High affinity ligands for 'diazepam-insensitive' benzodiazepine receptors

Domino-like transient dynamics at seizure onset in epilepsy

The International League Against Epilepsy (ILAE) groups seizures into "focal", "generalized" and "unknown" based on whether the seizure onset is confined to a brain region in one hemisphere, arises in several brain region simultaneously, or is not known, respectively. This separation fails to account for the rich diversity of clinically and experimentally observed spatiotemporal patterns of seizure onset and even less so for the properties of the brain networks generating them. We consider three different patterns of domino-like seizure onset in Idiopathic Generalized Epilepsy (IGE) and present a novel approach to classification of seizures. To understand how these patterns are generated on networks requires understanding of the relationship between intrinsic node dynamics and coupling between nodes in the presence of noise, which currently is unknown. We investigate this interplay here in the framework of domino-like recruitment across a network. In particular, we use a phenomenological model of seizure onset with heterogeneous coupling and node properties, and show that in combination they generate a range of domino-like onset patterns observed in the IGE seizures. We further explore the individual contribution of heterogeneous node dynamics and coupling by interpreting in-vitro experimental data in which the speed of onset can be chemically modulated. This work contributes to a better understanding of possible drivers for the spatiotemporal patterns observed at seizure onset and may ultimately contribute to a more personalized approach to classification of seizure types in clinical practice.

Initiation and slow propagation of epileptiform activity from ventral to dorsal medial entorhinal cortex is constrained by an inhibitory gradient

KEY POINTS

The medial entorhinal cortex (mEC) has an important role in initiation and propagation of seizure activity. Several anatomical relationships exist in neurophysiological properties of mEC neurons; however, in the context of hyperexcitability, previous studies often considered it as a homogeneous structure. Using multi-site extracellular recording techniques, ictal-like activity was observed along the dorso-ventral axis of the mEC in vitro in response to various ictogenic stimuli. This originated predominantly from ventral areas, spreading to dorsal mEC with a surprisingly slow velocity. Modulation of inhibitory tone was capable of changing the slope of ictal initiation, suggesting seizure propagation behaviours are highly dependent on levels of GABAergic function in this region. A distinct disinhibition model also showed, in the absence of inhibition, a prevalence for interictal-like initiation in ventral mEC, reflecting the intrinsic differences in mEC neurons. These findings suggest the ventral mEC is more prone to hyperexcitable discharge than the dorsal mEC, which may be relevant under pathological conditions.

ABSTRACT

The medial entorhinal cortex (mEC) has an important role in the generation and propagation of seizure activity. The organization of the mEC is such that a number of dorso-ventral relationships exist in neurophysiological properties of neurons. These range from intrinsic and synaptic properties to density of inhibitory connectivity. We examined the influence of these gradients on generation and propagation of epileptiform activity in the mEC. Using a 16-shank silicon probe array to record along the dorso-ventral axis of the mEC in vitro, we found 4-aminopyridine application produces ictal-like activity originating predominantly in ventral areas. This activity spreads to dorsal mEC at a surprisingly slow velocity (138 μm s^-1 ), while cross-site interictal-like activity appeared relatively synchronous. We propose that ictal propagation is constrained by differential levels of GABAergic control since increasing (diazepam) or decreasing (Ro19-4603) GABA_A receptor activation, respectively, reduced or increased the slope of ictal initiation. The observation that ictal activity is predominately generated in ventral mEC was replicated using a separate 0-Mg²⁺ model of epileptiform activity in vitro. By using a distinct disinhibition model (co-application of kainate and picrotoxin) we show that additional physiological features (for example intrinsic properties of mEC neurons) still produce a prevalence for interictal-like initiation in ventral mEC. These findings suggest that the ventral mEC is more likely to initiate hyperexcitable discharges than the dorsal mEC, and that seizure propagation is highly dependent on levels of GABAergic expression across the mEC.

Search for α3β₂/₃γ2 subtype selective ligands that are stable on human liver microsomes

Selective modulation of specific benzodiazepine receptor (BzR) gamma amino butyric acid-A (GABA(A)) receptor ion channels has been identified as an important method for separating out the variety of pharmacological effects elicited by BzR-related drugs. Importantly, it has been demonstrated that both α2β(2/3)γ2 (α2BzR) and α3BzR (and/or α2/α3) BzR subtype selective ligands exhibit anxiolytic effects with little or no sedation. Previously we have identified several such ligands; however, three of our parent ligands exhibited significant metabolic liability in rodents in the form of a labile ester group. Here eight analogs are reported which were designed to circumvent this liability by utilizing a rational replacement of the ester moiety based on medicinal chemistry precedents. In a metabolic stability study using human liver microsomes, four compounds were found to undergo slower metabolic transformation, as compared to their corresponding ester analogs. These compounds were also evaluated in in vitro efficacy assays. Additionally, bioisostere 11 was evaluated in a rodent model of anxiety. It exhibited anxiolytic activity at doses of 10 and 100mg/kg and was devoid of sedative properties.

