Pharmacological properties of GABAA receptors containing gamma1 subunits

Total Synthesis of (15R)- and (15S)-Prostaglandin A2

From both pharmaceutical and structural perspectives, the large family of prostaglandins represent a truly remarkable class of natural products. Prostaglandin A2 is a tissue hormone naturally found in human seminal plasma and in the sea whip Plexaura homomalla with yet poorly understood biological or therapeutic effects. Herein, a novel strategy for the stereoselective construction of both naturally occurring prostaglandin A2 epimers and first insights into their functional effects on the major inhibitory neurotransmitter γ-aminobutyric acid (GABA) type A receptors (GABAAR) are provided. The synthesis of both epimers was achieved in only 11 steps starting from commercially available 2,5-dimethoxy-tetrahydrofuran employing an organocatalytic domino-aldol reaction, a Mizoroki-Heck reaction, a Wittig reaction as well as an oxidation-decarboxylation sequence. The (15R)-epimer significantly reduced GABA-induced currents through GABAA receptors while its (15S)-epimer did not show any significant effect. These data suggest that (15R)-PGA2 might serve as a novel scaffold for the development of selective GABAA receptor modulators.

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.

Clerodane Diterpenes from Casearia corymbosa as Allosteric GABAA Receptor Modulators

An EtOAc extract of Casearia corymbosa leaves led to an allosteric potentiation of the GABA signal in a fluorometric imaging plate reader (FLIPR) assay on Chinese hamster ovary (CHO) cells stably expressing GABAA receptors with an α1β2γ2 subunit composition. The activity was tracked by HPLC-based activity profiling, and four known (2, 3, 4, and 8) and five new clerodane-type diterpenoids (1, 5-7, and 9) were isolated. Compounds 1-8 were obtained from the active time window. The absolute configuration of all compounds was established by ECD. Compounds 3, 7, and 8 exhibited EC50 values of 0.5, 4.6, and 1.4 μM, respectively. To explore possible binding sites at the receptor, the most abundant diterpenoid 8 was tested in combination with diazepam, etazolate, and allopregnanolone. An additive potentiation of the GABA signal was observed with these compounds, while the effect of 8 was not inhibited by flumazenil, a negative allosteric modulator at the benzodiazepine binding site. Finally, the activity was validated in voltage clamp studies on Xenopus laevis oocytes transiently expressing GABAA receptors of the α1β2γ2S and α1β2 subtypes. Compound 8 potentiated GABA-induced currents with both receptor subunit compositions [EC50 (α1β2γ2S) = 43.6 μM; Emax = 809% and EC50 (α1β2) = 57.6 μM; Emax = 534%]. The positive modulation of GABA-induced currents was not inhibited by flumazenil, thereby confirming an allosteric modulation independent of the benzodiazepine binding site.

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.

Voltage dependent allosteric modulation of IPSCs by benzodiazepines

GABA_A receptors (GABA_AR) are inhibitory ion channels ubiquitously expressed in the central nervous system and play critical roles in brain development and function. Benzodiazepines are positive allosteric modulators of GABA_AR, enhancing channel opening frequency when GABA is bound to the receptor. Midazolam is a commonly used benzodiazepine. It is frequently used for premature infants, but the long-term consequences of its use in this patient population are not well established. Here, we studied the acute effects of midazolam on immature synapses. Using a rodent organotypic hippocampal slice preparation, we evaluated how midazolam affects inhibitory synaptic transmission onto CA1 pyramidal neurons. We found that 1 μM midazolam enhances evoked inhibitory post synaptic currents (eIPSCs) at a holding potential of -60 mV. Similarly, 1 μM midazolam enhances miniature IPSCs (mIPSCs) in CA1 pyramidal neurons at holding potentials of -60 mV and -30 mV. At depolarized holding potentials, however, midazolam no longer enhances mIPSCs. Depolarization of the postsynaptic cell by itself increases mIPSC decay, which occludes the allosteric effects of midazolam. These results provide insight into how a benzodiazepine and membrane voltage may modulate GABA_AR function in developing circuits.

GABA Receptors and the Pharmacology of Sleep

Current GABAergic sleep-promoting medications were developed pragmatically, without making use of the immense diversity of GABA_A receptors. Pharmacogenetic experiments are leading to an understanding of the circuit mechanisms in the hypothalamus by which zolpidem and similar compounds induce sleep at α2βγ2-type GABA_A receptors. Drugs acting at more selective receptor types, for example, at receptors containing the α2 and/or α3 subunits expressed in hypothalamic and brain stem areas, could in principle be useful as hypnotics/anxiolytics. A highly promising sleep-promoting drug, gaboxadol, which activates αβδ-type receptors failed in clinical trials. Thus, for the time being, drugs such as zolpidem, which work as positive allosteric modulators at GABA_A receptors, continue to be some of the most effective compounds to treat primary insomnia.

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.

