Interaction of allosteric ligands with GABAA receptors containing one, two, or three different subunits

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Integrated workflow for the identification of new GABAA R positive allosteric modulators based on the in silico screening with further in vitro validation. Case study using Enamine's stock chemical space

Numerous studies reported an association between GABAA R subunit genes and epilepsy, eating disorders, autism spectrum disorders, neurodevelopmental disorders, and bipolar disorders. This study was aimed to find some potential positive allosteric modulators and was performed by combining the in silico approach with further in vitro evaluation of its real activity. We started from the GABAA R-diazepam complexes and assembled a lipid embedded protein ensemble to refine it via molecular dynamics (MD) simulation. Then we focused on the interaction of α1β2γ2 with some Z-drugs (non-benzodiazepine compounds) using an Induced Fit Docking (IFD) into the relaxed binding site to generate a pharmacophore model. The pharmacophore model was validated with a reference set and applied to decrease the pre-filtered Enamine database before the main docking procedure. Finally, we succeeded in identifying a set of compounds, which met all features of the docking model. The aqueous solubility and stability of these compounds in mouse plasma were assessed. Then they were tested for the biological activity using the rat Purkinje neurons and CHO cells with heterologously expressed human α1β2γ2 GABAA receptors. Whole-cell patch clamp recordings were used to reveal the GABA induced currents. Our study represents a convenient and tunable model for the discovery of novel positive allosteric modulators of GABAA receptors. A High-throughput virtual screening of the largest available database of chemical compounds resulted in the selection of 23 compounds. Further electrophysiological tests allowed us to determine a set of 3 the most outstanding active compounds. Considering the structural features of leader compounds, the study can develop into the MedChem project soon.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

8-Substituted Triazolobenzodiazepines: In Vitro and In Vivo Pharmacology in Relation to Structural Docking at the α1 Subunit-Containing GABAA Receptor

In order to develop improved anxiolytic drugs, 8-substituted analogs of triazolam were synthesized in an effort to discover compounds with selectivity for α2/α3 subunit-containing GABAA subtypes. Two compounds in this series, XLi-JY-DMH (6-(2-chlorophenyl)-8-ethynyl-1-methyl-4H-benzo [f][1,2,4]triazolo[4,3-a][1,4]diazepine) and SH-TRI-108 [(E)-8-ethynyl-1-methyl-6-(pyridin-2-yl)-4H-benzo [f][1,2,4]triazolo[4,3-a][1,4]diazepine], were evaluated for in vitro and in vivo properties associated with GABAA subtype-selective ligands. In radioligand binding assays conducted in transfected HEK cells containing rat αXβ3γ2 subtypes (X = 1,2,3,5), no evidence of selectivity was obtained, although differences in potency relative to triazolam were observed overall (triazolam > XLi-JY-DMH > SH-TRI-108). In studies with rat αXβ3γ2 subtypes (X = 1,2,3,5) using patch-clamp electrophysiology, no differences in maximal potentiation of GABA-mediated Cl- current was obtained across subtypes for any compound. However, SH-TRI-108 demonstrated a 25-fold difference in functional potency between α1β3γ2 vs. α2β3γ2 subtypes. We evaluated the extent to which this potency difference translated into behavioral pharmacological differences in monkeys. In a rhesus monkey conflict model of anxiolytic-like effects, triazolam, XLi-JY-DMH, and SH-TR-108 increased rates of responding attenuated by shock (anti-conflict effect) but also attenuated non-suppressed responding. In a squirrel monkey observation procedure, both analogs engendered a profile of sedative-motor effects similar to that reported previously for triazolam. In molecular docking studies, we found that the interactions of the 8-ethynyl triazolobenzodiazepines with the C-loop of the α1GABAA site was stronger than that of imidazodiazepines XHe-II-053 and HZ-166, which may account for the non-sedating yet anxiolytic profile of these latter compounds when evaluated in previous studies.

Intricacies of GABAA Receptor Function: The Critical Role of the β3 Subunit in Norm and Pathology

Neuronal intracellular chloride ([Cl⁻]_i) is a key determinant in γ-aminobutyric acid type A (GABA)ergic signaling. γ-Aminobutyric acid type A receptors (GABA_ARs) mediate both inhibitory and excitatory neurotransmission, as the passive fluxes of Cl⁻ and HCO₃⁻ via pores can be reversed by changes in the transmembrane concentration gradient of Cl⁻. The cation–chloride co-transporters (CCCs) are the primary systems for maintaining [Cl⁻]_i homeostasis. However, despite extensive electrophysiological data obtained in vitro that are supported by a wide range of molecular biological studies on the expression patterns and properties of CCCs, the presence of ontogenetic changes in [Cl⁻]_i—along with the consequent shift in GABA reversal potential—remain a subject of debate. Recent studies showed that the β3 subunit possesses properties of the P-type ATPase that participates in the ATP-consuming movement of Cl⁻ via the receptor. Moreover, row studies have demonstrated that the β3 subunit is a key player in GABA_AR performance and in the appearance of serious neurological disorders. In this review, we discuss the properties and driving forces of CCCs and Cl⁻, HCO₃⁻ATPase in the maintenance of [Cl⁻]_i homeostasis after changes in upcoming GABA_AR function. Moreover, we discuss the contribution of the β3 subunit in the manifestation of epilepsy, autism, and other syndromes.

L-Proline-catalyzed regioselective C1 arylation of tetrahydroisoquinolines through a multicomponent reaction under solvent-free conditions

Here we disclose the C1 arylation of tetrahydroisoquinolines (THIQ) through regioselective C(sp3)-H functionalization using a multicomponent reaction. The reaction was performed by reacting THIQ, aldehydes and aminopyrazoles or indoles under neat conditions with l-proline as a catalyst. The regioselectivity of the products was confirmed by X-ray analysis and spectroscopic data. The formation of an azomethine ylide intermediate is crucial for obtaining the regioselectivity.

