxin loop III may contribute to the efficient interaction of some long-chain neurotoxins with GABA_AR. One of isolated toxins manifested different affinity to two binding sites on Torpedo nAChR.

A critical residue in the α1M2-M3 linker regulating mammalian GABAA receptor pore gating by diazepam

Benzodiazepines (BZDs) are a class of widely prescribed psychotropic drugs that modulate activity of GABA_A receptors (GABA_ARs), neurotransmitter-gated ion channels critical for synaptic transmission. However, the physical basis of this modulation is poorly understood. We explore the role of an important gating domain, the α₁M2–M3 linker, in linkage between the BZD site and pore gate. To probe energetics of this coupling without complication from bound agonist, we use a gain of function mutant (α₁L9'Tβ₂γ_2L) directly activated by BZDs. We identify a specific residue whose mutation (α₁V279A) more than doubles the energetic contribution of the BZD positive modulator diazepam (DZ) to pore opening and also enhances DZ potentiation of GABA-evoked currents in a wild-type background. In contrast, other linker mutations have little effect on DZ efficiency, but generally impair unliganded pore opening. Our observations reveal an important residue regulating BZD-pore linkage, thereby shedding new light on the molecular mechanism of these drugs.

The Z-Drugs Zolpidem, Zaleplon, and Eszopiclone Have Varying Actions on Human GABA A Receptors Containing γ1, γ2, and γ3 Subunits

γ-Aminobutyric-acid type A (GABA _A ) receptors expressing the γ1 or γ3 subunit are only found within a few regions of the brain, some of which are involved in sleep. No known compounds have been reported to selectively target γ1- or γ3-containing GABA _A receptors. Pharmacological assessments of this are conflicting, possibly due to differences in experimental models, conditions, and exact protocols when reporting efficacies and potencies. In this study, we evaluated the modulatory properties of five non-benzodiazepine Z-drugs (zaleplon, indiplon, eszopiclone, zolpidem, and alpidem) used in sleep management and the benzodiazepine, diazepam on human α1β2γ receptors using all three γ subtypes. This was accomplished using concatenated GABA _A pentamers expressed in Xenopus laevis oocytes and measured via two-electrode voltage clamp. This approach removes the potential for single subunits to form erroneous receptors that could contribute to the pharmacological assessment of these compounds. No compound tested had significant effects on γ1-containing receptors below 10 μM. Interestingly, zaleplon and indiplon were found to modulate γ3-containing receptors equally as efficacious as γ2-containing receptors. Furthermore, zaleplon had a higher potency for γ3- than for γ2-containing receptors, indicating certain therapeutic effects could occur via these γ3-containing receptors. Eszopiclone modulated γ3-containing receptors with reduced efficacy but no reduction in potency. These data demonstrate that the imidazopyridines zaleplon and indiplon are well suited to further investigate potential γ3 effects on sleep in vivo.

Design, Synthesis, and Pharmacological Evaluation of Novel β2/3 Subunit-Selective γ-Aminobutyric Acid Type A (GABAA) Receptor Modulators

Subunit-selective modulation of γ-aminobutyric acid type A receptors (GABA_AR) is considered to exert fewer side effects compared to unselective clinically used drugs. Here, the β2/3 subunit-selective GABA_AR modulators valerenic acid (VA) and loreclezole (LOR) guided the synthesis of novel subunit-selective ligands with simplified structures. We studied their effects on GABA_ARs expressed in Xenopus laevis oocytes using two-microelectrode voltage clamp technique. Five compounds showed significantly more efficacious modulation of GABA-evoked currents than VA and LOR with retained potency and selectivity. Compound 18 [( E)-2-Cyano-3-(2,4-dichlorophenyl)but-2-enamide] induced the highest maximal modulation of GABA-induced chloride currents ( E_max: 3114 ± 242%), while 12 [( Z)-3-(2,4-dichlorophenyl)but-2-enenitrile] displayed the highest potency (EC₅₀: 13 ± 2 μM). Furthermore, in hippocampal neurons 12 facilitated phasic and tonic GABAergic inhibition, and in vivo studies revealed significantly more potent protection against pentylenetetrazole (PTZ)-induced seizures compared to VA and LOR. Collectively, compound 12 constitutes a novel, simplified, and subunit-selective GABA_AR modulator with low-dose anticonvulsant activity.

An intersubunit electrostatic interaction in the GABAA receptor facilitates its responses to benzodiazepines

Benzodiazepines are positive allosteric modulators of the GABA_A receptor (GABA_AR), acting at the α-γ subunit interface to enhance GABA_AR function. GABA or benzodiazepine binding induces distinct conformational changes in the GABA_AR. The molecular rearrangements in the GABA_AR following benzodiazepine binding remain to be fully elucidated. Using two molecular models of the GABA_AR, we identified electrostatic interactions between specific amino acids at the α-γ subunit interface that were broken by, or formed after, benzodiazepine binding. Using two-electrode voltage clamp electrophysiology in Xenopus laevis oocytes, we investigated these interactions by substituting one or both amino acids of each potential pair. We found that Lys¹⁰⁴ in the α₁ subunit forms an electrostatic bond with Asp⁷⁵ of the γ₂ subunit after benzodiazepine binding and that this bond stabilizes the positively modified state of the receptor. Substitution of these two residues to cysteine and subsequent covalent linkage between them increased the receptor's sensitivity to low GABA concentrations and decreased its response to benzodiazepines, producing a GABA_AR that resembles a benzodiazepine-bound WT GABA_AR. Breaking this bond restored sensitivity to GABA to WT levels and increased the receptor's response to benzodiazepines. The α₁ Lys¹⁰⁴ and γ₂ Asp⁷⁵ interaction did not play a role in ethanol or neurosteroid modulation of GABA_AR, suggesting that different modulators induce different conformational changes in the receptor. These findings may help explain the additive or synergistic effects of modulators acting at the GABA_AR.

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.

The novel isoxazoline ectoparasiticide lotilaner (Credelio™): a non-competitive antagonist specific to invertebrates γ-aminobutyric acid-gated chloride channels (GABACls)

BACKGROUND

The isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and, to a lesser extent, of inhibitory glutamate-gated chloride channels (GluCls). Lotilaner (Credelio™), a novel representative of this chemical class, is currently evaluated for its excellent ectoparasiticide properties.

METHODS

In this study, we investigated the molecular mode of action and pharmacology of lotilaner. We report the successful gene identification, cDNA cloning and functional expression in Xenopus oocytes of Drosohpila melanogaster (wild type and dieldrin/fipronil-resistant forms), Lepeophtheirus salmonis (an ectoparasite copepod crustacean of salmon), Rhipicephalus microplus and Canis lupus familiaris GABACls. Automated Xenopus oocyte two-electrode voltage clamp electrophysiology was used to assess GABACls functionality and to compare ion channel inhibition by lotilaner with that of established insecticides addressing GABACls as targets.

RESULTS

In these assays, we demonstrated that lotilaner is a potent non-competitive antagonist of insects (fly) GABACls. No cross-resistance with dieldrin or fipronil resistance mutations was detected, suggesting that lotilaner might bind to a site at least partly different from the one bound by known GABACl blockers. Using co-application experiments, we observed that lotilaner antagonism differs significantly from the classical open channel blocker fipronil. We finally confirmed for the first time that isoxazoline compounds are not only powerful antagonists of GABACls of acari (ticks) but also of crustaceans (sea lice), while no activity on a dog GABA_A receptor was observed up to a concentration of 10 μM.