Sigmoid colon mucosal gene expression supports alterations of neuronal signaling in irritable bowel syndrome with constipation

Peripheral factors likely play a role in at least a subset of irritable bowel syndrome (IBS) patients. Few studies have investigated mucosal gene expression using an unbiased approach. Here, we performed mucosal gene profiling in a sex-balanced sample to identify relevant signaling pathways and gene networks and compare with publicly available profiling data from additional cohorts. Twenty Rome III+ IBS patients [10 IBS with constipation (IBS-C), 10 IBS with diarrhea (IBS-D), 5 men/women each), and 10 age-/sex-matched healthy controls (HCs)] underwent sigmoidoscopy with biopsy for gene microarray analysis, including differential expression, weighted gene coexpression network analysis (WGCNA), gene set enrichment analysis, and comparison with publicly available data. Expression levels of 67 genes were validated in an expanded cohort, including the above samples and 18 additional participants (6 each of IBS-C, IBS-D, HCs) using NanoString nCounter technology. There were 1,270 differentially expressed genes (FDR < 0.05) in IBS-C vs. HCs but none in IBS or IBS-D vs. HCs. WGNCA analysis identified activation of the cAMP/protein kinase A signaling pathway. Nine of 67 genes were validated by the NanoString nCounter technology (FDR < 0.05) in the expanded sample. Comparison with publicly available microarray data from the Mayo Clinic and University of Nottingham supports the reproducibility of 17 genes from the microarray analysis and three of nine genes validated by nCounter in IBS-C vs. HCs. This study supports the involvement of peripheral mechanisms in IBS-C, particularly pathways mediating neuronal signaling. NEW & NOTEWORTHY Peripheral factors play a role in the pathophysiology of irritable bowel syndrome (IBS), which, to date, has been mostly evident in IBS with diarrhea. Here, we show that sigmoid colon mucosal gene expression profiles differentiate IBS with constipation from healthy controls. These profiling data and analysis of additional cohorts also support the concept that peripheral neuronal pathways contribute to IBS pathophysiology.

Molecular tools for GABAA receptors: High affinity ligands for β1-containing subtypes

γ-Aminobutyric acid type A (GABA_A) receptors are pentameric GABA-gated chloride channels that are, in mammalians, drawn from a repertoire of 19 different genes, namely α1-6, β1-3, γ1-3, δ, ε, θ, π and ρ1-3. The existence of this wide variety of subunits as well as their diverse assembly into different subunit compositions result in miscellaneous receptor subtypes. In combination with the large number of known and putative allosteric binding sites, this leads to a highly complex pharmacology. Recently, a novel binding site at extracellular α+/β- interfaces was described as the site of modulatory action of several pyrazoloquinolinones. In this study we report a highly potent ligand from this class of compounds with pronounced β1-selectivity that mainly lacks α-subunit selectivity. It constitutes the most potent β1-selective positive allosteric modulatory ligand with known binding site. In addition, a proof of concept pyrazoloquinolinone ligand lacking the additional high affinity interaction with the benzodiazepine binding site is presented. Ultimately, such ligands can be used as invaluable molecular tools for the detection of β1-containing receptor subtypes and the investigation of their abundance and distribution.

α2 Subunit-Containing GABAA Receptor Subtypes Are Upregulated and Contribute to Alcohol-Induced Functional Plasticity in the Rat Hippocampus

Alcohol (EtOH) intoxication causes changes in the rodent brain γ-aminobutyric acid receptor (GABA_AR) subunit composition and function, playing a crucial role in EtOH withdrawal symptoms and dependence. Building evidence indicates that withdrawal from acute EtOH and chronic intermittent EtOH (CIE) results in decreased EtOH-enhanced GABA_AR δ subunit-containing extrasynaptic and EtOH-insensitive α1βγ2 subtype synaptic GABA_ARs but increased synaptic α4βγ2 subtype, and increased EtOH sensitivity of GABA_AR miniature postsynaptic currents (mIPSCs) correlated with EtOH dependence. Here we demonstrate that after acute EtOH intoxication and CIE, upregulation of hippocampal α4βγ2 subtypes, as well as increased cell-surface levels of GABA_AR α2 and γ1 subunits, along with increased α2β1γ1 GABA_AR pentamers in hippocampal slices using cell-surface cross-linking, followed by Western blot and coimmunoprecipitation. One-dose and two-dose acute EtOH treatments produced temporal plastic changes in EtOH-induced anxiolysis or withdrawal anxiety, and the presence or absence of EtOH-sensitive synaptic currents correlated with cell surface peptide levels of both α4 and γ1(new α2) subunits. CIE increased the abundance of novel mIPSC patterns differing in activation/deactivation kinetics, charge transfer, and sensitivity to EtOH. The different mIPSC patterns in CIE could be correlated with upregulated highly EtOH-sensitive α2βγ subtypes and EtOH-sensitive α4βγ2 subtypes. Naïve α4 subunit knockout mice express EtOH-sensitive mIPSCs in hippocampal slices, correlating with upregulated GABA_AR α2 (and not α4) subunits. Consistent with α2, β1, and γ1 subunits genetically linked to alcoholism in humans, our findings indicate that these new α2-containing synaptic GABA_ARs could mediate the maintained anxiolytic response to EtOH in dependent individuals, rat or human, contributing to elevated EtOH consumption.

Fast and Slow Inhibition in the Visual Thalamus Is Influenced by Allocating GABAA Receptors with Different γ Subunits

Cell-type specific differences in the kinetics of inhibitory postsynaptic conductance changes (IPSCs) are believed to impact upon network dynamics throughout the brain. Much attention has focused on how GABA_A receptor (GABA_AR) α and β subunit diversity will influence IPSC kinetics, but less is known about the influence of the γ subunit. We have examined whether GABA_AR γ subunit heterogeneity influences IPSC properties in the thalamus. The γ2 subunit gene was deleted from GABA_ARs selectively in the dorsal lateral geniculate nucleus (dLGN). The removal of the γ2 subunit from the dLGN reduced the overall spontaneous IPSC (sIPSC) frequency across all relay cells and produced an absence of IPSCs in a subset of relay neurons. The remaining slower IPSCs were both insensitive to diazepam and zinc indicating the absence of the γ2 subunit. Because these slower IPSCs were potentiated by methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), we propose these IPSCs involve γ1 subunit-containing GABA_AR activation. Therefore, γ subunit heterogeneity appears to influence the kinetics of GABA_AR-mediated synaptic transmission in the visual thalamus in a cell-selective manner. We suggest that activation of γ1 subunit-containing GABA_ARs give rise to slower IPSCs in general, while faster IPSCs tend to be mediated by γ2 subunit-containing GABA_ARs.

