Affinity of various ligands for GABAA receptors containing alpha 4 beta 3 gamma 2, alpha 4 gamma 2, or alpha 1 beta 3 gamma 2 subunits

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].

α2-containing GABAA receptors expressed in hippocampal region CA3 control fast network oscillations

GABA(A) receptors are critically involved in hippocampal oscillations. GABA(A) receptor α1 and α2 subunits are differentially expressed throughout the hippocampal circuitry and thereby may have distinct contributions to oscillations. It is unknown which GABA(A) receptor α subunit controls hippocampal oscillations and where these receptors are expressed. To address these questions we used transgenic mice expressing GABA(A) receptor α1 and/or α2 subunits with point mutations (H101R) that render these receptors insensitive to allosteric modulation at the benzodiazepine binding site, and tested how increased or decreased function of α subunits affects hippocampal oscillations. Positive allosteric modulation by zolpidem prolonged decay kinetics of hippocampal GABAergic synaptic transmission and reduced the frequency of cholinergically induced oscillations. Allosteric modulation of GABAergic receptors in CA3 altered oscillation frequency in CA1, while modulation of GABA receptors in CA1 did not affect oscillations. In mice having a point mutation (H101R) at the GABA(A) receptor α2 subunit, zolpidem effects on cholinergically induced oscillations were strongly reduced compared to wild-type animals, while zolpidem modulation was still present in mice with the H101R mutation at the α1 subunit. Furthermore, genetic knockout of α2 subunits strongly reduced oscillations, whereas knockout of α1 subunits had no effect. Allosteric modulation of GABAergic receptors was strongly reduced in unitary connections between fast spiking interneurons and pyramidal neurons in CA3 of α2H101R mice, but not of α1H101R mice, suggesting that fast spiking interneuron to pyramidal neuron synapses in CA3 contain α2 subunits. These findings suggest that α2-containing GABA(A) receptors expressed in the CA3 region provide the inhibition that controls hippocampal rhythm during cholinergically induced oscillations.

Receptors with low affinity for neurosteroids and GABA contribute to tonic inhibition of granule cells in epileptic animals

Neurosteroid sensitivity of GABA(A) receptor mediated inhibition of the hippocampal dentate granule cells (DGCs) is reduced in animal models of temporal lobe epilepsy. However, the properties and subunit composition of GABA(A) receptors mediating tonic inhibition in DGCs of epileptic animals have not been described. In the DGCs of epileptic animals, allopregnanolone and L-655708 sensitivity of holding current was diminished and δ subunit was retained in the endoplasmic reticulum and its surface expression was decreased the in the hippocampus. Ro15-4513 and lanthanum had distinct effects on holding current recorded from DGCs of control and epileptic animals suggesting that the pharmacological properties of GABA(A) receptors maintaining tonic inhibition in DGCs of epileptic animals were similar to those containing the α4βxγ2 subunits. Furthermore, surface expression of the α4 subunit increased and a larger fraction of the subunit co-immunoprecipitated with theγ2 subunit in hippocampi of epileptic animals. Together, these studies revealed that functional α4βxδ and α5βxγ2 receptors were reduced in the hippocampi of epileptic animals and that novel α4bxγ2 receptors contributed to the maintenance of tonic inhibition. The presence of α4βxγ2 receptors resulted in low GABA affinity and neurosteroid sensitivity of tonic currents in the DGCs of epileptic animals that could potentially increase seizure vulnerability. These receptors may represent a novel therapeutic target for anticonvulsant drugs without sedative actions.

The relevance of neuroactive steroids in schizophrenia, depression, and anxiety disorders