Ectopic GABAA receptor β3 subunit determines Cl- / HCO 3 - -ATPase and chloride transport in HEK 293FT cells

Neuronal intracellular chloride concentration ([Cl^- ]_i ) is a crucial determinant of transmission mediated by the γ-aminobutyric acid type A receptor (GABA_A R), which subserves synaptic and extrasynaptic inhibition as well as excitation. The Cl^- ion is the main carrier of charge through the GABA_A R; however, bicarbonate ions ( HCO 3 - ) flowing in the opposite direction can also contribute to the net current. The direction of Cl^- and HCO 3 - fluxes is determined by the underlying electrochemical gradient, which is controlled by Cl^- transporters and channels. Accumulating evidence suggests that active mechanisms of chloride transport across the GABA_A R pore can underlie the regulation of [Cl^- ]_i . Measurement of Cl^- / HCO 3 - -ATPase activity and Cl^- transport in HEK 293FT cells expressing homomeric or heteromeric GABA_A R ensembles (α2, β3, or γ2) with fluorescent dye for chloride demonstrated that receptor subtypes containing the β3 subunit show enzymatic activity and participate in GABA-mediated or ATP-dependent Cl^- transport. GABA-mediated flow of Cl^- ions into and out of the cells occurred for a short time period but then rapidly declined. However, Cl^- ion flux was stabilized for a long time period in the presence of HCO 3 - ions. The reconstituted β3 subunit isoform, purified as a fusion protein, confirmed that β3 is critical for ATPase; however, only the triplet variant showed the full receptor function. The high sensitivity of the enzyme to γ-phosphate inhibitors led us to postulate that the β3 subunit is catalytic. Our discovery of a GABA_A R type that requires ATP consumption for chloride movement provides new insight into the molecular mechanisms of inhibitory signaling.

Alterations in GABAA Receptor Subunit Expression in the Amygdala and Entorhinal Cortex in Human Temporal Lobe Epilepsy

The amygdala has long been implicated in the pathophysiology of human temporal lobe epilepsy (TLE). The different nuclei of this complex structure are interconnected and share reciprocal connections with the hippocampus and other brain structures, partly via the entorhinal cortex. Expression of GABAA receptor subunits α1, α2, α3, α5, β2, β2/3, and γ2 was evaluated by immunohistochemistry in amygdala specimens and the entorhinal cortex of 12 TLE patients and 12 autopsy controls. A substantial decrease in the expression of α1, α2, α3, and β2/3 subunits was found in TLE cases, accompanied by an increase of γ2 subunit expression in many nuclei. In the entorhinal cortex, the expression of all GABAA receptor subunits was decreased except for the α1 subunit, which was increased on cellular somata. The overall reduction in α subunit expression may lead to decreased sensitivity to GABA and its ligands and compromise phasic inhibition, whereas upregulation of the γ2 subunit might influence clustering and kinetics of receptors and impair tonic inhibition. The description of these alterations in the human amygdala is important for the understanding of network changes in TLE as well as the development of subunit-specific therapeutic agents for the treatment of this disease.

DNA Origami Scaffolds as Templates for Functional Tetrameric Kir3 K+ Channels

In native systems, scaffolding proteins play important roles in assembling proteins into complexes to transduce signals. This concept is yet to be applied to the assembly of functional transmembrane protein complexes in artificial systems. To address this issue, DNA origami has the potential to serve as scaffolds that arrange proteins at specific positions in complexes. Herein, we report that Kir3 K⁺ channel proteins are assembled through zinc-finger protein (ZFP)-adaptors at specific locations on DNA origami scaffolds. Specific binding of the ZFP-fused Kir3 channels and ZFP-based adaptors on DNA origami were confirmed by atomic force microscopy and gel electrophoresis. Furthermore, the DNA origami with ZFP binding sites nearly tripled the K⁺ channel current activity elicited by heterotetrameric Kir3 channels in HEK293T cells. Thus, our method provides a useful template to control the oligomerization states of membrane protein complexes in vitro and in living cells.

Recent developments on ultrasound-assisted one-pot multicomponent synthesis of biologically relevant heterocycles

Heterocycles are the backbone of organic compounds. Specially, N- &O-containing heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. Multicomponent reactions (MCRs) are becoming valuable tool for synthesizing structurally diverse molecular entities. On the other hand, the last decade has seen a tremendous outburst in modifying chemical processes to make them sustainable for the betterment of our environment. The application of ultrasound in organic synthesis is fulfilling some of the goals of 'green and sustainable chemistry' as it has some advantages over the traditional thermal methods in terms of reaction rates, yields, purity of the products, product selectivity, etc. Therefore the synthesis of biologically relevant heterocycles using one-pot multi-component technique coupled with the application of ultrasound is one of the thrusting areas in the 21st Century among the organic chemists. The present review deals with the "up to date" developments on ultrasound assisted one-pot multi-component synthesis of biologically relevant heterocycles reported so far.

GABA, Selank, and Olanzapine Affect the Expression of Genes Involved in GABAergic Neurotransmission in IMR-32 Cells

Clinical studies have shown that Selank had an anxiolytic effect comparable to that of classical benzodiazepine drugs, which can enhance the inhibitory effect of GABA by allosteric modulation of GABA_A receptors. These data suggest that the molecular mechanism of the effect of Selank may also be related to its ability to affect the performance of the GABAergic system. To test this hypothesis, we studied the changes in expression of 84 genes involved in the functioning of the GABAergic system and in the processes of neurotransmission in the culture of neuroblastoma IMR-32 cells using qPCR method. As test substances, in addition to Selank, we selected the major GABA_A receptor ligand, GABA, the atypical antipsychotic, olanzapine, and combinations of these compounds (Selank and GABA; Selank and olanzapine). We found no changes in the mRNA levels of the genes studied under the effect of Selank. The combined effect of GABA and Selank led to nearly complete suppression of changes in expression of genes in which mRNA levels changed under the effect of GABA. When Selank was used in conjunction with olanzapine, the expression alterations of more genes were observed compared with olanzapine alone. The data obtained indicate that Selank has no direct effect on the mRNA levels of the GABAergic system genes in neuroblastoma IMR-32 cells. At the same time, our results partially confirm the hypothesis that the peptide may affect the interaction of GABA with GABA_A receptors. Our data also suggest that Selank may enhance the effect of olanzapine on the expression of the genes studied.