Zolpidem is a potent stoichiometry-selective modulator of α1β3 GABAA receptors: evidence of a novel benzodiazepine site in the α1-α1 interface

Zolpidem is not a typical GABA_A receptor hypnotic. Unlike benzodiazepines, zolpidem modulates tonic GABA currents in the rat dorsal motor nucleus of the vagus, exhibits residual effects in mice lacking the benzodiazepine binding site, and improves speech, cognitive and motor function in human patients with severe brain injury. The receptor by which zolpidem mediates these effects is not known. In this study we evaluated binary α1β3 GABA_A receptors in either the 3α1:2β3 or 2α1:3β3 subunit stoichiometry, which differ by the existence of either an α1-α1 interface, or a β3-β3 interface, respectively. Both receptor stoichiometries are readily expressed in Xenopus oocytes, distinguished from each other by using GABA, zolpidem, diazepam and Zn²⁺. At the 3α1:2β3 receptor, clinically relevant concentrations of zolpidem enhanced GABA in a flumazenil-sensitive manner. The efficacy of diazepam was significantly lower compared to zolpidem. No modulation by either zolpidem or diazepam was detected at the 2α1:3β3 receptor, indicating that the binding site for zolpidem is at the α1-α1 interface, a site mimicking the classical α1-γ2 benzodiazepine site. Activating α1β3 (3α1:2β3) receptors may, in part, mediate the physiological effects of zolpidem observed under distinct physiological and clinical conditions, constituting a potentially attractive drug target.

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.

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.

Nitrogenated honokiol derivatives allosterically modulate GABAA receptors and act as strong partial agonists

In traditional Asian medicinal systems, preparations of the root and stem bark of Magnolia species are widely used to treat anxiety and other nervous disturbances. The biphenyl-type neolignan honokiol together with its isomer magnolol are the main constituents of Magnolia bark extracts. We have previously identified a nitrogen-containing honokiol derivative (3-acetylamino-4'-O-methylhonokiol, AMH) as a high efficient modulator of GABAA receptors. Here we further elucidate the structure-activity relation of a series of nitrogenated biphenyl-neolignan derivatives by analysing allosteric modulation and agonistic effects on α1β2γ2S GABAA receptors. The strongest IGABA enhancement was induced by compound 5 (3-acetamido-4'-ethoxy-3',5-dipropylbiphenyl-2-ol, Emax: 123.4±9.4% of IGABA-max) and 6 (5'-amino-2-ethoxy-3',5-dipropylbiphenyl-4'-ol, Emax: 117.7±13.5% of IGABA-max). Compound 5 displayed, however, a significantly higher potency (EC50=1.8±1.1 μM) than compound 6 (EC50=20.4±4.3 μM). Honokiol, AMH and four of the derivatives induced significant inward currents in the absence of GABA. Strong partial agonists were honokiol (inducing 78±6% of IGABA-max), AMH (63±6%), 5'-amino-2-O-methylhonokiol (1) (59±1%) and 2-methoxy-5'-nitro-3',5-dipropylbiphenyl-4'-ol (3) (52±1%). 3-N-Acetylamino-4'-ethoxy-3',5-dipropyl-biphenyl-4'-ol (5) and 3-amino-4'-ethoxy-3',5-dipropyl-biphenyl-4'-ol (7) were less efficacious but even more potent (5: EC50=6.9±1.0 μM; 7: EC50=33.2±5.1 μM) than the full agonist GABA.

Functional characterization of the 1,5-benzodiazepine clobazam and its major active metabolite N-desmethylclobazam at human GABA(A) receptors expressed in Xenopus laevis oocytes

The 1,5-benzodiazepine clobazam is indicated for the adjunctive treatment of seizures associated with Lennox-Gastaut syndrome in patients 2 years of age or older in the United States, and for treatment of anxiety and various forms of epilepsy elsewhere. Clobazam has been reported to exhibit different in vivo adverse effects and addiction liability profile than the classic 1,4-benzodiazepines. In this study, it was investigated whether the in vitro pharmacological properties of clobazam and its major active metabolite N-desmethylclobazam could explain some of these clinical differences. The functional properties of the two 1,5-benzodiazepines were characterized at the human γ-aminobutyric acid type A receptor (GABA_AR) subtypes α₁β₂γ_2S, α₂β₂γ_2S, α₃β₂γ_2S, α₅β₂γ_2S and α₆β₂δ expressed in Xenopus laevis oocytes by use of two-electrode voltage-clamp electrophysiology and compared to those exhibited by the 1,4-benzodiazepine clonazepam. All three compounds potentiated GABA EC₂₀-evoked responses through the α_1,2,3,5β₂γ_2S GABA_ARs in a reversible and concentration-dependent manner, with each displaying similar EC₅₀ values at the four subtypes. Furthermore, the degrees of potentiation of the GABA EC₂₀ currents through the four receptors mediated by saturating modulator concentrations did not differ substantially for any of the three benzodiazepines. The three compounds were substantially less potent (200-3900 fold) as positive allosteric modulators at the α₆β₂δ GABA_AR than at the α_1,2,3,5β₂γ_2S receptors. Interestingly, however, clobazam and especially N-desmethylclobazam were highly efficacious potentiators of α₆β₂δ receptor signaling. Although this activity component is unlikely to contribute to the in vivo effects of clobazam/N-desmethylclobazam, the 1,5-benzodiazepine could constitute an interesting lead for novel modulators targeting this low-affinity binding site in GABA_ARs. In conclusion, the non-selective modulation exerted by clobazam, N-desmethylclobazam and clonazepam at the α₁β₂γ_2S, α₂β₂γ_2S, α₃β₂γ_2S and α₅β₂γ_2S GABA_ARs indicate that the observed clinical differences between clobazam and 1,4-benzodiazepines are likely to arise from factors other than their respective pharmacological properties at the GABA_ARs as investigated here.