1. Neuroactive steroids are steroid hormones that exert rapid, nongenomic effects at ligand-gated ion channels. There is increasing awareness of the possible role of these steroids in the pathology and manifestation of symptoms of psychiatric disorders. The aim of this paper is to review the current knowledge of neuroactive steroid functioning in the central nervous system, and to assess the role of neuroactive steroids in the pathophysiology and treatment of symptoms of schizophrenia, depression, and anxiety disorders. Particular emphasis will be placed on GABAA receptor modulation, given the extensive knowledge of the interactions between this receptor complex, neuroactive steroids, and psychiatric illness. 2. A brief description of neuroactive steroid metabolism is followed by a discussion of the interactions of neuroactive steroids with acute and chronic stress and the HPA axis. Preclinical and clinical studies related to psychiatric disorders that have been conducted on neuroactive steroids are also described. 3. Plasma concentrations of some neuroactive steroids are altered in individuals suffering from schizophrenia, depression, or anxiety disorders compared to values in healthy controls. Some drugs used to treat these disorders have been reported to alter plasma and brain concentrations in clinical and preclinical studies, respectively. 4. Further research is warranted into the role of neuroactive steroids in the pathophysiology of psychiatric illnesses and the possible role of these steroids in the successful treatment of these disorders.

Organization of the torus longitudinalis in the rainbow trout (Oncorhynchus mykiss): an immunohistochemical study of the GABAergic system and a DiI tract-tracing study

The torus longitudinalis (TL) is a tectum-associated structure of actinopterygian fishes. The organization of the TL of rainbow trout was studied with Nissl staining, Golgi methods, immunocytochemistry with antibodies to gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD), and the GABA(A) receptor subunits delta and beta2/beta 3, and with tract tracing methods. Two types of neuron were characterized: medium-sized GABAergic neurons and small GABA-negative granule cells. GABA(A) receptor subunit delta-like immunoreactivity delineated two different TL regions, ventrolateral and central. Small GABAergic cells were also observed in marginal and periventricular strata of the optic tectum. These results indicate the presence of local GABAergic inhibitory circuits in the TL system. For tract-tracing, a lipophilic dye (DiI) was applied to the TL and to presumed toropetal nuclei or toral targets. Toropetal neurons were observed in the optic tectum, in pretectal (central, intermediate, and paracommissural) nuclei, in the subvalvular nucleus, and associated with the pretectocerebellar tract. Torofugal fibers were numerous in the stratum marginale of the optic tectum. Toropetal pretectal nuclei also project to the cerebellum, and a few TL cells project to the cerebellar corpus. The pyramidal cells of the trout tectum were also studied by Golgi methods and local DiI labeling. The connections of trout TL revealed here were more similar to those recently reported in carp and holocentrids (Ito et al. [2003] J. Comp. Neurol. 457:202-211; Xue et al. [2003] J. Comp. Neurol. 462:194-212), than to those reported in earlier studies. However, important differences in organization of toropetal nuclei were noted between salmonids and these other teleosts.

Specific subtypes of GABAA receptors mediate phasic and tonic forms of inhibition in hippocampal pyramidal neurons

The main inhibitory neurotransmitter in the mammalian brain, GABA, mediates multiple forms of inhibitory signals, such as fast and slow inhibitory postsynaptic currents and tonic inhibition, by activating a diverse family of ionotropic GABA(A) receptors (GABA(A)Rs). Here, we studied whether distinct GABA(A)R subtypes mediate these various forms of inhibition using as approach mice carrying a point mutation in the alpha-subunit rendering individual GABA(A)R subtypes insensitive to diazepam without altering their GABA sensitivity and expression of receptors. Whole cell patch-clamp recordings were performed in hippocampal pyramidal cells from single, double, and triple mutant mice. Comparing diazepam effects in knock-in and wild-type mice allowed determining the contribution of alpha1, alpha2, alpha3, and alpha5 subunits containing GABA(A)Rs to phasic and tonic forms of inhibition. Fast phasic currents were mediated by synaptic alpha2-GABA(A)Rs on the soma and by synaptic alpha1-GABA(A)Rs on the dendrites. No contribution of alpha3- or alpha5-GABA(A)Rs was detectable. Slow phasic currents were produced by both synaptic and perisynaptic GABA(A)Rs, judged by their strong sensitivity to blockade of GABA reuptake. In the CA1 area, but not in the subiculum, perisynaptic alpha5-GABA(A)Rs contributed to slow phasic currents. In the CA1 area, the diazepam-sensitive component of tonic inhibition also involved activation of alpha5-GABA(A)Rs and slow phasic and tonic signals shared overlapping pools of receptors. These results show that the major forms of inhibitory neurotransmission in hippocampal pyramidal cells are mediated by distinct GABA(A)Rs subtypes.