Recent Developments on Ultrasound-Assisted Synthesis of Bioactive N-Heterocycles at Ambient Temperature

N-Heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. The last decade has seen a tremendous practice to carry out reactions at ambient temperature avoiding harsh reaction conditions. By applying ultrasonic radiation in organic synthesis we can make synthetic protocols more sustainable and can carry out reactions at room temperature avoiding the traditional thermal harsh reaction conditions. Therefore the synthesis of biologically relevant N-heterocycles at room temperature under the influence of ultrasonic irradiation is one of the advancing areas in the 21st century among organic chemists. The present review summarises the latest developments on ultrasound-assisted synthesis of biologically relevant N-heterocycles at ambient temperature.

Human α1β3γ2L gamma-aminobutyric acid type A receptors: High-level production and purification in a functional state

Gamma-aminobutyric acid type A receptors (GABA(A)Rs) are the most important inhibitory chloride ion channels in the central nervous system and are major targets for a wide variety of drugs. The subunit compositions of GABA(A)Rs determine their function and pharmacological profile. GABAA Rs are heteropentamers of subunits, and (α1)2 (β3)2 (γ2L)1 is a common subtype. Biochemical and biophysical studies of GABA(A)Rs require larger quantities of receptors of defined subunit composition than are currently available. We previously reported high-level production of active human α1β3 GABA(A)R using tetracycline-inducible stable HEK293 cells. Here we extend the strategy to receptors containing three different subunits. We constructed a stable tetracycline-inducible HEK293-TetR cell line expressing human (N)-FLAG-α1β3γ2L-(C)-(GGS)3 GK-1D4 GABA(A)R. These cells achieved expression levels of 70-90 pmol [(3)H]muscimol binding sites/15-cm plate at a specific activity of 15-30 pmol/mg of membrane protein. Incorporation of the γ2 subunit was confirmed by the ratio of [(3)H]flunitrazepam to [(3)H]muscimol binding sites and sensitivity of GABA-induced currents to benzodiazepines and zinc. The α1β3γ2L GABA(A)Rs were solubilized in dodecyl-D-maltoside, purified by anti-FLAG affinity chromatography and reconstituted in CHAPS/asolectin at an overall yield of ∼ 30%. Typical purifications yielded 1.0-1.5 nmoles of [(3)H]muscimol binding sites/60 plates. Receptors with similar properties could be purified by 1D4 affinity chromatography with lower overall yield. The composition of the purified, reconstituted receptors was confirmed by ligand binding, Western blot, and proteomics. Allosteric interactions between etomidate and [(3)H]muscimol binding were maintained in the purified state.

Characterization of the picrotoxin site of GABAA receptors

This unit describes an in vitro assay for characterization of the picrotoxin site of GABAA receptors in rat brain membranes using various radioligands. Methods and representative data for Scatchard analysis (Kd, Bmax determination), association kinetics, dissociation kinetics, and competition assays (IC50, Ki determination) are included.

A propofol binding site on mammalian GABAA receptors identified by photolabeling

Propofol is the most important intravenous general anesthetic in current clinical use. It acts by potentiating GABAA (γ-aminobutyric acid type A) receptors, but where it binds to this receptor is not known and has been a matter of some debate. We synthesized a new propofol analog photolabeling reagent whose biological activity is very similar to that of propofol. We confirmed that this reagent labeled known propofol binding sites in human serum albumin that have been identified using X-ray crystallography. Using a combination of protiated and deuterated versions of the reagent to label mammalian receptors in intact membranes, we identified a new binding site for propofol in GABAA receptors consisting of both β3 homopentamers and α1β3 heteropentamers. The binding site is located within the β subunit at the interface between the transmembrane domains and the extracellular domain and lies close to known determinants of anesthetic sensitivity in the transmembrane segments TM1 and TM2.

Characterization of the picrotoxin site of GABAA receptors

This protocol describes an in vitro assay for characterization of the picrotoxin site of GABA(A) receptors in rat brain membranes using various radioligands. Methods and representative data for Scatchard analysis (Kd, Bmax determination), association kinetics, dissociation kinetics and competition assays (IC50, Ki determination) are included in the protocol.

Association between leptin levels and doses of clomethiazole during alcohol withdrawal: a pilot study

AIMS

The issue of leptin as a putative state marker of alcohol use and its role in craving has been raised in the last few years. Recently, a strong GABA-ergic modulation of leptin was postulated. The aim of the pilot study was to examine leptin levels in correlation with the strongly GABA-mimetic active substance clomethiazole. The main hypothesis was that higher doses of the strong GABA-mimetic clomethiazole are positively correlated with higher leptin levels.

METHODS

Twenty-eight alcohol-dependent patients (3 females, median age 36 years) undergoing alcohol withdrawal were included. In 18 patients with and 10 without clomethiazole, serum leptin was analyzed at day 1 and day 7 of alcohol withdrawal. Both groups did not differ by age, BMI, or alcohol use characteristics.