Tolerance to the rate-increasing and not rate-decreasing effects of pregnanolone in rats

Chronic treatment with benzodiazepines, which positively modulate γ-aminobutyric acidA (GABAA) receptors, can lead to the development of tolerance. Similar effects might also occur during chronic treatment with positive modulators acting at other sites on GABAA receptors (e.g. neuroactive steroids). In this study, tolerance and cross tolerance were examined in seven rats treated daily with the neuroactive steroid pregnanolone (25.6 mg/kg/day) and responding under a fixed ratio 10 schedule of food presentation. Dose-effect curves were determined for positive GABAA modulators (pregnanolone, flunitrazepam, midazolam, and pentobarbital), and other drugs (ketamine and morphine) before, during, and after chronic treatment. Initially, daily pregnanolone administration increased responding; although tolerance developed to the rate-increasing effects after 14 weeks, tolerance did not develop to the rate-decreasing effects. The potencies of pregnanolone, midazolam, and morphine to decrease responding did not change during treatment, whereas flunitrazepam was more potent and pentobarbital and ketamine were less potent during treatment as compared to before treatment. Pregnanolone and midazolam were more potent after treatment than before treatment. The development of tolerance to the rate-increasing effects of pregnanolone indicates that neuroadaptations occur during chronic treatment; the fact that tolerance develops to only some effects suggests that the behavioral consequences of these neuroadaptations are limited.

Benzodiazepine-site pharmacology on GABAA receptors in histaminergic neurons

BACKGROUND AND PURPOSE

The histaminergic tuberomamillary nucleus (TMN) of the posterior hypothalamus controls the cognitive aspects of vigilance which is reduced by common sedatives and anxiolytics. The receptors targeted by these drugs in histaminergic neurons are unknown. TMN neurons express nine different subunits of the GABAA receptor (GABAA R) with three α- (α1, α2 and α5) and two γ- (γ1, γ 2) subunits, which confer different pharmacologies of the benzodiazepine-binding site.

EXPERIMENTAL APPROACH

We investigated the actions of zolpidem, midazolam, diazepam, chlordiazepoxide, flumazenil (Ro15-1788) and methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) in TMN neurons using mouse genetics, electrophysiological and molecular biological methods.

KEY RESULTS

We find the sensitivity of GABAA R to zolpidem, midazolam and DMCM significantly reduced in TMN neurons from γ2F77I mice, but modulatory activities of diazepam, chlordiazepoxide and flumazenil not affected. Potencies and efficacies of these compounds are in line with the dominance of α2- and α1-subunit containing receptors associated with γ2- or γ1-subunits. Functional expression of the γ1-subunit is supported by siRNA-based knock-down experiments in γ2F77I mice.

CONCLUSIONS AND IMPLICATIONS

GABAA R of TMN neurons respond to a variety of common sedatives with a high affinity binding site (γ2F77I) involved. The γ1-subunit likely contributes to the action of common sedatives in TMN neurons. This study is relevant for understanding the role of neuronal histamine and benzodiazepines in disorders of sleep and metabolism.

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.

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.

Efficient modulation of γ-aminobutyric acid type A receptors by piperine derivatives

Piperine activates TRPV1 (transient receptor potential vanilloid type 1 receptor) receptors and modulates γ-aminobutyric acid type A receptors (GABAAR). We have synthesized a library of 76 piperine analogues and analyzed their effects on GABAAR by means of a two-microelectrode voltage-clamp technique. GABAAR were expressed in Xenopus laevis oocytes. Structure-activity relationships (SARs) were established to identify structural elements essential for efficiency and potency. Efficiency of piperine derivatives was significantly increased by exchanging the piperidine moiety with either N,N-dipropyl, N,N-diisopropyl, N,N-dibutyl, p-methylpiperidine, or N,N-bis(trifluoroethyl) groups. Potency was enhanced by replacing the piperidine moiety by N,N-dibutyl, N,N-diisobutyl, or N,N-bistrifluoroethyl groups. Linker modifications did not substantially enhance the effect on GABAAR. Compound 23 [(2E,4E)-5-(1,3-benzodioxol-5-yl)-N,N-dipropyl-2,4-pentadienamide] induced the strongest modulation of GABAA (maximal GABA-induced chloride current modulation (IGABA-max = 1673% ± 146%, EC50 = 51.7 ± 9.5 μM), while 25 [(2E,4E)-5-(1,3-benzodioxol-5-yl)-N,N-dibutyl-2,4-pentadienamide] displayed the highest potency (EC50 = 13.8 ± 1.8 μM, IGABA-max = 760% ± 47%). Compound 23 induced significantly stronger anxiolysis in mice than piperine and thus may serve as a starting point for developing novel GABAAR modulators.

The behavioral pharmacology of zolpidem: evidence for the functional significance of α1-containing GABA(A) receptors

RATIONALE

Zolpidem is a positive allosteric modulator of γ-aminobutyric acid (GABA) with preferential binding affinity and efficacy for α1-subunit containing GABA(A) receptors (α1-GABA(A)Rs). Over the last three decades, a variety of animal models and experimental procedures have been used in an attempt to relate the behavioral profile of zolpidem and classic benzodiazepines (BZs) to their interaction with α1-GABA(A)Rs.