Proenkephalin A and the gamma-aminobutyric acid A receptor pi subunit: expression, localization, and dynamic changes in human secretory endometrium

OBJECTIVE

To compare mRNA and protein levels of proenkephalin A (PEA) and gamma-aminobutyric acid A receptor pi subunit (piGABA-R) in human secretory endometrium before and during receptivity and to determine the cell phenotypes where they are expressed.

DESIGN

Prospective and observational, comparing prereceptive vs. receptive stages of secretory endometrium within the same nonconceptional menstrual cycle.

SETTING

University and non-governmental organization (NGO)-based academic and clinical-research facilities.

PATIENT(S)

Seven healthy, multiparous, surgically sterilized women with spontaneous regular menstrual cycles.

INTERVENTION(S)

Endometrial biopsies were obtained on LH+3 and LH+7 within the same cycle.

MAIN OUTCOME MEASURE(S)

Levels of PEA and piGABA-R mRNA were determined by real-time PCR, and protein presence, by immunofluorescence.

RESULT(S)

The mRNA level of PEA fell, whereas that of piGABA-R increased, during endometrial receptivity. Positive immunostaining of PEA was found in the luminal and glandular epithelium, whereas that of piGABA-R was in luminal epithelium and stromal cells.

CONCLUSION(S)

The discrete cell-phenotype localization and timing of the changes in the level of PEA and of piGABA-R mRNA and protein suggest an important role for these molecules in switching the human endometrium from a refractory to a receptive state.

Characterization of the interaction of indiplon, a novel pyrazolopyrimidine sedative-hypnotic, with the GABAA receptor

Clinically used benzodiazepine and nonbenzodiazepine sedative-hypnotic agents for the treatment of insomnia produce their therapeutic effects through allosteric enhancement of the effects of the inhibitory neurotransmitter GABA at the GABA(A) receptor. Indiplon is a novel pyrazolopyrimidine sedative-hypnotic agent, currently in development for insomnia. Using radioligand binding studies, indiplon inhibited the binding of [(3)H]Ro 15-1788 (flumazenil) to rat cerebellar and cerebral cortex membranes with high affinity (K(i) values of 0.55 and 0.45 nM, respectively). [(3)H]Indiplon binding to rat cerebellar and cerebral cortex membranes was reversible and of high affinity, with K(D) values of 1.01 and 0.45 nM, respectively, with a pharmacological specificity consistent with preferential labeling of GABA(A) receptors containing alpha1 subunits. In "GABA shift" experiments and in measurements of GABA-induced chloride conductance in rat cortical neurons in culture, indiplon behaved as an efficacious potentiator of GABA(A) receptor function. In both the radioligand binding and electrophysiological experiments, indiplon had a higher affinity than zolpidem or zaleplon. These in vitro properties are consistent with the in vivo properties of indiplon as an effective sedative-hypnotic acting through allosteric potentiation of the GABA(A) receptor.

Synthesis of beta- and gamma-carbolines by the palladium-catalyzed iminoannulation of alkynes

A variety of substituted beta- and gamma-carbolines have been prepared in moderate to excellent yields by the palladium-catalyzed annulation of internal and terminal acetylenes by the tert-butylimines of N-substituted 3-iodoindole-2-carboxaldehydes and 2-haloindole-3-carboxaldehydes, respectively. This annulation chemistry is effective for a wide range of alkynes, including aryl-, alkyl-, hydroxymethyl-, ethoxycarbonyl-, and trimethylsilyl-substituted alkynes. When an unsymmetrical internal alkyne is employed, this method generally gives two regioisomers. When a terminal alkyne is employed, only one regioisomer has been isolated. This palladium-catalyzed annulation chemistry has also been successfully applied to the synthesis of two biologically interesting beta-carboline alkaloids, ZK93423 and abecarnil (ZK112119).