RESULTS

In the clomethiazole group, significant correlations were found between leptin levels at day 1 and clomethiazole dose (p = 0.004), clomethiazole and leptin at day 1/BMI (p = 0.009) and leptin at day 1 and clomethiazole/body surface (p = 0.006). Furthermore, patients with higher clomethiazole doses demonstrated significant higher leptin levels at day 1 (p = 0.044) and day 7 (p = 0.046).

CONCLUSIONS

Our pilot data show a strong association between leptin levels and clomethiazole doses, thus supporting further research.

Differential effects of etifoxine on anxiety-like behaviour and convulsions in BALB/cByJ and C57BL/6J mice: any relation to overexpression of central GABAA receptor beta2 subunits?

Dysfunction of GABAergic transmission related to abnormal expression of GABA(A) receptor subunits in specific brain regions underlies some pathological anxiety states. Besides involvement of the benzodiazepine recognition site of GABA(A) receptor in the expression of anxiety-like behaviour, the roles of the β(2)/β(3) subunits are not well characterized. To address this issue, the experimental design of this study utilized the GABAergic compound etifoxine (with a preferential effectiveness after binding to a specific site at β(2)/β(3) subunits) tested in two inbred mouse strains: BALB/cByJ and C57BL/6J mice using three behavioural paradigms (light/dark box, elevated plus maze and restraint stress-induced small intestinal transit inhibition) and the t-butylbicyclophosphorothionate-induced convulsions model. Etifoxine plasma and brain levels and β(2)/β(3) mRNAs and protein expression levels in various brain regions were compared between the two strains. The two mouse strains differed markedly in basal anxiety level. Etifoxine exhibited more pronounced anxiolytic and anticonvulsant effects in the BALB/cByJ mice compared to the C57BL/6J mice. The etifoxine brain/plasma ratios of the two strains were not different. Beta2 subunit mRNA and protein expression levels were around 25 and 10% higher respectively in the anterodorsal nucleus of the thalamus and the CA3 field of hippocampus of BALB/cByJ mice compared to C57BL/6J mice. Beta3 subunit mRNA and protein expression levels did not differ between the two strains. Based on these results, it is suggested that overexpression of GABA(A) receptor β(2) subunit in BALB/cByJ mice relative to C57BL/6j mice contributes to the dysfunction in GABA(A) transmission in regions of brain known to regulate responses to stress. The dysregulated GABA(A) function in BALB/cByJ mice may be corrected by the administration of etifoxine.

High-level expression and purification of Cys-loop ligand-gated ion channels in a tetracycline-inducible stable mammalian cell line: GABAA and serotonin receptors

The human neuronal Cys-loop ligand-gated ion channel superfamily of ion channels are important determinants of human behavior and the target of many drugs. It is essential for their structural characterization to achieve high-level expression in a functional state. The aim of this work was to establish stable mammalian cell lines that enable high-level heterologous production of pure receptors in a state that supports agonist-induced allosteric conformational changes. In a tetracycline-inducible stable human embryonic kidney cells (HEK293S) cell line, GABA(A) receptors containing α1 and β3 subunits could be expressed with specific activities of 29-34 pmol/mg corresponding to 140-170 pmol/plate, the highest expression level reported so far. Comparable figures for serotonin (5-HT(3A)) receptors were 49-63 pmol/mg and 245-315 pmol/plate. The expression of 10 nmol of either receptor in suspension in a bioreactor required 0.3-3.0 L. Both receptor constructs had a FLAG epitope inserted at the N-terminus and could be purified in one step after solubilization using ANTI-FLAG affinity chromatography with yields of 30-40%. Purified receptors were functional. Binding of the agonist [(3)H]muscimol to the purified GABA(A)R was enhanced allosterically by the general anesthetic etomidate, and purified 5-hydroxytryptamine-3A receptor supported serotonin-stimulated cation flux when reconstituted into lipid vesicles.

Gel-based mass spectrometric analysis of recombinant GABA(A) receptor subunits representing strongly hydrophobic transmembrane proteins

GABA(A) receptors are the major inhibitory transmitter receptors in mammalian brain and are composed of several protein subunits that can belong to different subunit classes, leading to enormous heterogeneity. To establish techniques for the analysis of GABA(A) receptors in complex mixtures such as brain tissue, recombinant receptors composed of alpha1 and His-tagged beta3 subunits expressed in insect cells were purified by affinity chromatography and run on blue native gels. After denaturing, receptors were subjected to one- or two-dimensional electrophoresis in SDS-gels. Proteins were cleaved by multienzyme proteolysis and subjected to nano-ESI-LC-MS/MS. Both GABA(A) receptor subunits were well-separated and unambiguously identified by sequence coverage of 99.1% (alpha1) and 92.9% (beta3).

Enhanced macroscopic desensitization shapes the response of alpha4 subtype-containing GABAA receptors to synaptic and extrasynaptic GABA

Up-regulation of the GABAA receptor alpha4 subunit subtype has been consistently shown in multiple animal models of chronic epilepsy. This isoform is expressed in both thalamus and hippocampus and is likely to play a significant role in regulating corticothalamic and hippocampal rhythms. However, little is known about its physiological properties, thus limiting understanding of the role of alpha4 subtype-containing GABAA receptors in normal and abnormal physiology. We used rapid GABA application to recombinant GABAA receptors expressed in HEK293T cells to compare the macroscopic kinetic properties of alpha4beta3gamma2L receptors to those of the more widely distributed alpha1beta3gamma2L receptors. These receptor currents had similar peak current amplitudes and GABA EC50 values. However, alpha4beta3gamma2L currents activated more slowly when exposed to submaximal GABA concentrations, had more fast desensitization (tau = 15-100 ms), and had less residual current during long GABA applications. In addition, alpha4beta3gamma2L currents deactivated more slowly than alpha1beta3gamma2L currents. Peak currents evoked by repetitive, brief GABA applications were more strongly attenuated for alpha4beta3gamma2L currents than alpha1beta3gamma2L currents. Moreover, the time required to recover from desensitization was prolonged in alpha4beta3gamma2L currents compared to alpha1beta3gamma2L currents. We also found that exposure to prolonged low levels of GABA, similar to those that might be present in the extrasynaptic space, greatly suppressed the response of alpha4beta3gamma2L currents to higher concentrations of GABA, while alpha1beta3gamma2L currents were less affected by exposure to low levels of GABA. Taken together, these data suggest that alpha4beta3gamma2L receptors have unique kinetic properties that limit the range of GABA applications to which they can respond maximally. While similar to alpha1beta3gamma2L receptors in their ability to respond to brief and low frequency synaptic inputs, alpha4beta3gamma2L receptors are less efficacious when exposed to prolonged tonic GABA or during repetitive stimulation, as may occur during learning and seizures.