CONCLUSIONS

Together, these results demonstrate that lotilaner is a non-competitive antagonist specific to invertebrate's γ-aminobutyric acid-gated chloride channels (GABACls). They contribute to our understanding of the mode of action of this new ectoparasiticide compound.

Comparison of γ-Aminobutyric Acid, Type A (GABAA), Receptor αβγ and αβδ Expression Using Flow Cytometry and Electrophysiology: EVIDENCE FOR ALTERNATIVE SUBUNIT STOICHIOMETRIES AND ARRANGEMENTS

The subunit stoichiometry and arrangement of synaptic αβγ GABAA receptors are generally accepted as 2α:2β:1γ with a β-α-γ-β-α counterclockwise configuration, respectively. Whether extrasynaptic αβδ receptors adopt the analogous β-α-δ-β-α subunit configuration remains controversial. Using flow cytometry, we evaluated expression levels of human recombinant γ2 and δ subunits when co-transfected with α1 and/or β2 subunits in HEK293T cells. Nearly identical patterns of γ2 and δ subunit expression were observed as follows: both required co-transfection with α1 and β2 subunits for maximal expression; both were incorporated into receptors primarily at the expense of β2 subunits; and both yielded similar FRET profiles when probed for subunit adjacency, suggesting similar underlying subunit arrangements. However, because of a slower rate of δ subunit degradation, 10-fold less δ subunit cDNA was required to recapitulate γ2 subunit expression patterns and to eliminate the functional signature of α1β2 receptors. Interestingly, titrating γ2 or δ subunit cDNA levels progressively altered GABA-evoked currents, revealing more than one kinetic profile for both αβγ and αβδ receptors. This raised the possibility of alternative receptor isoforms, a hypothesis confirmed using concatameric constructs for αβγ receptors. Taken together, our results suggest a limited cohort of alternative subunit arrangements in addition to canonical β-α-γ/δ-β-α receptors, including β-α-γ/δ-α-α receptors at lower levels of γ2/δ expression and β-α-γ/δ-α-γ/δ receptors at higher levels of expression. These findings provide important insight into the role of GABAA receptor subunit under- or overexpression in disease states such as genetic epilepsies.

Immunochemical Localization of GABAA Receptor Subunits in the Freshwater Polyp Hydra vulgaris (Cnidaria, Hydrozoa)

γ-aminobutyric acid (GABA) receptors, responding to GABA positive allosteric modulators, are present in the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa), one of the most primitive metazoans to develop a nervous system. We examined the occurrence and distribution of GABA_A receptor subunits in Hydra tissues by western blot and immunohistochemistry. Antibodies against different GABA_A receptor subunits were used in Hydra membrane preparations. Unique protein bands, inhibited by the specific peptide, appeared at 35, 60, ∼50 and ∼52 kDa in membranes incubated with α3, β1, γ3 or δ antibodies, respectively. Immunohistochemical screening of whole mount Hydra preparations revealed diffuse immunoreactivity to α3, β1 or γ3 antibodies in tentacles, hypostome, and upper part of the gastric region; immunoreactive fibers were also present in the lower peduncle. By contrast, δ antibodies revealed a strong labeling in the lower gastric region and peduncle, as well as in tentacles. Double labeling showed colocalization of α3/β1, α3/γ3 and α3/δ immunoreactivity in granules or cells in tentacles and gastric region. In the peduncle, colocalization of both α3/β1 and α3/γ3 immunoreactivity was found in fibers running horizontally above the foot. These data indicate that specific GABA_A receptor subunits are present and differentially distributed in Hydra body regions. Subunit colocalization suggests that Hydra GABA receptors are heterologous multimers, possibly sub-serving different physiological activities.

Analysis of β-Subunit-dependent GABAA Receptor Modulation and Behavioral Effects of Valerenic Acid Derivatives

Valerenic acid (VA)-a β2/3-selective GABA type A (GABAA) receptor modulator-displays anxiolytic and anticonvulsive effects in mice devoid of sedation, making VA an interesting drug candidate. Here we analyzed β-subunit-dependent enhancement of GABA-induced chloride currents (IGABA) by a library of VA derivatives and studied their effects on pentylenetetrazole (PTZ)-induced seizure threshold and locomotion. Compound-induced IGABA enhancement was determined in oocytes expressing α1β1γ2S, α1β2γ2S, or α1β3γ2S receptors. Effects on seizure threshold and locomotion were studied using C57BL/6N mice and compared with saline-treated controls. β2/3-selective VA derivatives such as VA-amide (VA-A) modulating α1β3γ2S (VA-A: Emax = 972 ± 69%, n = 6, P < 0.05) and α1β2γ2S receptors (Emax = 1119 ± 72%, n = 6, P < 0.05) more efficaciously than VA (α1β3γ2S: VA: Emax = 632 ± 88%, n = 9 versus α1β2γ2S: VA: Emax = 721 ± 68%, n = 6) displayed significantly more pronounced seizure threshold elevation than VA (saline control: 40.4 ± 1.4 mg/kg PTZ versus VA 10 mg/kg: 49.0 ± 1.8 mg/kg PTZ versus VA-A 3 mg/kg: 57.9 ± 1.9 mg/kg PTZ, P < 0.05). Similarly, VA's methylamide (VA-MA) enhancing IGABA through β3-containing receptors more efficaciously than VA (Emax = 1043 ± 57%, P < 0.01, n = 6) displayed stronger anticonvulsive effects. Increased potency of IGABA enhancement and anticonvulsive effects at lower doses compared with VA were observed for VA-tetrazole (α1β3γ2S: VA-TET: EC50 = 6.0 ± 1.0 μM, P < 0.05; VA-TET: 0.3 mg/kg: 47.3 ± 0.5 mg/kg PTZ versus VA: 10 mg/kg: 49.0 ± 1.8 mg/kg PTZ, P < 0.05). At higher doses (≥10 mg/kg), VA-A, VA-MA, and VA-TET reduced locomotion. In contrast, unselective VA derivatives induced anticonvulsive effects only at high doses (30 mg/kg) or did not display any behavioral effects. Our data indicate that the β2/3-selective compounds VA-A, VA-MA, and VA-TET induce anticonvulsive effects at low doses (≤10 mg/kg), whereas impairment of locomotion was observed at doses ≥10 mg/kg.

Novel positive allosteric modulators of GABAA receptors with anesthetic activity

GABA_A receptors are the main inhibitory neurotransmitter receptors in the brain and are targets for numerous clinically important drugs such as benzodiazepines, anxiolytics and anesthetics. We previously identified novel ligands of the classical benzodiazepine binding pocket in α₁β₂γ₂ GABA_A receptors using an experiment-guided virtual screening (EGVS) method. This screen also identified novel ligands for intramembrane low affinity diazepam site(s). In the current study we have further characterized compounds 31 and 132 identified with EGVS as well as 4-O-methylhonokiol. We investigated the site of action of these compounds in α₁β₂γ₂ GABA_A receptors expressed in Xenopus laevis oocytes using voltage-clamp electrophysiology combined with a benzodiazepine site antagonist and transmembrane domain mutations. All three compounds act mainly through the two β+/α− subunit transmembrane interfaces of the GABA_A receptors. We then used concatenated receptors to dissect the involvement of individual β+/α− interfaces. We further demonstrated that these compounds have anesthetic activity in a small aquatic animal model, Xenopus laevis tadpoles. The newly identified compounds may serve as scaffolds for the development of novel anesthetics.