Comparison of the effects of zaleplon, zolpidem, and triazolam at various GABA(A) receptor subtypes

The pyrazolopyrimidine zaleplon is a hypnotic agent that acts at the benzodiazepine recognition site of GABA(A) receptors. Zaleplon, like the hypnotic agent zolpidem but unlike classical benzodiazepines, exhibits preferential affinity for type I benzodiazepine (BZ(1)/omega(1)) receptors in binding assays. The modulatory action of zaleplon at GABA(A) receptors has now been compared with those of zolpidem and the triazolobenzodiazepine triazolam. Zaleplon potentiated GABA-evoked Cl(-) currents in Xenopus oocytes expressing human GABA(A) receptor subunits with a potency that was higher at alpha1beta2gamma2 receptors than at alpha2- or alpha3-containing receptors. Zolpidem, but not triazolam, also exhibited selectivity for alpha1-containing receptors. However, the potency of zaleplon at these various receptors was one-third to one-half that of zolpidem. Zaleplon and zolpidem also differed in their actions at receptors containing the alpha5 or gamma3 subunit. Zaleplon, zolpidem, and triazolam exhibited similar patterns of efficacy among the different receptor subtypes. The affinities of zaleplon for [(3)H]flunitrazepam or t-[(35)S]butylbicyclophosphorothionate ([(35)S]TBPS) binding sites in rat brain membranes were lower than those of zolpidem or triazolam. Furthermore, zaleplon, unlike zolpidem, exhibited virtually no affinity for the peripheral type of benzodiazepine receptor.

Functional consequences of the loss of high affinity agonist binding to gamma-aminobutyric acid type A receptors. Implications for receptor desensitization

We reported previously that tyrosine 62 of the beta2 subunit of the gamma-aminobutyric acid, type A (GABA(A)) receptor is an important determinant of high affinity agonist binding and that recombinant alpha1beta2gamma2(L) receptors carrying the Y62S mutation lack measurable high affinity sites for [3H]muscimol. We have now examined the effects of disrupting these sites on the macroscopic desensitization properties of receptors expressed in Xenopus oocytes. Desensitization was measured by the ability of low concentrations of bath-perfused agonist to reduce the current responses elicited by subsequent challenges with saturating concentrations of GABA. Wild-type receptors were desensitized by pre-perfused muscimol with an IC50 approximately 0.7 microm, which correlates well with the lower affinity sites for this agonist that are measured in direct binding studies. Receptors carrying the beta2 Y62S and Y62F mutations desensitized at slightly higher (2-7-fold) agonist concentrations. However, at low perfusate concentrations, the Y62S-containing receptor recovered from the desensitized state even in the continued presence of agonist. The characteristics of desensitization in the wild-type and mutant receptors lead us to suggest that the major role of the high affinity agonist-binding site(s) of the GABA(A) receptor is not to induce desensitization but rather to stabilize the desensitized state once it has been formed.

Increase in expression of the GABA(A) receptor alpha(4) subunit gene induced by withdrawal of, but not by long-term treatment with, benzodiazepine full or partial agonists

The effects of long-term exposure to, and subsequent withdrawal of, diazepam or imidazenil (full and partial agonists of the benzodiazepine receptor, respectively) on the abundance of GABA(A) receptor subunit mRNAs and peptides were investigated in rat cerebellar granule cells in culture. Exposure of cells to 10 microM diazepam for 5 days significantly reduced the amounts of alpha(1) and gamma(2) subunit mRNAs, and had no effect on the amount of alpha(4) mRNA. These effects were accompanied by a decrease in the levels of alpha(1) and gamma(2) protein and by a reduction in the efficacy of diazepam with regard to potentiation of GABA-evoked Cl- current. Similar long-term treatment with 10 microM imidazenil significantly reduced the abundance of only the gamma(2)S subunit mRNA and had no effect on GABA(A) receptor function. Withdrawal of diazepam or imidazenil induced a marked increase in the amount of alpha(4) mRNA; withdrawal of imidazenil also reduced the amounts of alpha(1) and gamma(2) mRNAs. In addition, withdrawal of diazepam or imidazenil was associated with a reduced ability of diazepam to potentiate GABA action. These data give new insights into the different molecular events related to GABA(A) receptor gene expression and function produced by chronic treatment and withdrawal of benzodiazepines with full or partial agonist properties.