Changes in neocortical and hippocampal GABAA receptor subunit distribution during brain maturation and aging

gamma-Aminobutyric acid type A (GABA(A)) receptors are the most important inhibitory receptors in the central nervous system, playing a pivotal role in the regulation of brain excitability. The pentameric receptor is commonly composed of different alpha, beta, and gamma subunits which mediate the function and pharmacology of the receptor and show regional- and temporal-specific expression patterns. Under varying physiological and pathophysiological conditions, this diversity allows a multitude of adaptive changes in subunit composition leading to distinct biological and pharmacological properties of the receptor. Here, we investigated the expression of five major GABA(A) receptors subunits (alpha1, alpha2, alpha3, alpha5, gamma2) in early postnatal, adult, and aged rat brains. Immunohistochemistry was performed at postnatal day 10, 30, 60, 90, 180, 360, and 540. Morphological and semi-quantitative evaluations of regional optical densities revealed specific regional and temporal expression patterns for all subunits. The study clearly demonstrated that changes in GABA(A) receptor distribution not only occur in the early postnatal cortex and hippocampal formation but also during later periods in the adolescent and aging brain. These findings contribute to a better understanding of age-related changes in brain excitability and further elucidate the distinct pharmacological effects of different GABAergic drugs in young and elderly patients.

Determinants of amentoflavone interaction at the GABA(A) receptor

We investigated the recognition properties of different GABA(A) receptor subtypes and mutant receptors for the biflavonoid amentoflavone, a constituent of St. John's Wort. Radioligand binding studies showed that amentoflavone recognition paralleled that of the classical benzodiazepine diazepam in that it had little or no affinity for alpha4- or alpha6-containing receptors. Lysine and alanine substitutions at position 101 of the rat alpha1 subunit resulted in a complete loss of competitive amentoflavone binding, but functional analysis of the alanine mutant expressed with beta2 and gamma2 subunits in Xenopus oocytes revealed no significant difference in the negative modulation of GABA-induced currents brought about by amentoflavone. Furthermore, elimination of the gamma subunit had no effect on the negative modulation of these currents. This negative modulation was also observed at alpha1beta1gamma2 GABA(A) receptors and is therefore not likely mediated by the loreclezole site. These results suggest a complex mechanism of amentoflavone interaction at GABA(A) receptors.

Effect of chronic administration of ethanol on GABAA receptor assemblies derived from α2-, α3-, β2- and γ2-subunits in the rat cerebral cortex

Chronic administration of ethanol decreased the immunoprecipitation of the [(3)H]flunitrazepam binding activity for GABA(A) receptor assemblies derived from alpha(2)-, alpha(3)- and gamma(2)-subunits in the rat cerebral cortex. However, the [(3)H]muscimol binding sites derived from these subunits were not affected. Thus, chronic ethanol causes the down-regulation of the benzodiazepine sites derived from the alpha(2)-, alpha(3)- and gamma(2)-subunits without affecting the GABA binding sites.

Subunit composition and quantitative importance of GABA(A) receptor subtypes in the cerebellum of mouse and rat

In cerebellum, 13 different GABA(A) receptor subunits are expressed. The number of different receptor subtypes formed in this tissue, their subunit composition and their quantitative importance so far has not been determined. In the present study, immunodepletion by immunoaffinity chromatography, as well as immunoprecipitation and western blot analysis was performed using 13 different subunit-specific antibodies to provide an overview on the subunit composition and abundance of GABA(A) receptor subtypes in mouse and rat cerebellum. Results obtained indicate that alpha1betaxgamma2, alpha1alpha6betaxgamma2, alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta are the major GABA(A) receptor subtypes present in the cerebellum. In addition, small amounts of alpha1betaxdelta receptors and a series of minor receptor subtypes containing alpha2, alpha3, alpha4, alpha5, gamma1 or gamma3 subunits are also present in the cerebellum. Whereas the abundance of alpha1alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta receptors is different in mouse and rat cerebellum, that of other receptors is quite similar in these tissues. Data obtained for the first time provide an overview on the GABA(A) receptor subtypes present in the cerebellum and represent the basis for further studies investigating changes in receptor expression and composition under pathological conditions.

Changes of GABA receptors and dopamine turnover in the postmortem brains of parkinsonians with levodopa-induced motor complications

Brain samples from 14 Parkinson's disease patients, 10 of whom developed motor complications (dyskinesias and/or wearing-off) on dopaminomimetic therapy, and 11 controls were analyzed. Striatal 3beta-(4-(125)I-iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([(125)I]RTI-121) -specific binding to dopamine transporter and concentration of dopamine were markedly decreased, but no association between level of denervation and development of motor complications was observed. The homovanillic acid/dopamine ratio of concentrations was higher in putamen of patients with wearing-off compared to those without. Striatal (35)S-labeled t-butylbicyclophosphorothionate ([(35)S]TBPS) and [(3)H]flunitrazepam binding to GABA(A) receptors were unchanged in patients with Parkinson's disease, whereas [(125)I]CGP 64213 -specific binding to GABA(B) receptors was decreased in the putamen and external segment of the globus pallidus of parkinsonian patients compared with controls. [(3)H]Flunitrazepam binding was increased in the putamen of patients with wearing-off compared to those without. [(35)S]TBPS-specific binding was increased in the ventral internal globus pallidus of dyskinetic subjects. These data suggest altered dopamine metabolism and increased GABA(A) receptors in the putamen related to the pathophysiology of wearing-off. The present results also suggest that an up-regulation of GABA(A) receptors in the internal globus pallidus is linked to the pathogenesis of levodopa-induced dyskinesias.