Functional sites involved in modulation of the GABAA receptor channel by the intravenous anesthetics propofol, etomidate and pentobarbital

GABAA receptors are the major inhibitory neurotransmitter receptors in the brain and are the target for many clinically important drugs. Among the many modulatory compounds are also the intravenous anesthetics propofol and etomidate, and barbiturates. The mechanism of receptor modulation by these compounds is of mayor relevance. The site of action of these compounds has been located to subunit interfaces in the intra-membrane region of the receptor. In α1β2γ2 GABAA receptors there are five such interfaces, two β+/α- and one each of α+/β-, α+/γ- and γ+/β- subunit interfaces. We have used reporter mutations located in the second trans-membrane region in different subunits to probe the effects of changes at these subunit interfaces on modulation by propofol, etomidate and pentobarbital. We provide evidence for the fact that each of these compounds either modulates through a different set of subunit interfaces or through the same set of subunit interfaces to a different degree. As a GABAA receptor pentamer harbors two β+/α- subunit interfaces, we used concatenated receptors to dissect the contribution of individual interfaces and show that only one of these interfaces is important for receptor modulation by etomidate.

A new chromanone derivative isolated from Hypericum lissophloeus (Hypericaceae) potentiates GABAA receptor currents in a subunit specific fashion

A phytochemical investigation of the lipophilic extract of Hypericum lissophloeus (smoothbark St. John's wort, Hypericaceae) was conducted, resulting in the isolation and identification of a new chromanone derivative: 5,7-dihydroxy-2,3-dimethyl-6-(3-methyl-but-2-enyl)-chroman-4-one (1). This compound was demonstrated to act as a potent stimulator of currents elicited by GABA in recombinant α1β2γ2 GABAA receptors, with a half-maximal potentiation observed at a concentration of about 4μM and a maximal potentiation of >4000%. Significant potentiation was already evident at a concentration as low as 0.1μM. Extent of potentiation strongly depends on the type of α subunit, the type of β subunit and the presence of the γ subunit.

Identification of the putative binding pocket of valerenic acid on GABAA receptors using docking studies and site-directed mutagenesis

Background and Purpose

β2/3‐subunit‐selective modulation of GABA_A receptors by valerenic acid (VA) is determined by the presence of transmembrane residue β2/3N265. Currently, it is not known whether β2/3N265 is part of VA's binding pocket or is involved in the transduction pathway of VA's action. The aim of this study was to clarify the localization of VA's binding pocket on GABA_A receptors.

Experimental Approach

Docking and a structure‐based three‐dimensional pharmacophore were employed to identify candidate amino acid residues that are likely to interact with VA. Selected amino acid residues were mutated, and VA‐induced modulation of the resulting GABA_A receptors expressed in Xenopus oocytes was analysed.

Key Results

A binding pocket for VA at the β⁺/α⁻ interface encompassing amino acid β3N265 was predicted. Mutational analysis of suggested amino acid residues revealed a complete loss of VA's activity on β3M286W channels as well as significantly decreased efficacy and potency of VA on β3N265S and β3F289S receptors. In addition, reduced efficacy of VA‐induced I_GABA enhancement was also observed for α1M235W, β3R269A and β3M286A constructs.

Conclusions and Implications

Our data suggest that amino acid residues β3N265, β3F289, β3M286, β3R269 in the β3 subunit, at or near the etomidate/propofol binding site(s), form part of a VA binding pocket. The identification of the binding pocket for VA is essential for elucidating its pharmacological effects and might also help to develop new selective GABA_A receptor ligands.

The Direct Actions of GABA, 2'-Methoxy-6-Methylflavone and General Anaesthetics at β3γ2L GABAA Receptors: Evidence for Receptors with Different Subunit Stoichiometries

2'-Methoxy-6-methylflavone (2'MeO6MF) is an anxiolytic flavonoid which has been shown to display GABAA receptor (GABAAR) β2/3-subunit selectivity, a pharmacological profile similar to that of the general anaesthetic etomidate. Electrophysiological studies suggest that the full agonist action of 2'MeO6MF at α2β3γ2L GABAARs may mediate the flavonoid's in vivo effects. However, we found variations in the relative efficacy of 2'MeO6MF (2'MeO6MF-elicited current responses normalised to the maximal GABA response) at α2β3γ2L GABAARs due to the presence of mixed receptor populations. To understand which receptor subpopulation(s) underlie the variations observed, we conducted a systematic investigation of 2'MeO6MF activity at all receptor combinations that could theoretically form (α2, β3, γ2L, α2β3, α2γ2L, β3γ2L and α2β3γ2L) in Xenopus oocytes using the two-electrode voltage clamp technique. We found that 2'MeO6MF activated non-α-containing β3γ2L receptors. In an attempt to establish the optimal conditions to express a uniform population of these receptors, we found that varying the relative amounts of β3:γ2L subunit mRNAs resulted in differences in the level of constitutive activity, the GABA concentration-response relationships, and the relative efficacy of 2'MeO6MF activation. Like 2'MeO6MF, general anaesthetics such as etomidate and propofol also showed distinct levels of relative efficacy across different injection ratios. Based on these results, we infer that β3γ2L receptors may form with different subunit stoichiometries, resulting in the complex pharmacology observed across different injection ratios. Moreover, the discovery that GABA and etomidate have direct actions at the α-lacking β3γ2L receptors raises questions about the structural requirements for their respective binding sites at GABAARs.

Assembly, trafficking and function of α1β2γ2 GABAA receptors are regulated by N-terminal regions, in a subunit-specific manner

GABAA receptors are pentameric ligand-gated ion channels that mediate inhibitory fast synaptic transmission in the central nervous system. Consistent with recent pentameric ligand-gated ion channels structures, sequence analysis predicts an α-helix near the N-terminus of each GABAA receptor subunit. Preceding each α-helix are 8-36 additional residues, which we term the N-terminal extension. In homomeric GABAC receptors and nicotinic acetylcholine receptors, the N-terminal α-helix is functionally essential. Here, we determined the role of the N-terminal extension and putative α-helix in heteromeric α1β2γ2 GABAA receptors. This role was most prominent in the α1 subunit, with deletion of the N-terminal extension or further deletion of the putative α-helix both dramatically reduced the number of functional receptors at the cell surface. Conversely, deletion of the β2 or γ2 N-terminal extension had little effect on the number of functional cell surface receptors. Additional deletion of the putative α-helix in the β2 or γ2 subunits did, however, decrease both functional cell surface receptors and incorporation of the γ2 subunit into mature receptors. In the β2 subunit only, α-helix deletions affected GABA sensitivity and desensitization. Our findings demonstrate that N-terminal extensions and α-helices make key subunit-specific contributions to assembly, consistent with both regions being involved in inter-subunit interactions. N-terminal α-helices and preceding sequences of eukaryotic pentameric ligand-gated ion channels are absent in prokaryotic homologues, suggesting they may not be functionally essential. Here, we show that in heteropentameric α1β2γ2 GABAA receptors, the role of these segments is highly subunit dependent. The extension preceding the α-helix in the α subunit is crucial for assembly and trafficking, but is of little importance in β and γ subunits. Indeed, robust receptor levels remain when the extension and α-helix are removed in β or γ subunits.