Characterization of novel benzodiazepine ligands in Spodotera frugiperda (Sf-9) insect cells

The goals of the present work were to characterize the binding profile of nine benzodiazepine ligands in Spodotera frugiperda (Sf-9) insect cells expressing specific gamma aminobutyric acid (A) (GABA(A)) receptor subunit combinations and compare the affinities to those for the receptors in the rat cerebellum. Three recombinant baculovirus constructs, each harboring a different GABA(A) receptor subunit, were introduced into insect cells by simultaneous infection. Saturation and competition binding assays were carried out in membranes from Sf-9 cells infected with either alpha1beta2gamma2 or alpha6beta2gamma2 subunit combinations. The affinities of the ligands to the alpha1beta2gamma2 or alpha6beta2gamma2 receptors expressed in Sf-9 cells were similar to the affinities previously determined for the alpha1 or alpha6 subunit-containing GABA(A) receptors in the rat cerebellum, respectively, thus confirming the previously assigned receptor types in the cerebellum.

Effects of chronic salicylate on GABAergic activity in rat inferior colliculus

It is well accepted that salicylate ototoxicity results in reversible tinnitus in humans. Salicylate-induced tinnitus may be an example of plasticity of the central auditory system and could potentially serve as a model to further understand mechanisms of tinnitus generation. This study examined levels of glutamic acid decarboxylase (GAD) and the binding characteristics of the GABA(A) receptor in auditory brainstem structures of Long-Evans rats chronically treated with salicylate. Western blotting revealed a significant 63% (P<0.008) elevation of GAD levels in the inferior colliculus (IC) of salicylate-treated subjects. This occurred in subjects demonstrating behavioral evidence of tinnitus. Muscimol saturation analysis was indicative of a salicylate-related increase in receptor affinity. Linear regression of [(3)H]muscimol saturation analysis data revealed a significant (P<0.05) reduction in K(d) values in whole IC (-48%), as well as in the central nucleus of IC (CIC, -58%) and combined external and dorsal cortex of IC (E/DCIC, -46%). The number of GABA(A) binding sites (B(max)) were also significantly (P<0.05) decreased. These changes were observed only in central auditory structures. This suggests that GAD expression and GABA(A) receptor binding characteristics may be altered with chronic exposure to sodium salicylate and these changes may represent aberrant plasticity clinically experienced as tinnitus.

Characterization of benzodiazepine receptors in the cerebellum

1. The goals of the work reported here were to further characterize benzodiazepine/GABA(A) (BDZR) receptor heterogeneity in the cerebellum and to measure the affinities and selectivities of structurally diverse benzodiazepines at each site identified. 2. Five chemical families were included in these studies. These were 1,4-benzodiazepines (flunitrazepam), imidazobenzodiazepines (RO15-1788 and RO15-4513 and RO16-6028), beta-carbolines (Abecarnil) and pyrazoloquinolines (CGS 8216, CGS 9895 and CGS 9896). 3. Saturation and competition binding assays were combined with powerful data analysis software developed in our laboratory. Among the capabilities of this software is the identification of multiple binding sites for a cold ligand using a non-selective labeled ligand that binds with equal, but high, affinity to all the binding sites 4. Saturation binding assays using either [3H]-RO15-1788 or [3H]-RO15-4513 revealed only one apparent binding site, with a higher affinity for RO15-4513 than for RO15-1788. However, using [3H]-RO15-4513 for the competition binding studies in the cerebellum, together with our data analysis software, led to the identification of two distinct binding sites with equal densities for the diverse benzodiazepines studied. 5. In rat cerebellum one of the sites identified corresponds to GABA(A) receptors exhibiting alpha1 subunit pharmacology and the other to GABA(A) receptors exhibiting alpha6 subunit pharmacology. In general, the diverse families of BDZR ligands studied had much lower affinities for the alpha6 containing receptors.

Unraveling the identity of benzodiazepine binding sites in rat hipppocampus and olfactory bulb

The goals of the work reported here were (i) to identify distinct GABA(A)/benzodiazepine receptors in the rat hippocampus and olfactory bulb using receptor binding assays, and (ii) to determine the affinities and selectivities of benzodiazepine receptor ligands from structurally diverse chemical families at each site identified. These studies were aided by the use of software AFFINITY ANALYSIS SYSTEM, developed in our laboratory for analysis of receptor binding data that allows the determination of receptor heterogeneity using non-selective radioligands. Saturation binding assays using [3H]RO15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1, 5-a]-[1,4]benzodiazepine-3-carboxylate) revealed two binding sites in each of these two tissues. The higher affinity site corresponds to alpha(5) subunit-containing GABA(A) receptor and the lower affinity site to a combination of alpha(1), alpha(2), and alpha(3) subunit-containing receptors. These results should be useful in the challenging task of identifying the various functional GABA(A) receptors in the central nervous system, and in providing a link between receptor affinities and in vivo activities of the GABA(A)/benzodiazepine receptor ligands studied.