Pharmacophore models for GABA(A) modulators: implications in CNS drug discovery

IMPORTANCE OF THE FIELD

GABA(A) ion channel is a validated drug target, implicated in the pathophysiology of various neurological and psychiatric disorders. Structural investigations on GABA(A) are currently precluded in the absence of experimentally resolved structure. Pharmacophore modeling circumvents such issues and proves to be a powerful and successful method in drug discovery.

AREAS COVERED IN THIS REVIEW

The present reviews encompass pharmacophoric models available in the literature for the orthosteric GABA and the allosteric benzodiazepine binding site. Success stories from these simplistic pharmacophore models in scaffold hopping and strategic lead optimization have been highlighted. Recent advances in pharmacophore modeling that can leverage CNS drug discovery programs and deliver astounding results have been reviewed.

WHAT THE READER WILL GAIN

Readers are bound to gain a comprehensive insight on different computational techniques used by different groups to arrive at simple, yet sophisticated pharmacophore models. In the absence of experimentally unresolved active site geometry of GABA(A), these models will provide the reader an opportunity to translate these pharmacophoric features to the microscopic phenomenon of supramolecular ligand interaction.

TAKE HOME MESSAGE

Pharmacophore modeling has now evolved as a mainstay approach for lead generation and optimization in drug discovery programs. Of late, many advances in pharmacophore perception have emerged. Such advancements should be used to confront activity profiling and early stage risk assessment in a high-throughput fashion. Extending such technologies has the potential not only to reduce time and cost, but also to prevent late stage attrition in drug discovery.

An Overview of the CNS-Pharmacodynamic Profiles of Nonselective and Selective GABA Agonists

Various α_2,3 subtype selective partial GABA-A agonists are in development to treat anxiety disorders. These compounds are expected to be anxiolytic with fewer undesirable side effects, compared to nonselective GABA-A agonists like benzodiazepines. Several α_2,3 subtype selective and nonselective GABA-A agonists have been examined in healthy volunteers, using a battery addressing different brain domains. Data from five placebo-controlled double-blind studies were pooled. Lorazepam 2 mg was the comparator in three studies. Three α_2,3-selective GABAA agonists (i.e., TPA023, TPACMP2, SL65.1498), one α₁-selective GABAA agonists (zolpidem), and another full agonist (alprazolam) were examined. Pharmacological selectivity was assessed by determination of regression lines for the change from baseline of saccadic-peak-velocity- (ΔSPV-) relative effect, relative to changes in different pharmacodynamic endpoints (ΔPD). SPV was chosen for its sensitivity to the anxiolysis of benzodiazepines. Slopes of the ΔSPV-ΔPD relations were consistently lower with the α_2,3 selective GABA-A agonists than with lorazepam, indicating that their PD effects are less than their SPV-effects. The ΔSPV-ΔPD relations of lorazepam were comparable to alprazolam. Zolpidem showed relatively higher impairments in ΔPD relative to ΔSPV, but did not significantly differ from lorazepam. These PD results support the pharmacological selectivity of the α_2,3-selective GABA-A agonists, implying an improved therapeutic window.

γ-Hydroxybutyrate and the GABAergic footprint: a metabolomic approach to unpicking the actions of GHB

Gamma-hydroxybutyrate is found both naturally in the brain and self-administered as a drug of abuse. It has been reported to act at endogenous γ-hydroxybutyrate (GHB) receptors and GABA(B) receptors [GABA(B)R], and may also be metabolized to GABA. Here, the metabolic fingerprints of a range of concentrations of GHB were measured in brain cortical tissue slices and compared with those of ligands active at GHB and GABA-R using principal components analysis (PCA) to identify sites of GHB activity. Low concentrations of GHB (1.0 μM) produced fingerprints similar to those of ligands active at GHB receptors and α4-containing GABA(A)R. A total of 10 μM GHB clustered proximate to mainstream GABAergic synapse ligands, such as 1.0 μM baclofen, a GABA(B)R agonist. Higher concentrations of GHB (30 μM) clustered with GABA(C)R agonists and the metabolic responses induced by blockade of the GABA transporter-1 (GAT1). The metabolic responses induced by 60 and 100 μM GHB were mimicked by simultaneous blockade of GAT1 and GAT3, addition of low concentrations of GABA(C)R antagonists, or increasing cytoplasmic GABA concentrations by incubation with the GABA transaminase inhibitor vigabatrin. These data suggest that at concentrations > 30 μM, GHB may be active via metabolism to GABA, which is then acting upon an unidentified GABAergic master switch receptor (possibly a high-affinity extrasynaptic receptor), or GHB may itself be acting directly on an extrasynaptic GABA-R, capable of turning off large numbers of cells. These results offer an explanation for the steep dose-response curve of GHB seen in vivo, and suggest potential target receptors for further investigation.