Structural analogues of the natural products magnolol and honokiol as potent allosteric potentiators of GABA(A) receptors

Biphenylic compounds related to the natural products magnolol and 4'-O-methylhonokiol were synthesized, evaluated and optimized as positive allosteric modulators (PAMs) of GABA(A) receptors. The most efficacious compounds were the magnolol analog 5-ethyl-5'-hexylbiphenyl-2,2'-diol (45) and the honokiol analogs 4'-methoxy-5-propylbiphenyl-2-ol (61), 5-butyl-4'-methoxybiphenyl-2-ol (62) and 5-hexyl-4'-methoxybiphenyl-2-ol (64), which showed a most powerful potentiation of GABA-induced currents (up to 20-fold at a GABA concentration of 3μM). They were found not to interfere with the allosteric sites occupied by known allosteric modulators, such as benzodiazepines and N-arachidonoylglycerol. These new PAMs will be useful as pharmacological tools and may have therapeutic potential for mono-therapy, or in combination, for example, with GABA(A) receptor agonists.

A cycloartane glycoside derived from Actaea racemosa L. modulates GABAA receptors and induces pronounced sedation in mice

23-O-Acetylshengmanol 3-O-β-D-xylopyranoside (Ac-SM) isolated from Actaea racemosa L.-an herbal remedy for the treatment of mild menopausal disorders-has been recently identified as a novel efficacious modulator of GABAA receptors composed of α1-, β2-, and γ2S-subunits. In the present study, we analyzed a potential subunit-selective modulation of GABA-induced chloride currents (IGABA) at GABA concentrations eliciting 3-8% of the maximal GABA response (EC3-8) through nine GABAA receptor isoforms expressed in Xenopus laevis oocytes by Ac-SM with two-microelectrode voltage clamp and behavioral effects 30 minutes after intraperitoneal application in a mouse model. Efficacy of IGABA enhancement by Ac-SM displayed a mild α-subunit dependence with α2β2γ2S (maximal IGABA potentiation [Emax] = 1454 ± 97%) and α5β2γ2S (Emax = 1408 ± 87%) receptors being most efficaciously modulated, followed by slightly weaker IGABA enhancement through α1β2γ2S (Emax = 1187 ± 166%), α3β2γ2S (Emax = 1174 ± 218%), and α6β2γ2S (Emax = 1171 ± 274%) receptors and less pronounced effects on receptors composed of α4β2γ2S (Emax = 752 ± 53%) subunits, whereas potency was not affected by the subunit composition (EC50 values ranging from α1β2γ2S = 35.4 ± 12.3 µM to α5β2γ2S = 50.9 ± 11.8 µM). Replacing β2- with β1- or β3-subunits as well as omitting the γ2S-subunit affected neither efficacy nor potency of IGABA enhancement by Ac-SM. Ac-SM shifted the GABA concentration-response curve toward higher GABA sensitivity (about 3-fold) and significantly increased the maximal GABA response by 44 ± 13%, indicating a pharmacological profile distinct from a pure allosteric GABAA receptor modulator. In mice, Ac-SM significantly reduced anxiety-related behavior in the elevated plus maze test at a dose of 0.6 mg/kg, total ambulation in the open field test at doses ≥6 mg/kg, stress-induced hyperthermia at doses ≥0.6 mg/kg, and significantly elevated seizure threshold at doses ≥20 mg/kg body weight. High efficacy and long biologic half-life of Ac-SM suggest that potential cumulative sedative side effects upon repetitive intake of A. racemosa L. preparations might not be negligible.

Moderate concentrations of 4-O-methylhonokiol potentiate GABAA receptor currents stronger than honokiol

BACKGROUND

Magnolia bark preparations from Magnolia officinalis of Asian medicinal systems are known for their muscle relaxant effect and anticonvulsant activity. These CNS related effects are ascribed to the presence of the biphenyl-type neolignans honokiol and magnolol that exert a potentiating effect on GABAA receptors. 4-O-methylhonokiol isolated from seeds of the North-American M. grandiflora was compared to honokiol for its activity to potentiate GABAA receptors and its GABAA receptor subtype-specificity was established.

METHODS

Different recombinant GABAA receptors were functionally expressed in Xenopus oocytes and electrophysiological techniques were used determine to their modulation by 4-O-methylhonokiol.

RESULTS

3μM 4-O-methylhonokiol is shown here to potentiate responses of the α₁β₂γ₂ GABAA receptor about 20-fold stronger than the same concentration of honokiol. In the present study potentiation by 4-O-methylhonokiol is also detailed for 12 GABAA receptor subtypes to assess GABAA receptor subunits that are responsible for the potentiating effect.

CONCLUSION

The much higher potentiation of GABAA receptors at identical concentrations of 4-O-methylhonokiol as compared to honokiol parallels previous observations made in other systems of potentiated pharmacological activity of 4-O-methylhonokiol over honokiol.

GENERAL SIGNIFICANCE

The results point to the use of 4-O-methylhonokiol as a lead for GABAA receptor potentiation and corroborate the use of M. grandiflora seeds against convulsions in Mexican folk medicine.

Esters of valerenic acid as potential prodrugs

Valerenic acid (VA) is a β_2/3 subunit-specific modulator of γ-aminobutyric acid (GABA) type A (GABA_A) receptors inducing anxiolysis. Here we analyze if VA-esters can serve as prodrugs and if different ester structures have different in vitro/in vivo effects. Modulation of GABA_A receptors expressed in Xenopus oocytes was studied with 2-microelectrode-voltage-clamp. Anxiolytic effects of the VA-esters were studied on male C57BL/6N mice by means of the elevated plus maze-test; anticonvulsant properties were deduced from changes in seizure threshold upon pentylenetetrazole infusion. VA was detected in plasma confirming hydrolysis of the esters and release of VA in vivo. Esterification significantly reduced the positive allosteric modulation of GABA_A (α₁β₃γ_2S) receptors in vitro. in vivo, the studied VA-ester derivatives induced similar or even stronger anxiolytic and anticonvulsant action than VA. While methylation and propylation of VA resulted in faster onset of anxiolysis, the action of VA-ethylester was longer lasting, but occurred with a significant delay. The later finding is in line with the longer lasting anticonvulsant effects of this compound. The estimated VA plasma concentrations provided first insight into the release kinetics from different VA-esters. This might be an important step for its future clinical application as a potential non-sedative anxiolytic and anticonvulsant.

Do N-arachidonyl-glycine (NA-glycine) and 2-arachidonoyl glycerol (2-AG) share mode of action and the binding site on the β2 subunit of GABAA receptors?

NA-glycine is an endogenous lipid molecule with analgesic properties, which is structurally similar to the endocannabinoids 2-AG and anandamide but does not interact with cannabinoid receptors. NA-glycine has been suggested to act at the G-protein coupled receptors GPR18 and GPR92. Recently, we have described that NA-glycine can also modulate recombinant α₁β₂γ₂ GABA_A receptors. Here we characterize in more detail this modulation and investigate the relationship of its binding site with that of the endocannabinoid 2-AG.

GABAA receptor modulation by piperine and a non-TRPV1 activating derivative

The action of piperine (the pungent component of pepper) and its derivative SCT-66 ((2E,4E)-5-(1,3-benzodioxol-5-yl))-N,N-diisobutyl-2,4-pentadienamide) on different gamma-aminobutyric acid (GABA) type A (GABA_A) receptors, transient-receptor-potential-vanilloid-1 (TRPV1) receptors and behavioural effects were investigated.