Development of a 3D pharmacophore for nonspecific ligand recognition of alpha1, alpha2, alpha3, alpha5, and alpha6 containing GABA(A)/benzodiazepine receptors

Transfected cells containing GABA(A) benzodiazepine receptors (BDZRs) have been utilized to systematically determine the affinity of ligands at alpha1, alpha2, alpha3, alpha5 and alpha6 subtypes in combination with beta2 and gamma2. All but a few of the ligands thus far studied have relatively high affinities for each of these alpha subtype receptors. Thus, these ligands must contain common stereochemical properties favorable for recognition by each of the subtype combinations. In the present work, such a common three-dimensional (3D) pharmacophore for recognition of alpha1, alpha2, alpha3, alpha5 and alpha6 containing GABA(A)/BDZRs types of receptors has been developed and assessed, using as a database receptor affinities measured in transfected cells for 27 diverse compounds. The 3D-recognition pharmacophore developed consists of three proton accepting groups, a hydrophobic group, and the centroid of an aromatic ring found in a common geometric arrangement in the 19 nonselective ligands used. Three tests were made to assess this pharmacophore: (i) Four low affinity compounds were used as negative controls, (ii) Four high affinity compounds, excluded from the pharmacophore development, were used as compounds for pharmacophore validation, (iii) The 3D pharmacophore was used to search 3D databases. The results of each of these types of assessments provided robust validation of the 3D pharmacophore. This 3D pharmacophore can now be used to discover novel nonselective ligands that could be activation selective at different behavioral end points. Additionally, it may serve as a guide in the design of more selective ligands, by determining if candidate ligands proposed for synthesis conform to this pharmacophore and selecting those that do not for further experimental assessment.

Pharmacology of recombinant human GABA(A) receptor subtypes measured using a novel pH-based high-throughput functional efficacy assay

To facilitate the discovery of novel compounds that modulate human GABA(A) receptor function, we have developed a high throughput functional assay using a fluorescence imaging system. L(tk-) cells expressing combinations of human GABA(A) receptor subunits were incubated with the pH-sensitive dye 2',7'bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein, then washed and placed in a 96-well real-time fluorescence plate reader. In buffer adjusted to pH 6.9 there was a robust and persisting acidification response to addition of GABA, which was antagonised by the GABA(A) receptor antagonist bicuculline. The concentration-response relationship for GABA was modulated by allosteric ligands, including benzodiazepine (BZ) site agonists and inverse agonists. The effects of BZ site ligands on the pH response to GABA for receptors containing alpha1beta3gamma2, alpha3beta3gamma2 or alpha5beta3gamma2 subunits were well correlated with results from electrophysiological studies on the same receptor subunit combinations expressed in Xenopus oocytes. Most modulatory compounds tested were found to be relatively unselective across the three subunit combinations tested; however, some showed subtype-dependent efficacy, such as diazepam, which had highest agonist effects on the alpha3beta3gamma2 subtype, substantial but lesser agonism on alpha1beta3gamma2 and still substantial but the least agonism on alpha5beta3gamma2. This indicates that the alpha subunit within the recombinant receptor expressed in L(tk-) cells can affect the efficacy of the response to some BZ compounds. Inhibitors of Na(+)/Cl(-) cotransport, anion/anion exchange and the gastric type of H(+)/K(+) ATPase potently inhibited GABA-evoked acidification, indicating that multiple transporters are involved in the GABA-evoked pH change. This novel fluorescence-based high throughput functional assay allows the rapid characterization of allosteric ligands acting on human GABA(A) receptors.