OBJECTIVES

This paper reviews the results of rodent and non-human primate studies that have evaluated the effects of zolpidem on motor behaviors, anxiety, memory, food and fluid intake, and electroencephalogram (EEG) sleep patterns. Also included are studies that examined zolpidem's discriminative, reinforcing, and anticonvulsant effects as well as behavioral signs of tolerance and withdrawal.

RESULTS

The literature reviewed indicates that α1-GABA(A)Rs play a principle role in mediating the hypothermic, ataxic-like, locomotor- and memory-impairing effects of zolpidem and BZs. Evidence also suggests that α1-GABA(A)Rs play partial roles in the hypnotic, EEG sleep, anticonvulsant effects, and anxiolytic-like of zolpidem and diazepam. These studies also indicate that α1-GABA(A)Rs play a more prominent role in mediating the discriminative stimulus, reinforcing, hyperphagic, and withdrawal effects of zolpidem and BZs in primates than in rodents.

CONCLUSIONS

The psychopharmacological data from both rodents and non-human primates suggest that zolpidem has a unique pharmacological profile when compared with classic BZs. The literature reviewed here provides an important framework for studying the role of different GABA(A)R subtypes in the behavioral effects of BZ-type drugs and helps guide the development of new pharmaceutical agents for disorders currently treated with BZ-type drugs.

HPLC-based activity profiling for GABAA receptor modulators in Adenocarpus cincinnatus

In a two-microelectrode voltage clamp assay with Xenopus laevis oocytes, a dichloromethane extract of Adenocarpus cincinnatus roots and tubers (Leguminosae) enhanced the GABA-induced chloride current (IGABA) through receptors of the subtype α1β2γ2s by 126.5 ± 25.1% when tested at 100 μg/mL. By means of HPLC-based activity profiling, 15 flavonoid and isoflavonoid derivatives, including eight new compounds, were identified in the active fractions of the extract. Isoflavone 11 and pterocarpans 2 and 8 showed promising activity in the oocyte assay, with EC50 values between 2.8 ± 1.4 and 18.8 ± 2.3 μM. Maximal potentiation of IGABA ranged between 490% and 640%. This is the first report of pterocarpans as GABAA receptor modulators.

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.

The Genetics, Neurogenetics and Pharmacogenetics of Addiction

Addictions are prevalent psychiatric disorders that confer remarkable personal and social burden. Despite substantial evidence for their moderate, yet robust, heritability (approx. 50%), specific genetic mechanisms underlying their development and maintenance remain unclear. The goal of this selective review is to highlight progress in unveiling the genetic underpinnings of addiction. First, we revisit the basis for heritable variation in addiction before reviewing the most replicable candidate gene findings and emerging signals from genomewide association studies for alcohol, nicotine and cannabis addictions. Second, we survey the modest but growing field of neurogenetics examining how genetic variation influences corticostriatal structure, function, and connectivity to identify neural mechanisms that may underlie associations between genetic variation and addiction. Third, we outline how extant genomic findings are being used to develop and refine pharmacotherapies. Finally, as sample sizes for genetically informed studies of addiction approach critical mass, we posit five exciting possibilities that may propel further discovery (improved phenotyping, rare variant discovery, gene-environment interplay, epigenetics, and novel neuroimaging designs).

Phytochemical profiling of Curcuma kwangsiensis rhizome extract, and identification of labdane diterpenoids as positive GABAA receptor modulators

An ethyl acetate extract of Curcuma kwangsiensis S.G. Lee & C.F. Liang (Zingiberaceae) rhizomes (100 μg/ml) enhanced the GABA-induced chloride current (IGABA) through GABAA receptors of the α1β2γ2S subtype by 79.0±7.0%. Potentiation of IGABA was measured using the two-microelectrode voltage-clamp technique and Xenopus laevis oocytes. HPLC-based activity profiling of the crude extract led to the identification of 11 structurally related labdane diterpenoids, including four new compounds. Structure elucidation was achieved by comprehensive analysis of on-line (LC-PDA-ESI-TOF-MS) and off-line (microprobe 1D and 2D NMR) spectroscopic data. The absolute configuration of the compounds was established by comparison of experimental and calculated ECD spectra. Labdane diterpenes represent a new class of plant secondary metabolites eliciting positive GABAA receptor modulation. The highest efficiency was observed for zerumin A (maximum potentiation of IGABA by 309.4±35.6%, and EC50 of 24.9±8.8 μM).

Expression of the γ2-subunit distinguishes synaptic and extrasynaptic GABA(A) receptors in NG2 cells of the hippocampus

NG2 cells are equipped with transmitter receptors and receive direct synaptic input from glutamatergic and GABAergic neurons. The functional impact of these neuron-glia synapses is still unclear. Here, we combined functional and molecular techniques to analyze properties of GABA(A) receptors in NG2 cells of the juvenile mouse hippocampus. GABA activated slowly desensitizing responses in NG2 cells, which were mimicked by muscimol and inhibited by bicuculline. To elucidate the subunit composition of the receptors we tested its pharmacological properties. Coapplication of pentobarbital, benzodiazepines, and zolpidem all significantly increased the GABA-evoked responses. The presence of small tonic currents indicated the presence of extrasynaptic GABA(A) receptors. To further analyze the subunit expression, single cell transcript analysis was performed subsequent to functional characterization of NG2 cells. The subunits α1, α2, β3, γ1, and γ2 were most abundantly expressed, matching properties resulting from pharmacological characterization. Importantly, lack of the γ2-subunit conferred a high Zn²⁺ sensitivity to the GABA(A) receptors of NG2 cells. Judging from the zolpidem sensitivity, postsynaptic GABA(A) receptors in NG2 cells contain the γ2-subunit, in contrast to extrasynaptic receptors, which were not modulated by zolpidem. To determine the effect of GABA(A) receptor activation on membrane potential, perforated patch recordings were obtained from NG2 cells. In the current-clamp mode, GABA depolarized the cells to approximately -30 mV, indicating a higher intracellular Cl⁻ concentration (∼50 mM) than previously reported. GABA-induced depolarization in NG2 cells might trigger Ca²⁺ influx through voltage-activated Ca²⁺ channels.