GABA_A receptor subtypes and TRPV1 receptors were expressed in Xenopus laevis oocytes. Modulation of GABA-induced chloride currents (I_GABA) by piperine and SCT-66 and activation of TRPV1 was studied using the two-microelectrode-voltage-clamp technique and fast perfusion. Their effects on explorative behaviour, thermoregulation and seizure threshold were analysed in mice. Piperine acted with similar potency on all GABA_A receptor subtypes (EC₅₀ range: 42.8 ± 7.6 μM (α₂β₂)–59.6 ± 12.3 μM (α₃β₂)). I_GABA modulation by piperine did not require the presence of a γ_2S-subunit, suggesting a binding site involving only α and β subunits. I_GABA activation was slightly more efficacious on receptors formed from β_2/3 subunits (maximal I_GABA stimulation through α₁β₃ receptors: 332 ± 64% and α₁β₂: 271 ± 36% vs. α₁β₁: 171 ± 22%, p < 0.05) and α₃-subunits (α₃β₂: 375 ± 51% vs. α₅β₂:136 ± 22%, p < 0.05). Replacing the piperidine ring by a N,N-diisobutyl residue (SCT-66) prevents interactions with TRPV1 and simultaneously increases the potency and efficiency of GABA_A receptor modulation. SCT-66 displayed greater efficacy on GABA_A receptors than piperine, with different subunit-dependence. Both compounds induced anxiolytic, anticonvulsant effects and reduced locomotor activity; however, SCT-66 induced stronger anxiolysis without decreasing body temperature and without the proconvulsive effects of TRPV1 activation and thus may serve as a scaffold for the development of novel GABA_A receptor modulators.

Ginsenoside Rg₃ regulates GABAA receptor channel activity: involvement of interaction with the γ₂ subunit

Ginseng exhibits beneficial effects on GABAA receptor-related anxiety and sleep disorders. However, little is known regarding the cellular and molecular bases of the ginseng action on GABAA receptor. The present study was performed to elucidate the molecular mechanism of the ginseng effect on GABAA receptor. The effect of ginsenoside Rg₃ (Rg₃), one of the active ingredients of ginseng, on γ-aminobutyric acid (GABA)A receptor channel activity was examined in Xenopus oocytes using two-electrode voltage-clamp technique. Rg₃ itself evoked an inward current in Xenopus oocytes expressing GABAA receptor subunits (α₁β₁γ₂) and the Rg₃ itself-elicited inward current was only selective to γ₂ subunit expression ratio, since Rg₃ alone had no effects in oocytes expressing other subunits such as γ₁, γ₃, δ, or ε. Co-treatment of Rg₃ with GABA enhanced GABA receptor (α₁β₁γ₂)-mediated inward currents (IGABA) but Rg₃-mediated IGABA enhancement was independent on γ₂. Rg₃ itself-elicited inward current was blocked by GABAA receptor antagonist. The present results indicate that Rg₃-induced GABAA receptor activation via the γ₂ subunit and IGABA enhancement by Rg₃ might be one of the molecular bases of ginseng effects on GABAA receptor.

Potency of GABA at human recombinant GABA(A) receptors expressed in Xenopus oocytes: a mini review

GABAA receptors are members of the ligand-gated ion channel superfamily that mediate inhibitory neurotransmission in the central nervous system. They are thought to be composed of 2 alpha (α), 2 beta (β) subunits and one other such as a gamma (γ) or delta (δ) subunit. The potency of GABA is influenced by the subunit composition. However, there are no reported systematic studies that evaluate GABA potency on a comprehensive number of subunit combinations expressed in Xenopus oocytes, despite the wide use of this heterologous expression system in structure-function studies and drug discovery. Thus, the aim of this study was to conduct a systematic characterization of the potency of GABA at 43 human recombinant GABA(A) receptor combinations expressed in Xenopus oocytes using the two-electrode voltage clamp technique. The results show that the α-subunits and to a lesser extent, the β-subunits influence GABA potency. Of the binary and ternary combinations with and without the γ2L subunit, the α6/γ2L-containing receptors were the most sensitive to GABA, while the β2- or β3-subunit conferred higher sensitivity to GABA than receptors containing the β1-subunit with the exception of the α2β1γ2L and α6β1γ2L subtypes. Of the δ-subunit containing GABA(A) receptors, α4/δ-containing GABA(A) receptors displayed highest GABA sensitivity, with mid-nanomolar concentrations activating α4β1δ and α4β3δ receptors. At α4β2δ, GABA had low micromolar activity.

Relative neurotoxicity of ivermectin and moxidectin in Mdr1ab (-/-) mice and effects on mammalian GABA(A) channel activity

The anthelmintics ivermectin (IVM) and moxidectin (MOX) display differences in toxicity in several host species. Entrance into the brain is restricted by the P-glycoprotein (P-gp) efflux transporter, while toxicity is mediated through the brain GABA(A) receptors. This study compared the toxicity of IVM and MOX in vivo and their interaction with GABA(A) receptors in vitro. Drug toxicity was assessed in Mdr1ab(−/−) mice P-gp-deficient after subcutaneous administration of increasing doses (0.11–2.0 and 0.23–12.9 µmol/kg for IVM and MOX in P-gp-deficient mice and half lethal doses (LD₅₀) in wild-type mice). Survival was evaluated over 14-days. In Mdr1ab(−/−) mice, LD₅₀ was 0.46 and 2.3 µmol/kg for IVM and MOX, respectively, demonstrating that MOX was less toxic than IVM. In P-gp-deficient mice, MOX had a lower brain-to-plasma concentration ratio and entered into the brain more slowly than IVM. The brain sublethal drug concentrations determined after administration of doses close to LD₅₀ were, in Mdr1ab(−/−) and wild-type mice, respectively, 270 and 210 pmol/g for IVM and 830 and 740–1380 pmol/g for MOX, indicating that higher brain concentrations are required for MOX toxicity than IVM. In rat α1β2γ2 GABA channels expressed in Xenopus oocytes, IVM and MOX were both allosteric activators of the GABA-induced response. The Hill coefficient was 1.52±0.45 for IVM and 0.34±0.56 for MOX (p<0.001), while the maximum potentiation caused by IVM and MOX relative to GABA alone was 413.7±66.1 and 257.4±40.6%, respectively (p<0.05), showing that IVM causes a greater potentiation of GABA action on this receptor. Differences in the accumulation of IVM and MOX in the brain and in the interaction of IVM and MOX with GABA(A) receptors account for differences in neurotoxicity seen in intact and Mdr1-deficient animals. These differences in neurotoxicity of IVM and MOX are important in considering their use in humans.

Ivermectin (IVM) is used for onchocerciasis mass drug administration and is important for control of lymphatic filariasis, strongyloidiases and Scarcoptes mange in humans. It is widely used for parasite control in livestock. Moxidectin (MOX) is being evaluated against Onchocerca volvulus in humans and is also widely used in veterinary medicine. Both anthelmintics are macrocyclic lactones (MLs) that act on ligand-gated chloride channels and share similar spectra of activity. Nevertheless, there are marked differences in their pharmacokinetics, pharmacodynamics and toxicity. Usually, both MLs are remarkably safe drugs. However, there are reports of severe adverse events to IVM, in some humans with high Loa loa burdens, and IVM can be neurotoxic in animals with defects in P-glycoproteins (P-gp) in the blood-brain barrier. We have compared the in vivo neurotoxicity of IVM and MOX in P-gp-deficient mice and their accumulation in brain. We also investigated their effects on mammalian GABA receptors. We show that MOX has a wider margin of safety than IVM, even when the blood-brain barrier function is impaired, and that the neurotoxicity in vivo is related to different effects of the drugs on GABA-gated channels. These observations contribute to understanding ML toxicity and open new perspectives for possible MOX use in humans.