GABAA receptor subunit composition and functional properties of Cl- channels with differential sensitivity to zolpidem in embryonic rat hippocampal cells

Using flow cytometry in conjunction with a voltage-sensitive fluorescent indicator dye (oxonol), we have identified and separated embryonic hippocampal cells according to the sensitivity of their functionally expressed GABAA receptors to zolpidem. Immunocytochemical and RT-PCR analysis of sorted zolpidem-sensitive (ZS) and zolpidem-insensitive (ZI) subpopulations identified ZS cells as postmitotic, differentiating neurons expressing alpha2, alpha4, alpha5, beta1, beta2, beta3, gamma1, gamma2, and gamma3 GABAA receptor subunits, whereas the ZI cells were neuroepithelial cells or newly postmitotic neurons, expressing predominantly alpha4, alpha5, beta1, and gamma2 subunits. Fluctuation analyses of macroscopic Cl- currents evoked by GABA revealed three kinetic components of GABAA receptor/Cl- channel activity in both subpopulations. We focused our study on ZI cells, which exhibited a limited number of subunits and functional channels, to directly correlate subunit composition with channel properties. Biophysical analyses of GABA-activated Cl- currents in ZI cells revealed two types of receptor-coupled channel properties: one comprising short-lasting openings, high affinity for GABA, and low sensitivity to diazepam, and the other with long-lasting openings, low affinity for GABA, and high sensitivity to diazepam. Both types of channel activity were found in the same cell. Channel kinetics were well modeled by fitting dwell time distributions to biliganded activation and included two open and five closed states. We propose that short- and long-lasting openings correspond to GABAA receptor/Cl- channels containing alpha4beta1gamma2 and alpha5beta1gamma2 subunits, respectively.

Diazepam enhancement of GABA-gated currents in binary and ternary GABAA receptors: relationship to benzodiazepine binding site density

Although the predominant GABAA receptor isoform in the adult rodent central nervous system is a ternary complex composed of alpha 1 beta 2/3 gamma 2-subunits, small populations of binary receptors lacking beta-subunits (i.e., complexes containing alpha gamma-subunits) have also been identified. When expressed in HEK 293 cells, recombinant GABAA receptors composed of either alpha 1 beta 2/3 gamma 2- or alpha 1 gamma 2-subunits form benzodiazepine-responsive, GABA-gated chloride channels. The objective of this study was to compare the ability of a prototypic benzodiazepine (diazepam) to augment GABA-gated chloride currents in these binary and ternary receptor isoforms. The potency of GABA was characteristically increased by diazepam (1 microM) in both receptor isoforms, but this increase was significantly greater (p < 0.05) in receptors composed of alpha 1 beta 2 gamma 2-subunits (approximately five- to sixfold) compared to alpha 1 gamma 2-subunits (approximately 2.2-fold). At GABA concentrations approximating its EC50 value (5 microM), the greater augmentation observed in ternary receptors was attributable to a higher efficacy of diazepam. Radioligand binding studies revealed that the Bmax of [3H]flunitrazepam was increased approximately 1.8- and 3.5-fold in cells expressing alpha 1 beta 2 gamma 2- and alpha 1 beta 3 gamma 2-subunits, respectively, compared to cells expressing alpha 1 gamma 2-subunits. A similar increase (approximately 3.8-fold) in the Bmax of [3H]Ro 15-4513 was observed in HEK 293 cells transiently transfected with cDNAs encoding alpha 6 beta 3 gamma 2-compared to alpha 6 gamma 2-subunits. The Kd values of these radioligands were not different in binary and ternary receptor isoforms. It is hypothesized that the greater efficacy of diazepam in alpha 1 beta 2 gamma 2 compared to alpha 1 gamma 2 GABAA receptors results from the higher benzodiazepine binding site density produced by the formation of a ternary complex.

Chronic zolpidem treatment alters GABA(A) receptor mRNA levels in the rat cortex

The effect of chronic zolpidem treatment on the steady-state levels of gamma-aminobutyric acidA alpha1-6, beta1-3 and gamma1-3 subunit mRNAs in rat cortex has been investigated. Male Sprague-Dawley rats were injected once daily, for 7 or 14 days, with 15 mg/kg of zolpidem in sesame oil vehicle. The levels of the alpha4 and beta1 subunit mRNAs were significantly increased after 7 days of treatment and the level of alpha1 subunit mRNA was significantly decreased after 14 days of treatment, as determined by solution hybridization. These results are compared to the previously determined effects of an equivalent schedule of treatment with diazepam.