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.

GABAergic transmission in hepatic encephalopathy

Hepatic encephalopathy (HE)(1) is a neuropsychiatric disorder caused by chronic or acute liver failure. Nearly thirty years ago a hypothesis was formulated explaining the neuropathology of HE by increased GABAergic tone. Recent progress in the GABAA-receptor (GABAAR) molecular pharmacology and biochemistry as well as the physiology of GABAergic transmission provided better understanding of GABA's role in health and disease. A detailed analysis of neuronal populations and their GABAergic afferents affected in HE is still missing. The slow progress in understanding the pathology of GABAergic transmission in HE is due to the high complexity of brain circuitries controlled by multiple types of GABAergic interneurons and the large variety of GABAAR, which are differently affected by pathological conditions and not yet fully identified. The mechanisms of action of the GABAAR agonist taurine, allosteric positive modulators (inhibitory neurosteroids, anaesthetics, benzodiazepines and histamine) and inhibitors of the GABAAR (excitatory neurosteroids, Ro15-4513) are discussed with respect to HE pathophysiology. Perspectives for GABAergic drugs in the symptomatic treatment of HE are suggested.

A novel GABA(A) receptor pharmacology: drugs interacting with the α(+) β(-) interface

GABA(A) receptors are ligand-gated chloride channels composed of five subunits that can belong to different subunit classes. The existence of 19 different subunits gives rise to a multiplicity of GABA(A) receptor subtypes with distinct subunit composition; regional, cellular and subcellular distribution; and pharmacology. Most of these receptors are composed of two α, two β and one γ2 subunits. GABA(A) receptors are the site of action of a variety of pharmacologically and clinically important drugs, such as benzodiazepines, barbiturates, neuroactive steroids, anaesthetics and convulsants. Whereas GABA acts at the two extracellular β(+) α(-) interfaces of GABA(A) receptors, the allosteric modulatory benzodiazepines interact with the extracellular α(+) γ2(-) interface. In contrast, barbiturates, neuroactive steroids and anaesthetics seem to interact with solvent accessible pockets in the transmembrane domain. Several benzodiazepine site ligands have been identified that selectively interact with GABA(A) receptor subtypes containing α2βγ2, α3βγ2 or α5βγ2 subunits. This indicates that the different α subunit types present in these receptors convey sufficient structural differences to the benzodiazepine binding site to allow specific interaction with certain benzodiazepine site ligands. Recently, a novel drug binding site was identified at the α(+) β(-) interface. This binding site is homologous to the benzodiazepine binding site at the α(+) γ2(-) interface and is thus also strongly influenced by the type of α subunit present in the receptor. Drugs interacting with this binding site cannot directly activate but only allosterically modulate GABA(A) receptors. The possible importance of such drugs addressing a spectrum of receptor subtypes completely different from that of benzodiazepines is discussed.

Loss of functional GABA(A) receptors in the Alzheimer diseased brain

The cholinergic and glutamatergic neurotransmission systems are known to be severely disrupted in Alzheimer's disease (AD). GABAergic neurotransmission, in contrast, is generally thought to be well preserved. Evidence from animal models and human postmortem tissue suggest GABAergic remodeling in the AD brain. Nevertheless, there is no information on changes, if any, in the electrophysiological properties of human native GABA receptors as a consequence of AD. To gain such information, we have microtransplanted cell membranes, isolated from temporal cortices of control and AD brains, into Xenopus oocytes, and recorded the electrophysiological activity of the transplanted GABA receptors. We found an age-dependent reduction of GABA currents in the AD brain. This reduction was larger when the AD membranes were obtained from younger subjects. We also found that GABA currents from AD brains have a faster rate of desensitization than those from non-AD brains. Furthermore, GABA receptors from AD brains were slightly, but significantly, less sensitive to GABA than receptors from non-AD brains. The reduction of GABA currents in AD was associated with reductions of mRNA and protein of the principal GABA receptor subunits normally present in the temporal cortex. Pairwise analysis of the transcripts within control and AD groups and analyses of the proportion of GABA receptor subunits revealed down-regulation of α1 and γ2 subunits in AD. In contrast, the proportions of α2, β1, and γ1 transcripts were up-regulated in the AD brains. Our data support a functional remodeling of GABAergic neurotransmission in the human AD brain.

Comparison of cell expression formats for the characterization of GABA(A) channels using a microfluidic patch clamp system

Ensemble recording and microfluidic perfusion are recently introduced techniques aimed at removing the laborious nature and low recording success rates of manual patch clamp. Here, we present assay characteristics for these features integrated into one automated electrophysiology platform as applied to the study of GABA(A) channels. A variety of cell types and methods of GABA(A) channel expression were successfully studied (defined as I(GABA)>500 pA), including stably transfected human embryonic kidney (HEK) cells expressing α(1)β(3)γ(2) GABA(A) channels, frozen ready-to-assay (RTA) HEK cells expressing α(1)β(3)γ(2) or α(3)β(3)γ(2) GABA(A) channels, transiently transfected HEK293T cells expressing α(1)β(3)γ(2) GABA(A) channels, and immortalized cultures of human airway smooth muscle cells endogenously expressing GABA(A) channels. Current measurements were successfully studied in multiple cell types with multiple modes of channel expression in response to several classic GABA(A) channel agonists, antagonists, and allosteric modulators. We obtained success rates above 95% for transiently or stably transfected HEK cells and frozen RTA HEK cells expressing GABA(A) channels. Tissue-derived immortalized cultures of airway smooth muscle cells exhibited a slightly lower recording success rate of 75% using automated patch, which was much higher than the 5% success rate using manual patch clamp technique by the same research group. Responses to agonists, antagonists, and allosteric modulators compared well to previously reported manual patch results. The data demonstrate that both the biophysics and pharmacologic characterization of GABA(A) channels in a wide variety of cell formats can be performed using this automated patch clamp system.