Influence of GABA(A) receptor α subunit isoforms on the benzodiazepine binding site

Classical benzodiazepines, such as diazepam, interact with α_xβ₂γ₂ GABA_A receptors, x = 1, 2, 3, 5 and modulate their function. Modulation of different receptor isoforms probably results in selective behavioural effects as sedation and anxiolysis. Knowledge of differences in the structure of the binding pocket in different receptor isoforms is of interest for the generation of isoform-specific ligands. We studied here the interaction of the covalently reacting diazepam analogue 3-NCS with α₁S204Cβ₂γ₂, α₁S205Cβ₂γ₂ and α₁T206Cβ₂γ₂ and with receptors containing the homologous mutations in α₂β₂γ₂, α₃β₂γ₂, α₅β_1/2γ₂ and α₆β₂γ₂. The interaction was studied using radioactive ligand binding and at the functional level using electrophysiological techniques. Both strategies gave overlapping results. Our data allow conclusions about the relative apposition of α₁S204Cβ₂γ₂, α₁S205Cβ₂γ₂ and α₁T206Cβ₂γ₂ and homologous positions in α₂, α₃, α₅ and α₆ with C-atom adjacent to the keto-group in diazepam. Together with similar data on the C-atom carrying Cl in diazepam, they indicate that the architecture of the binding site for benzodiazepines differs in each GABA_A receptor isoform α₁β₂γ₂, α₂β₂γ₂, α₃β₂γ₂, α₅β_1/2γ₂ and α₆β₂γ₂.

The cannabinoid CB1 receptor antagonists rimonabant (SR141716) and AM251 directly potentiate GABA(A) receptors

BACKGROUND AND PURPOSE

Rimonabant (SR141716) and the structurally related AM251 are widely used in pharmacological experiments as selective cannabinoid receptor CB(1) antagonists / inverse agonists. Concentrations of 0.5-10 µM are usually applied in in vitro experiments. We intended to show that these drugs did not act at GABA(A) receptors but found a significant positive allosteric modulation instead.

EXPERIMENTAL APPROACH

Recombinant GABA(A) receptors were expressed in Xenopus oocytes. Receptors were exposed to AM251 or rimonabant in the absence and presence of GABA. Standard electrophysiological techniques were used to monitor the elicited ionic currents.

KEY RESULTS

AM251 dose-dependently potentiated responses to 0.5 µM GABA at the recombinant α(1) β(2) γ(2) GABA(A) receptor with an EC(50) below 1 µM and a maximal potentiation of about eightfold. The Hill coefficient indicated that more than one binding site for AM251 was located in this receptor. Rimonabant had a lower affinity, but a fourfold higher efficacy. AM251 potentiated also currents mediated by α(1) β(2) , α(x) β(2) γ(2) (x = 2,3,5,6), α(1) β(3) γ(2) and α(4) β(2) δ GABA(A) receptors, but not those mediated by α(1) β(1) γ(2) . Interestingly, the CB(1) receptor antagonists LY320135 and O-2050 did not significantly affect α(1) β(2) γ(2) GABA(A) receptor-mediated currents at concentrations of 1 µM.

CONCLUSIONS AND IMPLICATIONS

This study identified rimonabant and AM251 as positive allosteric modulators of GABA(A) receptors. Thus, potential GABAergic effects of commonly used concentrations of these compounds should be considered in in vitro experiments, especially at extrasynaptic sites where GABA concentrations are low.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Two etomidate sites in α1β2γ2 γ-aminobutyric acid type A receptors contribute equally and noncooperatively to modulation of channel gating

BACKGROUND

Etomidate is a potent hypnotic agent that acts via γ-aminobutyric acid receptor type A (GABA(A)) receptors. Evidence supports the presence of two etomidate sites per GABA(A) receptor, and current models assume that each site contributes equally and noncooperatively to drug effects. These assumptions remain untested.

METHODS

We used concatenated dimer (β2-α1) and trimer (γ2-β2-α1) GABA(A) subunit assemblies that form functional α1β2γ2 channels, and inserted α1M236W etomidate site mutations into both dimers (β2-α1M236W) and trimers (γ2-β2-α1M236W). Wild-type or mutant dimers (D(wt) or D(αM236W)) and trimers (T(wt) or T(αM236W)) were coexpressed in Xenopus oocytes to produce four types of channels: D(wt)T(wt), D(αM236W)T(wt), D(wt)T(αM236W), and D(αM236W)T(αM236W). For each channel type, two-electrode voltage clamp was performed to quantitatively assess GABA EC(50), etomidate modulation (left shift), etomidate direct activation, and other functional parameters affected by αM236W mutations.

RESULTS

Concatenated wild-type D(wt)T(wt) channels displayed etomidate modulation and direct activation similar to α1β2γ2 receptors formed with free subunits. D(αM236W)T(αM236W) receptors also displayed altered GABA sensitivity and etomidate modulation similar to mutated channels formed with free subunits. Both single-site mutant receptors (D(αM236W)T(wt) and D(wt)T(αM236W)) displayed indistinguishable functional properties and equal gating energy changes for GABA activation (-4.9 ± 0.48 vs. -4.7 ± 0.48 kJ/mol, respectively) and etomidate modulation (-3.4 ± 0.49 vs. -3.7 ± 0.38 kJ/mol, respectively), which together accounted for the differences between D(wt)T(wt) and D(αM236W)T(αM236W) channels.

CONCLUSIONS

These results support the hypothesis that the two etomidate sites on α1β2γ2 GABA(A) receptors contribute equally and noncooperatively to drug interactions and gating effects.

2'-Methoxy-6-methylflavone: a novel anxiolytic and sedative with subtype selective activating and modulating actions at GABA(A) receptors

BACKGROUND AND PURPOSE

Flavonoids are known to have anxiolytic and sedative effects mediated via actions on ionotropic GABA receptors. We sought to investigate this further.

EXPERIMENTAL APPROACH

We evaluated the effects of 2'-methoxy-6-methylflavone (2'MeO6MF) on native GABA(A) receptors in new-born rat hippocampal neurons and determined specificity from 18 human recombinant GABA(A) receptor subtypes expressed in Xenopus oocytes. We used ligand binding, two-electrode voltage clamp and patch clamp studies together with behavioural studies.

KEY RESULTS

2'MeO6MF potentiated GABA at α2β1γ2L and all α1-containing GABA(A) receptor subtypes. At α2β2/3γ2L GABA(A) receptors, however, 2'MeO6MF directly activated the receptors without potentiating GABA. This activation was attenuated by bicuculline and gabazine but not flumazenil indicating a novel site. Mutation studies showed position 265 in the β1/2 subunit was key to whether 2'MeO6MF was an activator or a potentiator. In hippocampal neurons, 2'MeO6MF directly activated single-channel currents that showed the hallmarks of GABA(A) Cl(-) currents. In the continued presence of 2'MeO6MF the single-channel conductance increased and these high conductance channels were disrupted by the γ2(381-403) MA peptide, indicating that such currents are mediated by α2/γ2-containing GABA(A) receptors. In mice, 2'MeO6MF (1-100 mg·kg(-1) ; i.p.) displayed anxiolytic-like effects in two unconditioned models of anxiety: the elevated plus maze and light/dark tests. 2'MeO6MF induced sedative effects at higher doses in the holeboard, actimeter and barbiturate-induced sleep time tests. No myorelaxant effects were observed in the horizontal wire test.