Differential interaction of anaesthetics and antiepileptic drugs with neuronal Na+ channels, Ca2+ channels, and GABA(A) receptors

BACKGROUND

Current theories favour multiple agent-specific neuronal actions for both general anaesthetics and antiepileptic drugs, but the pharmacological properties that distinguish them are poorly understood. We compared the interactions of representative agents from each class on their putative targets using well-characterized radioligand binding assays.

METHODS

Synaptosomes or membranes prepared from rat cerebral cortex were used to analyse drug effects on [(35)S]t-butyl bicyclophosphorothionate ([(35)S]TBPS) binding to the picrotoxinin site of GABA(A) receptors, [(3)H]batrachotoxinin A 20-alpha benzoate ([(3)H]BTX-B) binding to site 2 of voltage-gated Na(+) channels, (+)-[methyl-(3)H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-methoxycarboxyl-2,6-dimethyl-3-pyridinecarboxylate ([(3)H]PN200-110; isradipine) binding to L-type Ca(2+) channels, and [cyclohexyl-2,3-(3)H](N)glibenclamide ([(3)H]GB) binding to K(ATP) channels.

RESULTS

I.V. anaesthetics other than ketamine preferentially inhibited [(35)S]TBPS binding (etomidate approximately equal alphaxalone > propofol > thiopental > pentobarbital). Volatile anaesthetics inhibited both [(35)S]TBPS and [(3)H]BTX-B binding with comparable potencies (halothane approximately equal isoflurane approximately equal enflurane). Antiepileptic drugs preferentially antagonized either [(35)S]TBPS (diazepam > phenobarbital) or [(3)H]BTX-B (phenytoin > carbamazepine) binding. Local anaesthetics (lidocaine, tertracaine) selectively antagonized [(3)H]BTX-B binding. None of the drugs tested were potent antagonists of [(3)H]PN200-110 or [(3)H]GB binding.

CONCLUSIONS

Comparative radioligand binding assays identified distinct classes of general anaesthetic and antiepileptic drugs based on their relative specificities for a defined target set. I.V. anaesthetics interacted preferentially with GABA(A) receptors, while volatile anaesthetics were essentially equipotent at Na(+) channels and GABA(A) receptors. Antiepileptic drugs could be classified by preferential actions at either Na(+) channels or GABA(A) receptors. Anaesthetics and antiepileptic drugs have agent-specific effects on radioligand binding. Both general anaesthetics and antiepileptic drugs interact with Na(+) channels and GABA(A) receptors at therapeutic concentrations, in most cases with little selectivity.

Identification of an amino acid sequence within GABA(A) receptor beta3 subunits that is important for receptor assembly

GABA(A) receptors are chloride ion channels that can be opened by GABA, the most important inhibitory transmitter in the CNS. In the mammalian brain the majority of these pentameric receptors is composed of two alpha, two beta and one gamma subunit. To achieve the correct order of subunits around the pore, each subunit must form specific contacts via its plus (+) and minus (-) side. To identify a sequence on the beta3 subunit important for assembly, we generated various full-length or truncated chimeric beta3 constructs and investigated their ability to assemble with alpha1 and gamma2 subunits. It was demonstrated that replacement of the sequence beta3(76-89) by the homologous alpha1 sequence impaired assembly with alpha1 but not with gamma2 subunits in alpha1beta3gamma2-GABA(A) receptors. Other experiments indicated that assembly was impaired via the beta3(-) side of the chimeric subunit. Within the sequence beta3(76-89) the sequence beta3(85-89) seemed to be of primary importance for assembly with alpha1 subunits. A comparison with the structure of the acetylcholine-binding protein supports the conclusion that the sequence beta3(85-89) is located at the beta3(-) side and indicates that it contains amino acid residues that might directly interact with the (+) side of the neighbouring alpha1 subunit.

Electrophysiological actions of gamma-aminobutyric acid and clomethiazole on recombinant GABA(A) receptors

Clomethiazole is a gamma-aminobutyric acid (GABA)-mimetic agent with anticonvulsant, sedative and neuroprotective properties. The pharmacological actions of clomethiazole that underlie its functional profile have not been fully explored, but are known to result from an interaction with the GABA(A) receptor. Here, we present a quantitative electrophysiological study of clomethiazole action at human recombinant GABA(A) receptors. Whole-cell currents were recorded from murine L(tk-) cells stably transfected with either alpha1, beta1 and gamma 2 or alpha1, beta2 and gamma 2 GABA(A) receptor subunits. Clomethiazole directly activated GABA(A) currents in alpha1/beta1/gamma 2- and alpha1/beta2/gamma 2-containing cells, with EC(50) values of 0.3 and 1.5 mM, respectively. A low concentration of clomethiazole (30 micro M) also potentiated the action of GABA in both cell types, equivalent to a 3-fold increase in potency and up to 1.8-fold increase in maximal current. Both direct activation and gamma-aminobutyric acid potentiation are likely to contribute to the in vivo profile of clomethiazole.