Ion channel pharmacology under flow: automation via well-plate microfluidics

Automated patch clamping addresses the need for high-throughput screening of chemical entities that alter ion channel function. As a result, there is considerable utility in the pharmaceutical screening arena for novel platforms that can produce relevant data both rapidly and consistently. Here we present results that were obtained with an innovative microfluidic automated patch clamp system utilizing a well-plate that eliminates the necessity of internal robotic liquid handling. Continuous recording from cell ensembles, rapid solution switching, and a bench-top footprint enable a number of assay formats previously inaccessible to automated systems. An electro-pneumatic interface was employed to drive the laminar flow of solutions in a microfluidic network that delivered cells in suspension to ensemble recording sites. Whole-cell voltage clamp was applied to linear arrays of 20 cells in parallel utilizing a 64-channel voltage clamp amplifier. A number of unique assays requiring sequential compound applications separated by a second or less, such as rapid determination of the agonist EC(50) for a ligand-gated ion channel or the kinetics of desensitization recovery, are enabled by the system. In addition, the system was validated via electrophysiological characterizations of both voltage-gated and ligand-gated ion channel targets: hK(V)2.1 and human Ether-à-go-go-related gene potassium channels, hNa(V)1.7 and 1.8 sodium channels, and (α1) hGABA(A) and (α1) human nicotinic acetylcholine receptor receptors. Our results show that the voltage dependence, kinetics, and interactions of these channels with pharmacological agents were matched to reference data. The results from these IonFlux™ experiments demonstrate that the system provides high-throughput automated electrophysiology with enhanced reliability and consistency, in a user-friendly format.

GABA(A) receptor modulators from Chinese herbal medicines traditionally applied against insomnia and anxiety

Several Chinese herbal medicines (CHMs) are used in the treatment of insomnia, restlessness, or anxiety. However, mechanisms underlying this effect and scientific proof for their traditional use is scarce. In the present study CHMs were screened for their ability to modulate GABA-induced chloride currents (I(GABA)), and active principles were isolated thus providing scientific evidence for their use as sedative and/or anxiolytic agents in CM. Herbal drugs were extracted successively with petroleum ether, ethyl acetate, methanol and water and further fractionated according to their bioactivity. The obtained extracts, fractions and finally pure compounds were tested for their ability to potentiate I(GABA) using the two-microelectrode voltage clamp technique on recombinant α₁β₂γ(2S) GABA(A) receptors expressed in Xenopus laevis oocytes. From all tested extracts the petroleum ether extract of Atractylodes macrocephala Koidz. rhizomes showed the strongest I(GABA) potentiation and was studied in more detail. This led to the isolation of the main components atractylenolide II and III, which seem to be responsible for the observed positive modulation of I(GABA) (166±12%, n=3 and 155±12%, n=3, respectively) in vitro. They were more active than the analogous compound atractylenolide I (96±3%, n=3) which differs in an additional double binding in position 9, 9a. Furthermore it could be shown that this effect is mediated independently of the benzodiazepine (BZ) binding site. In conclusion, A. macrocephala exerts its in vitro activity on recombinant GABA(A) receptors mainly through the two sesquiterpene lactones atractylenolide II and III (Fig. 1). This positive allosteric modulation of I(GABA) may partially be responsible for the traditional ethnopharmacological use of this herbal drug as a sedative.

Identification of GABA A receptor modulators in Kadsura longipedunculata and assignment of absolute configurations by quantum-chemical ECD calculations

A petroleum ether extract of Kadsura longipedunculata enhanced the GABA-induced chloride current (I(GABA)) by 122.5±0.3% (n=2) when tested at 100 μg/ml in Xenopuslaevis oocytes expressing GABA A receptors (α(1)β(2)γ(2S) subtype) in two-microelectrode voltage clamp measurements. Thirteen compounds were subsequently identified by HPLC-based activity profiling as responsible for GABA A receptor activity and purified in preparative scale. 6-Cinnamoyl-6,7-dihydro-7-myrceneol and 5,6-dihydrocuparenic acid were thereby isolated for the first time. The determination of the absolute stereochemistry of these compounds was achieved by comparison of experimental and calculated ECD spectra. All but one of the 13 isolated compounds from K. longipedunculata potentiated I(GABA) through GABA A receptors composed of α(1)β(2)γ(2S) subunits in a concentration-dependent manner. Potencies ranged from 12.8±3.1 to 135.6±85.7 μM, and efficiencies ranged from 129.7±36.8% to 885.8±291.2%. The phytochemical profiles of petroleum ether extracts of Kadsura japonica fruits (114.1±2.6% potentiation of I(GABA) at 100 μg/ml, n=2), and Schisandra chinensis fruits (inactive at 100 μg/ml) were compared by HPLC-PDA-ESIMS with that of K. longipedunculata.