Structural requirements for eszopiclone and zolpidem binding to the gamma-aminobutyric acid type-A (GABAA) receptor are different

The sleep-aids zolpidem and eszopiclone exert their effects by binding to and modulating gamma-aminobutyric acid type-A receptors (GABA(A)Rs), but little is known about the structural requirements for their actions. We made 24 cysteine mutations in the benzodiazepine (BZD) binding site of alpha(1)beta(2)gamma(2) GABA(A)Rs and measured zolpidem, eszopiclone, and BZD-site antagonist binding. Mutations in gamma(2)loop D and alpha(1)loops A and B altered the affinity of all ligands tested, indicating that these loops are important for BZD pocket structural integrity. In contrast, gamma(2)loop E and alpha(1)loop C mutations differentially affected ligand affinity, suggesting that these loops are important for ligand selectivity. In agreement with our mutagenesis data, eszopiclone docking yielded a single model stabilized by several hydrogen bonds. Zolpidem docking yielded three equally populated orientations with few polar interactions, suggesting that unlike eszopiclone, zolpidem relies more on shape recognition of the binding pocket than on specific residue interactions and may explain why zolpidem is highly alpha(1)- and gamma(2)-subunit selective.

A study of the structure-activity relationship of GABA(A)-benzodiazepine receptor bivalent ligands by conformational analysis with low temperature NMR and X-ray analysis

The stable conformations of GABA(A)-benzodiazepine receptor bivalent ligands were determined by low temperature NMR spectroscopy and confirmed by single crystal X-ray analysis. The stable conformations in solution correlated well with those in the solid state. The linear conformation was important for these dimers to access the binding site and exhibit potent in vitro affinity and was illustrated for alpha5 subtype selective ligands. Bivalent ligands with an oxygen-containing linker folded back upon themselves both in solution and the solid state. Dimers which are folded do not bind to Bz receptors.

Selective labelling of diazepam-insensitive GABAA receptors in vivo using [3H]Ro 15-4513

Classical benzodiazepines (BZs), such as diazepam, bind to GABAA receptors containing alpha1, alpha2, alpha3 or alpha5 subunits that are therefore described as diazepam-sensitive (DS) receptors. However, the corresponding binding site of GABAA receptors containing either an alpha4 or alpha6 subunit do not bind the classical BZs and are therefore diazepam-insensitive (DIS) receptors; a difference attributable to a single amino acid (histidine in alpha1, alpha2, alpha3 and alpha5 subunits and arginine in alpha4 and alpha6). Unlike classical BZs, the imidazobenzodiazepines Ro 15-4513 and bretazenil bind to both DS and DIS populations of GABAA receptors. In the present study, an in vivo assay was developed using lorazepam to fully occupy DS receptors such that [3H]Ro 15-4513 was then only able to bind to DIS receptors. When dosed i.v., [3H]Ro 15-4513 rapidly entered and was cleared from the brain, with approximately 70% of brain radioactivity being membrane-bound. Essentially all membrane binding to DS+DIS receptors could be displaced by unlabelled Ro 15-4513 or bretazenil, with respective ID50 values of 0.35 and 1.2 mg kg(-1). A dose of 30 mg kg(-1) lorazepam was used to block all DS receptors in a [3H]Ro 15-1788 in vivo binding assay. When predosed in a [3H]Ro 15-4513 binding assay, lorazepam blocked [3H]Ro 15-4513 binding to DS receptors, with the remaining binding to DIS receptors accounting for 5 and 23% of the total (DS plus DIS) receptors in the forebrain and cerebellum, respectively. The in vivo binding of [3H]Ro 15-4513 to DIS receptors in the presence of lorazepam was confirmed using alpha1H101R knock-in mice, in which alpha1-containing GABAA receptors are rendered diazepam insensitive by mutation of the histidine that confers diazepam sensitivity to arginine. In these mice, and in the presence of lorazepam, there was an increase of in vivo [3H]Ro 15-4513 binding in the forebrain and cerebellum from 4 and 15% to 36 and 59% of the total (i.e. DS plus DIS) [3H]Ro 15-4513 binding observed in the absence of lorazepam.