A novel site on gamma 3 subunits important for assembly of GABA(A) receptors

Gamma-aminobutyric acid, type A (GABA(A)) receptors are ligand-gated chloride channels and are the major inhibitory transmitter receptors in the central nervous system. The majority of these receptors is composed of two alpha, two beta, and one gamma subunits. To identify sequences important for subunit assembly, we generated C-terminally truncated and chimeric gamma(3) constructs. From their ability to associate with full-length alpha(1) and beta(3) subunits, we concluded that amino acid sequence gamma(3)(70-84) either directly interacts with alpha(1) or beta(3) subunits or stabilizes a contact site elsewhere in the protein. The observation that this sequence contains amino acid residues homologous to gamma(2) residues contributing to the benzodiazepine-binding site at the alpha(1)/gamma(2) interface suggested that in alpha(1)beta(3)gamma(3) receptors the sequence gamma(3)(70-84) is located at the alpha(1)/gamma(3) interface. In the absence of alpha(1) subunits this sequence might allow assembly of beta(3) with gamma(3) subunits. Other experiments indicated that sequences gamma(3)(86-95) and gamma(3)(94-107), which are homologous to previously identified sequences important for assembly of gamma(2) subunits, are also important for assembly of gamma(3) subunits. This indicates that during assembly of the GABA(A) receptor, more than one N-terminal sequence is important for binding to the same neighboring subunit. Whether the three sequences investigated are involved in direct interaction or stabilize other regions involved in intersubunit contacts has to be further studied.

N-Substituted 4-amino-3,3-dipropyl-2(3H)-furanones: new positive allosteric modulators of the GABA(A) receptor sharing electrophysiological properties with the anticonvulsant loreclezole

1,4-Addition of benzylamine to 2(5H)-furanone followed by dialkylation of the 3-position with allylbromide gave (+/-)-4-benzyl-3,3-diallyl-2(3H)-furanone (8), which served as the intermediate for the synthesis of various N-substituted 4-amino-3,3-dipropyl-2(3H)-furanones (+/-)-9a-l. The compounds were evaluated for their capacity to potentiate or inhibit GABA-evoked currents in Xenopus laevis oocytes expressing recombinant alpha1beta2gamma2 GABA(A) receptors. The benzyl, ethyl, and allyl carbamates ((R)-9a (100 microM), (+/-)-9b (100 microM), (+/-)-9c (200 microM)) stimulated GABA currents by 279 +/- 47%, 426 +/- 8%. and 765 +/- 61%, respectively, while the phenylcarboxamide (+/-)-9f (200 microM) stimulated currents by 420 +/- 33%. Concentration-response studies showed that compound 9c was approximately twice as potent in stimulating GABA currents as alpha-EMTBL (2), the most potent 3,3-dialkylbutyrolactone known to date. On the other hand, the N-sulfonyl analogues were much less active or even inhibited GABA-evoked currents. In vitro radioligand displacement studies on rat brain membranes showed that these compounds did not bind to the benzodiazepine or GABA recognition sites of the GABA(A) receptor. However, these compounds generally weakly displaced [(35)S]-TBPS (approximately 50% displacement at 100 microM), though potencies did not correlate with GABA current potentiation. Results obtained with alpha1beta1 and mutant alpha1beta2N265S receptors, which compared to alpha1beta2 receptors are both much less sensitive to current stimulation produced by the anticonvulsant loreclezole, suggest that at least some of these aminobutyrolactones, (e.g., 9a, 9c), and interestingly also alpha-EMTBL, share stimulatory properties with loreclezole.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Lack of allosteric modulation of striatal GABA(A) receptor binding and function after cocaine sensitization

GABA(A) receptor binding after repeated cocaine has been shown to be either increased as indicated by benzodiazepine binding or decreased as indicated by convulsant-site binding. We measured the GABA binding site with [3H]-muscimol binding to GABA(A) receptors and found no differences between saline- and cocaine-sensitized rats. Allosteric modulation of [3H]-muscimol binding with flunitrazepam was also unchanged after cocaine sensitization. In addition, [3H]-flunitrazepam binding and allosteric modulation of [3H]-flunitrazepam binding with GABA was unchanged after 1 day withdrawal from repeated cocaine. GABA(A) receptor function and allosteric modulation of GABA(A) receptor function measured by GABA-stimulated Cl(-) uptake was also unchanged after withdrawal from repeated cocaine. Finally, in vitro cocaine reduced GABA(A) receptor function in striatal microsacs of saline- and cocaine-treated rats. In conclusion, repeated cocaine did not change the coupling of the GABA(A) receptor between the GABA and benzodiazepine (BZD) binding site after 1 day withdrawal.

Differential Cross Talk of ROD Compounds with the Benzodiazepine Binding Site

We have recently identified a novel class of allosteric modulators of GABA(A) receptors, the ROD compounds that are structurally related to bicuculline. Here, the relationship of their site of action relative to other known modulatory sites of this receptor was investigated. Two types of ROD compounds, R1 (ROD164A, ROD185) and R2 (ROD222 and ROD259) could be differentiated. R1 compounds competitively inhibited binding of benzodiazepines in alpha1beta2gamma2 receptors, and their functional effects were partially inhibited by the benzodiazepine antagonist Ro15-1788 in a noncompetitive manner. The enhancement by an R1 compound was not additive with that by diazepam. R2 compounds in contrast failed to inhibit binding of benzodiazepines; the R2 compounds' functional effects were not inhibited by the benzodiazepine antagonist. The enhancement by an R2 compound was additive with that by diazepam. In contrast to benzodiazepines, both R1 and R2 type compounds were still able to enhance alpha1beta2 receptors. ROD164A in alpha1beta2gamma2 receptors was found to be partially antagonized by Ro15-1788 in a noncompetitive way. ROD178B did not affect gamma-aminobutyric acid induced currents, but was able to inhibit both enhancement by R1 and R2 type compounds as well as enhancement by diazepam. R1 and R2 type compounds as well as diazepam enhanced pentobarbital-induced currents in a Ro15-1788-sensitive way. We conclude that R1 type compounds act at the benzodiazepine binding site and additionally at a different R1 site, and that the R1, but not the R2 site is allosterically coupled to the benzodiazepine binding site. ROD178B is a competitive antagonist at the R1 site in that it shows allosteric interaction with the benzodiazepine binding site and displacement of benzodiazepines, and a negative allosteric modulator at the R2 site.