Genetic association of recovery from eating disorders: the role of GABA receptor SNPs

Follow-up studies of eating disorders (EDs) suggest outcomes ranging from recovery to chronic illness or death, but predictors of outcome have not been consistently identified. We tested 5151 single-nucleotide polymorphisms (SNPs) in approximately 350 candidate genes for association with recovery from ED in 1878 women. Initial analyses focused on a strictly defined discovery cohort of women who were over age 25 years, carried a lifetime diagnosis of an ED, and for whom data were available regarding the presence (n=361 ongoing symptoms in the past year, ie, 'ill') or absence (n=115 no symptoms in the past year, ie, 'recovered') of ED symptoms. An intronic SNP (rs17536211) in GABRG1 showed the strongest statistical evidence of association (p=4.63 × 10(-6), false discovery rate (FDR)=0.021, odds ratio (OR)=0.46). We replicated these findings in a more liberally defined cohort of women age 25 years or younger (n=464 ill, n=107 recovered; p=0.0336, OR=0.68; combined sample p=4.57 × 10(-6), FDR=0.0049, OR=0.55). Enrichment analyses revealed that GABA (γ-aminobutyric acid) SNPs were over-represented among SNPs associated at p<0.05 in both the discovery (Z=3.64, p=0.0003) and combined cohorts (Z=2.07, p=0.0388). In follow-up phenomic association analyses with a third independent cohort (n=154 ED cases, n=677 controls), rs17536211 was associated with trait anxiety (p=0.049), suggesting a possible mechanism through which this variant may influence ED outcome. These findings could provide new insights into the development of more effective interventions for the most treatment-resistant patients.

Identification and characterization of GABA(A) receptor modulatory diterpenes from Biota orientalis that decrease locomotor activity in mice

An ethyl acetate extract of Biota orientalis leaves potentiated GABA-induced control current by 92.6% ± 22.5% when tested at 100 μg/mL in Xenopus laevis oocytes expressing GABA(A) receptors (α₁β₂γ(2S) subtype) in two-microelectrode voltage clamp measurements. HPLC-based activity profiling was used to identify isopimaric acid (4) and sandaracopimaric acid (5) as the compounds largely responsible for the activity. Sandaracopimaradienolal (3) was characterized as a new natural product. Compounds 4 and 5 were investigated for GABA(A) receptor subtype selectivity at the subtypes α₁β₁γ(2S), α₁β₂γ(2S), α₁β₃γ(2S), α₂β₂γ(2S), α₃β₂γ(2S), and α₅β₂γ(2S). Sandaracopimaric acid (5) was significantly more potent than isopimaric acid (4) at the GABA(A) receptor subtypes α₁β₁γ(2S), α₂β₂γ(2S), and α₅β₂γ(2S) (EC₅₀4: 289.5 ± 82.0, 364.8 ± 85.0, and 317.0 ± 83.7 μM vs EC₅₀5: 48.1 ± 13.4, 31.2 ± 4.8, and 40.7 ± 14.7 μM). The highest efficiency was reached by 4 and 5 on α₂- and α₃-containing receptor subtypes. In the open field test, ip administration of 5 induced a dose-dependent decrease of locomotor activity in a range of 3 to 30 mg/kg body weight in mice. No significant anxiolytic-like activity was observed in doses between 1 and 30 mg/kg body weight in mice.

Modulation of GABAA-receptors by honokiol and derivatives: subtype selectivity and structure-activity relationship

A series of 31 analogues of the neolignan honokiol (a major constituent of Magnolia officinalis) was synthesized, and their effects on GABA(A) receptors expressed in Xenopus oocytes were investigated. Honokiol enhanced chloride currents (I(GABA)) through GABA(A) receptors of seven different subunit compositions with EC(50) values ranging from 23.4 μM (α(5)β(2)) to 59.6 μM (α(1)β(3)). Honokiol was most efficient on α(3)β(2) (maximal I(GABA) enhancement 2386%) > α(2)β(2) (1130%) > α(1)β(2) (1034%) > α(1)β(1) (260%)). On α(1)β(2)-receptors, N-substituted compounds were most active with 3-acetylamino-4'-O-methylhonokiol (31), enhancing I(GABA) by 2601% (EC(50) (α(1)β(2)) = 3.8 μM). Pharmacophore modeling gave a model with an overall classification accuracy of 91% showing three hydrophobic regions, one acceptor and one donor region. Unlike honokiol, 31 was most efficient on α(2)β(2)- (5204%) > α(3)β(2)- (3671%) > α(1)β(2)-receptors (2601%), suggesting a role of the acetamido group in subunit-dependent receptor modulation.

Insights into structure-activity relationship of GABAA receptor modulating coumarins and furanocoumarins

The coumarins imperatorin and osthole are known to exert anticonvulsant activity. We have therefore analyzed the modulation of GABA-induced chloride currents (I(GABA)) by a selection of 18 coumarin derivatives on recombinant α(1)β(2)γ(2S) GABA(A) receptors expressed in Xenopus laevis oocytes by means of the two-microelectrode voltage clamp technique. Osthole (EC(50)=14 ± 1 μM) and oxypeucedanin (EC(50)=25 ± 8 μM) displayed the highest efficiency with I(GABA) potentiation of 116 ± 4 % and 547 ± 56 %, respectively. I(GABA) enhancement by osthole and oxypeucedanin was not inhibited by flumazenil (1 μM) indicating an interaction with a binding site distinct from the benzodiazepine binding site. In general, prenyl residues are essential for the positive modulatory activity, while longer side chains or bulkier residues (e.g. geranyl residues) diminish I(GABA) modulation. Generation of a binary classification tree revealed the importance of polarisability, which is sufficient to distinguish actives from inactives. A 4-point pharmacophore model based on oxypeucedanin - comprising three hydrophobic and one aromatic feature - identified 6 out of 7 actives as hits. In summary, (oxy-)prenylated coumarin derivatives from natural origin represent new GABA(A) receptor modulators.