CONCLUSIONS AND IMPLICATIONS

2'MeO6MF will serve as a tool to study the complex nature of the activation and modulation of GABA(A) receptor subtypes.

Down-regulation of GABA(A) receptor via promiscuity with the vasoactive peptide urotensin II receptor. Potential involvement in astrocyte plasticity

GABA_A receptor (GABA_AR) expression level is inversely correlated with the proliferation rate of astrocytes after stroke or during malignancy of astrocytoma, leading to the hypothesis that GABA_AR expression/activation may work as a cell proliferation repressor. A number of vasoactive peptides exhibit the potential to modulate astrocyte proliferation, and the question whether these mechanisms may imply alteration in GABA_AR-mediated functions and/or plasma membrane densities is open. The peptide urotensin II (UII) activates a G protein-coupled receptor named UT, and mediates potent vasoconstriction or vasodilation in mammalian vasculature. We have previously demonstrated that UII activates a PLC/PIPs/Ca²⁺ transduction pathway, via both G_q and G_i/o proteins and stimulates astrocyte proliferation in culture. It was also shown that UT/G_q/IP₃ coupling is regulated by the GABA_AR in rat cultured astrocytes. Here we report that UT and GABA_AR are co-expressed in cerebellar glial cells from rat brain slices, in human native astrocytes and in glioma cell line, and that UII inhibited the GABAergic activity in rat cultured astrocytes. In CHO cell line co-expressing human UT and combinations of GABA_AR subunits, UII markedly depressed the GABA current (β₃γ₂>α₂β₃γ₂>α₂β₁γ₂). This effect, characterized by a fast short-term inhibition followed by drastic and irreversible run-down, is not relayed by G proteins. The run-down partially involves Ca²⁺ and phosphorylation processes, requires dynamin, and results from GABA_AR internalization. Thus, activation of the vasoactive G protein-coupled receptor UT triggers functional inhibition and endocytosis of GABA_AR in CHO and human astrocytes, via its receptor C-terminus. This UII-induced disappearance of the repressor activity of GABA_AR, may play a key role in the initiation of astrocyte proliferation.

Mutations in the GABAA receptor that mimic the allosteric ligand etomidate

Etomidate is a hydrophobic molecule, a potent general anesthetic, and the best understood drug in this group. Etomidate's target molecules are GABA(A) receptors, its site of action has been identified with photolabeling, and a quantitative allosteric coagonist model has emerged for etomidate effects on GABA(A) receptors. We have shown that when methionine residues that are thought to be adjacent to the etomidate site are mutated to tryptophan, that the bulky hydrophobic side-chains alter mutant GABA(A) receptor function in ways that mimic the effects of etomidate binding to wild-type receptors. Furthermore, these mutations reduce receptor modulation by etomidate. Both of these observations support the hypothesis that these methionine residues form part of the etomidate binding pocket.

Retrochalcone derivatives are positive allosteric modulators at synaptic and extrasynaptic GABA(A) receptors in vitro

BACKGROUND AND PURPOSE

Flavonoids, important plant pigments, have been shown to allosterically modulate brain GABA(A) receptors (GABA(A)Rs). We previously reported that trans-6,4'-dimethoxyretrochalcone (Rc-OMe), a hydrolytic derivative of the corresponding flavylium salt, displayed nanomolar affinity for the benzodiazepine binding site of GABA(A)Rs. Here, we evaluate the functional modulations of Rc-OMe, along with two other synthetic derivatives trans-6-bromo-4'-methoxyretrochalcone (Rc-Br) and 4,3'-dimethoxychalcone (Ch-OMe) on GABA(A)Rs.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp recordings were made to determine the effects of these derivatives on GABA(A)Rs expressed in HEK-293 cells and in hippocampal CA1 pyramidal and thalamic neurones from rat brain.

KEY RESULTS

Rc-OMe strongly potentiated GABA-evoked currents at recombinant α(1-4)β(2)γ(2s) and α(4)β(3)δ receptors but much less at α(1)β(2) and α(4)β(3). Rc-Br and Ch-OMe potentiated GABA-evoked currents at α(1)β(2)γ(2s). The potentiation by Rc-OMe was only reduced at α(1)H101Rβ(2)γ(2s) and α(1)β(2)N265Sγ(2s), mutations known to abolish the potentiation by diazepam and loreclezole respectively. The modulation of Rc-OMe and pentobarbital as well as by Rc-OMe and the neurosteroid 3α,21-dihydroxy-5α-pregnan-20-one was supra-additive. Rc-OMe modulation exhibited no apparent voltage-dependence, but was markedly dependent on GABA concentration. In neurones, Rc-Br slowed the decay of spontaneous inhibitory postsynaptic currents and both Rc-OMe and Rc-Br positively modulated synaptic and extrasynaptic diazepam-insensitive GABA(A)Rs.

CONCLUSIONS AND IMPLICATIONS

The trans-retrochalcones are powerful positive allosteric modulators of synaptic and extrasynaptic GABA(A)Rs. These novel modulators act through an original mode, thus making them putative drug candidates in the treatment of GABA(A)-related disorders in vivo.

Different residues in the GABAA receptor benzodiazepine binding pocket mediate benzodiazepine efficacy and binding

Benzodiazepines (BZDs) exert their therapeutic actions by binding to the GABA(A) receptor (GABA(A)R) and allosterically modulating GABA-induced chloride currents (I(GABA)). A variety of ligands with divergent structures bind to the BZD site, and the structural mechanisms that couple their binding to potentiation of I(GABA) are not well understood. In this study, we measured the effects of individually mutating 22 residues throughout the BZD binding pocket on the abilities of eszopiclone, zolpidem, and flurazepam to potentiate I(GABA). Wild-type and mutant α(1)β(2)γ(2) GABA(A)Rs were expressed in Xenopus laevis oocytes and analyzed using a two-electrode voltage clamp. GABA EC(50), BZD EC(50), and BZD maximal potentiation were measured. These data, combined with previous radioligand binding data describing the mutations' effects on BZD apparent binding affinities (J Neurosci 28:3490-3499, 2008; J Med Chem 51:7243-7252, 2008), were used to distinguish residues within the BZD pocket that contribute to BZD efficacy and BZD binding. We identified six residues whose mutation altered BZD maximal potentiation of I(GABA) (BZD efficacy) without altering BZD binding apparent affinity, three residues whose mutation altered binding but had no effect on BZD efficacy, and four residues whose mutation affected both binding and efficacy. Moreover, depending on the BZD ligand, the effects of some mutations were different, indicating that the structural mechanisms underlying the ability of BZD ligands with divergent structures to potentiate I(GABA) are distinct.

Disulphide trapping of the GABA(A) receptor reveals the importance of the coupling interface in the action of benzodiazepines

BACKGROUND AND SIGNIFICANCE

Although the functional effects of benzodiazepines (BZDs) on GABA(A) receptors have been well characterized, the structural mechanism by which these modulators alter activation of the receptor by GABA is still undefined.

EXPERIMENTAL APPROACH

We used disulphide trapping between engineered cysteines to probe BZD-induced conformational changes within the γ₂ subunit and at the α₁/γ₂ coupling interface (Loops 2, 7 and 9) of α₁β₂γ₂ GABA(A) receptors.