Detection and binding properties of GABA(A) receptor assembly intermediates

Density gradient centrifugation of native and recombinant gamma-aminobutyric acid, type A (GABA(A)) receptors was used to detect assembly intermediates. No such intermediates could be identified in extracts from adult rat brain or from human embryonic kidney (HEK) 293 cells transfected with alpha(1), beta(3), and gamma(2) subunits and cultured at 37 degrees C. However, subunit dimers, trimers, tetramers, and pentamers were found in extracts from the brain of 8-10-day-old rats and from alpha(1)beta(3)gamma(2) transfected HEK cells cultured at 25 degrees C. In both systems, alpha(1), beta(3), and gamma(2) subunits could be identified in subunit dimers, indicating that different subunit dimers are formed during GABA(A) receptor assembly. Co-transfection of HEK cells with various combinations of full-length and C-terminally truncated alpha(1) and beta(3) or alpha(1) and gamma(2) subunits and co-immunoprecipitation with subunit-specific antibodies indicated that even subunits containing no transmembrane domain can assemble with each other. Whereas alpha(1)gamma(2), alpha(1)Ngamma(2), alpha(1)gamma(2)N, and alpha(1)Ngamma(2)N, combinations exhibited specific [(3)H]Ro 15-1788 binding, specific [(3)H]muscimol binding could only be found in alpha(1)beta(3) and alpha(1)beta(3)N, but not in alpha(1)Nbeta(3) or alpha(1)Nbeta(3)N combinations. This seems to indicate that a full-length alpha(1) subunit is necessary for the formation of the muscimol-binding site and for the transduction of agonist binding into channel gating.

Role of human GABA(A) receptor beta3 subunit in insecticide toxicity

The gamma-aminobutyric acid type A (GABA(A)) receptor is the target for the major insecticides alpha-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [(3)H]ethynylbicycloorthobenzoate ([(3)H]EBOB). This study tests the hypothesis that GABA(A) receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamma2) and in combination in insect Sf9 cells. Binding parameters were similar for [(3)H]EBOB in the beta3 homooligomer, alpha1beta3gamma2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, alpha-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1beta3gamma2 subtype (IC50 values of 16-33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1beta3gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or alpha-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > alpha-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1beta3gamma2 receptors. Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.

Unsulfated and sulfated neurosteroids differentially modulate the binding characteristics of various radioligands of GABA(A) receptors following chronic ethanol administration

Dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) inhibited the binding of [(3)H]flunitrazepam (2 nM), [(3)H]muscimol (5 nM) and 4 nM [(35)S]t-butylbicyclophosphorothionate [(35)S]TBPS in the rat cerebellum as well as cerebral cortex. DHEAS-induced inhibition of binding of these radioligands (62% to 100%) was more pronounced as compared to that in the case of DHEA (5% to 31%). DHEAS, unlike DHEA, inhibited [(3)H]flunitrazepam binding significantly to a lesser extent in the cerebellum of ethanol-dependent rats as compared to the control group (I(max):82+/-1vs.92+/-2%, p<0.005). However, DHEA, unlike DHEAS, inhibited [(35)S]TBPS binding to a greater extent in the ethanol-dependent rat cerebellum as compared to the control group (I(max):31+/-2vs.19+/-2%, p<0.005). Furthermore, DHEA was more potent in inhibiting [(35)S]TBPS binding in the cerebellum (IC(50):55+/-5 vs. 74+/-7 microM, p<0.05) and cerebral cortex (IC(50):26+/-4vs.64+/-9 microM, p<0.05) of ethanol-dependent rats as compared to the control group. These observations indicate that unsulfated and sulfated androstane-steroids modulate the GABA(A) receptors in the control as well as the ethanol-dependent rats differentially, and also suggest that the androstane-steroid binding sites associated with the GABA(A) receptors play an important role during ethanol dependence.

Targeted disruption of the GABA(A) receptor delta subunit gene leads to an up-regulation of gamma 2 subunit-containing receptors in cerebellar granule cells

GABA(A) receptors are chloride channels composed of five subunits. Cerebellar granule cells express abundantly six subunits belonging to four subunit classes. These are assembled into a number of distinct receptors, but the regulation of their relative proportions is yet unknown. Here, we studied the composition of cerebellar GABA(A) receptors after targeted disruption of the delta subunit gene. In membranes and extracts of delta-/- cerebellum, [(3)H]muscimol binding was not significantly changed, whereas [(3)H]Ro15-4513 binding was increased by 52% due to an increase in diazepam-insensitive binding. Immunocytochemical and Western blot analysis revealed no change in alpha(6) subunits but an increased expression of gamma(2) subunits in delta-/- cerebellum. Immunoaffinity chromatography of cerebellar extracts indicated there was an increased coassembly of alpha(6) and gamma(2) subunits and that 24% of all receptors in delta-/- cerebellum did not contain a gamma subunit. Because 97% of delta subunits are coassembled with alpha(6) subunits in the cerebellum of wild-type mice, these results indicated that, in delta-/- mice, alpha(6)betagamma(2) and alphabeta receptors replaced delta subunit-containing receptors. The availability of the delta subunit, thus, influences the level of expression or the extent of assembly of the gamma(2) subunit, although these two subunits do not occur in the same receptor.