KEY RESULTS

Crosslinking γ₂ Loop 9 to γ₂β-strand 9 (via γ₂ S195C/F203C and γ₂ S187C/L206C) significantly decreased maximum potentiation by flurazepam, suggesting that modulation of GABA-induced current (I(GABA)) by flurazepam involves movements of γ₂ Loop 9 relative to γ₂β-strand 9. In contrast, tethering γ₂β-strand 9 to the γ₂ pre-M1 region (via γ₂S202C/S230C) significantly enhanced potentiation by both flurazepam and zolpidem, indicating γ₂S202C/S230C trapped the receptor in a more favourable conformation for positive modulation by BZDs. Intersubunit disulphide bonds formed at the α/γ coupling interface between α₁ Loop 2 and γ₂Loop 9 (α₁D56C/γ₂L198C) prevented flurazepam and zolpidem from efficiently modulating I(GABA) . Disulphide trapping α₁ Loop 2 (α₁D56C) to γ₂β-strand 1 (γ₂P64C) decreased maximal I(GABA) as well as flurazepam potentiation. None of the disulphide bonds affected the ability of the negative modulator, 3-carbomethoxy-4-ethyl-6,7-dimethoxy-β-carboline (DMCM), to inhibit I(GABA) .

CONCLUSIONS AND IMPLICATIONS

Positive modulation of GABA(A) receptors by BZDs requires reorganization of the loops in the α₁/γ₂ coupling interface. BZD-induced movements at the α/γ coupling interface likely synergize with rearrangements induced by GABA binding at the β/α subunit interfaces to enhance channel activation by GABA.

Bioactivity-guided isolation of GABA(A) receptor modulating constituents from the rhizomes of Actaea racemosa

Black cohosh (Actaea racemosa) is a frequently used herbal remedy for the treatment of mild climacteric symptoms. In the present study, the modulation of γ-aminobutryic acid (GABA)-induced chloride currents (I(GABA)) through GABA type A (GABA(A)) receptors by black cohosh extracts and isolated compounds was investigated. GABA(A) receptors, consisting of α(1), β(2), and γ(2S) subunits, were expressed in Xenopus laevis oocytes, and potentiation of I(GABA) was measured using the two-microelectrode voltage clamp technique. In a bioactivity-guided isolation procedure the positive modulation of I(GABA) could be restricted to the plant terpenoid fractions, resulting in the isolation of 11 cycloartane glycosides, of which four significantly (p < 0.05) enhanced I(GABA). The most efficient effect was observed for 23-O-acetylshengmanol 3-O-β-d-xylopyranoside (4, 100 μM), enhancing I(GABA) by 1692 ± 201%, while actein (1), cimigenol 3-O-β-d-xylopyranoside (6), and 25-O-acetylcimigenol 3-O-α-l-arabinopyranoside (8) were significantly less active. In the absence of GABA, only 4 induced small (not exceeding 1% of I(GABA-max)) chloride inward currents through GABA(A) receptors. It is hypothesized that the established positive allosteric modulation of GABA(A) receptors may contribute to beneficial effects of black cohosh extracts in the treatment of climacteric symptoms.

Alcohol- and alcohol antagonist-sensitive human GABAA receptors: tracking δ subunit incorporation into functional receptors

GABA(A) receptors (GABA(A)Rs) have long been a focus as targets for alcohol actions. Recent work suggests that tonic GABAergic inhibition mediated by extrasynaptic δ subunit-containing GABA(A)Rs is uniquely sensitive to ethanol and enhanced at concentrations relevant for human alcohol consumption. Ethanol enhancement of recombinant α4β3δ receptors is blocked by the behavioral alcohol antagonist 8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (Ro15-4513), suggesting that EtOH/Ro15-4513-sensitive receptors mediate important behavioral alcohol actions. Here we confirm alcohol/alcohol antagonist sensitivity of α4β3δ receptors using human clones expressed in a human cell line and test the hypothesis that discrepant findings concerning the high alcohol sensitivity of these receptors are due to difficulties incorporating δ subunits into functional receptors. To track δ subunit incorporation, we used a functional tag, a single amino acid change (H68A) in a benzodiazepine binding residue in which a histidine in the δ subunit is replaced by an alanine residue found at the homologous position in γ subunits. We demonstrate that the δH68A substitution confers diazepam sensitivity to otherwise diazepam-insensitive α4β3δ receptors. The extent of enhancement of α4β3δH68A receptors by 1 μM diazepam, 30 mM EtOH, and 1 μM β-carboline-3-carboxy ethyl ester (but not 1 μM Zn(2+) block) is correlated in individual recordings, suggesting that δ subunit incorporation into recombinant GABA(A)Rs varies from cell to cell and that this variation accounts for the variable pharmacological profile. These data are consistent with the notion that δ subunit-incorporation is often incomplete in recombinant systems yet is necessary for high ethanol sensitivity, one of the features of native δ subunit-containing GABA(A)Rs.

Monepantel allosterically activates DEG-3/DES-2 channels of the gastrointestinal nematode Haemonchus contortus

Monepantel is the first drug of a new family of anthelmintics, the amino acetonitrile derivatives (AAD), presently used to treat ruminants infected with gastrointestinal nematodes such as Haemonchus contortus. Monepantel shows an excellent tolerability in mammals and is active against multidrug-resistant parasites, indicating that its molecular target is absent or inaccessible in the host and is different from those of the classic anthelmintics. Genetic approaches with mutant nematodes have suggested acetylcholine receptors of the DEG-3 subfamily as the targets of AADs, an enigmatic clade of ligand-gated ion channels that is specific to nematodes and does not occur in mammals. Here we demonstrate direct interaction of monepantel, its major active metabolite monepantel sulfone, and other AADs with potential targets of the DEG-3 subfamily of acetylcholine receptors. H. contortus DEG-3/DES-2 receptors were functionally expressed in Xenopus laevis oocytes and were found to be preferentially activated by choline, to permeate monovalent cations, and to a smaller extent, calcium ions. Although monepantel and monepantel sulfone did not activate the channels by themselves, they substantially enhanced the late currents after activation of the channels with choline, indicating that these AADs are type II positive allosteric modulators of H. contortus DEG-3/DES-2 channels. It is noteworthy that the R-enantiomer of monepantel, which is inactive as an anthelmintic, inhibited the late currents after stimulation of H. contortus DEG-3/DES-2 receptors with choline. In summary, we present the first direct evidence for interaction of AADs with DEG-3-type acetylcholine receptors and discuss these findings in the context of anthelmintic action of AADs.

The short splice variant of the gamma 2 subunit acts as an external modulator of GABA(A) receptor function

GABA(A) receptors (GABA(A)Rs) regulate the majority of fast inhibition in the mammalian brain and are the target for multiple drug types, including sleep aids, anti-anxiety medication, anesthetics, alcohol, and neurosteroids. A variety of subunits, including the highly distributed gamma2, allow for pharmacologic and kinetic differences in particular brain regions. The two common splice variants gamma2S (short) and gamma2L (long) show different patterns of regional distribution both in adult brain and during the course of development, but show few notable differences when incorporated into pentameric receptors. However, results presented here show that the gamma2S variant can strongly affect both GABA(A)R pharmacology and kinetics by acting as an external modulator of fully formed receptors. Mutation of one serine residue can confer gamma2S-like properties to gamma2L subunits, and addition of a modified gamma2 N-terminal polypeptide to the cell surface recapitulates the pharmacological effect. Thus, rather than incorporation of a separate accessory protein as with voltage-gated channels, this is an example of an ion channel using a common subunit for dual purposes. The modified receptor properties conferred by accessory gamma2S have implications for understanding GABA(A)R pharmacology, receptor kinetics, stoichiometry, GABAergic signaling in the brain during development, and altered function in disease states such as epilepsy.