Pseudoreceptor Models and 3D-QSAR for Imidazobenzodiazepines at GABAA/BzR Subtypes αxβ3γ2 [x = 1−3, 5, and 6] via Flexible Atom Receptor Model

Since benzodiazepines have been used widely in the treatment of anxiety, sleeplessness, and epilepsy, the receptor sites for the benzodiazepine are of prime importance. Quantitative structure-activity relationship (QSAR) studies and receptor modeling via Flexible Atom Receptor Model (FLARM) for the binding affinities of a series of imidazobenzodiazepines at five recombinant receptor subtypes were carried out successfully. The 3D-QSAR models for all five receptor subtypes were examined by a set of test set and demonstrated their high predictability for affinities of imidazobenzodiazepines at five receptor subtypes. The pseudoreceptors yielded by FLARM were compared to the united pharmacophore/receptor model. The result shows that two hydrogen bonds and other regions in the united pharmacophore/receptor model are presented in the pseudoreceptors, which demonstrates the receptor modeling capability of FLARM. The models and pseudoreceptors can help design high affinity ligands on the GABA(A)/BZ receptor and understand the GABA(A) receptor.

The reinforcing properties of alcohol are mediated by GABA(A1) receptors in the ventral pallidum

It has been hypothesized that alcohol addiction is mediated, at least in part, by specific gamma-aminobutyric acid(A) (GABA(A)) receptors within the ventral pallidum (VP). Among the potential GABA(A) receptor isoforms regulating alcohol-seeking behaviors within the VP, the GABA(A) alpha1 receptor subtype (GABA(A1)) appears pre-eminent. In the present study, we developed beta-carboline-3-carboxylate-t-butyl ester (betaCCt), a mixed agonist-antagonist benzodiazepine (BDZ) site ligand, with binding selectivity at the A1 receptor to explore the functional role of VP(A1) receptors in the euphoric properties of alcohol. The in vivo actions of betaCCt were then determined following microinfusion into the VP, a novel alcohol reward substrate that primarily expresses the A1 receptor. In two selectively bred rodent models of chronic alcohol drinking (HAD-1, P rats), bilateral microinfusion of betaCCt (0.5-40 microg) produced marked reductions in alcohol-reinforced behaviors. Further, VP infusions of betaCCt exhibited both neuroanatomical and reinforcer specificity. Thus, no effects on alcohol-reinforced behaviors were observed following infusion in the nucleus accumbens (NACC)/caudate putamen (CPu), or on response maintained by saccharin. Parenteral-administered betaCCt (1-40 mg/kg) was equally effective and selective in reducing alcohol-reinforced behaviors in P and HAD-1 rats. Additional tests of locomotor activity revealed that betaCCt reversed the locomotor sedation produced by both chlordiazepoxide (10 mg/kg) and EtOH (1.25 g/kg), but was devoid of intrinsic effects when given alone. Studies in recombinant receptors expressed in Xenopus oocytes revealed that betaCCt acted as a low-efficacy partial agonist at alpha3beta3gamma2 and alpha4beta3gamma2 receptors and as a low-efficacy inverse agonist at alpha1beta3gamma2, alpha2beta3gamma2, and alpha5beta3gamma2 receptors. The present study indicates that betaCCt is capable of antagonizing the reinforcing and the sedative properties of alcohol. These anti-alcohol properties of betaCCt are primarily mediated via the GABA(A1) receptor. betaCCt may represent a prototype of a pharmacotherapeutic agent to effectively reduce alcohol drinking behavior in human alcoholics.

Metabolite analysis of [11C]Ro15-4513 in mice, rats, monkeys and humans

We performed in vitro and in vivo assays of the metabolism of [(11)C]Ro15-4513 over time in the plasma of mice, rats, monkeys and humans, using a radio-HPLC equipped with a sensitive positron detector, in order to compare the metabolic rates of the radiopharmaceutical agent among the different animal species and to establish a highly sensitive analytical method for the radiotracer agent. We also examined the metabolism of [(11)C]Ro15-4513 in the brain tissue of mice and rats. The analytical method used in this study permitted detection of even extremely low levels of radioactivity (approximately 5,000 dpm). In vitro experiments revealed that [(11)C]Ro15-4513 in the blood was metabolized to hydrolysate [(11)C]A. The species were classified in descending order of the metabolic rate of the radiotracer in vitro as follows; mice, rats, and monkeys/humans. In the in vitro experiment, the percentage of the unchanged drug in the plasma at 60 minutes postdose was 9% in mice, 70% in rats, 97% in monkeys, and 98% in humans. In vivo metabolite analysis in the blood showed the presence of two radioactive metabolites, consisting of one hydrolysate [(11)C]A and another unidentified substance. The species were classified in descending order of the metabolic rate of the radiotracer in vivo as follows; mice, rats/humans, and monkeys. The percentage of the unchanged drug in the plasma was 6% in mice, 21% in rats, 26% in humans, and 40% in monkeys. Furthermore, the in vitro and in vivo experiments conducted to analyze the metabolism of [(11)C]Ro15-4513 in the brain tissue of mice and rats revealed that the radiotracer was metabolized to some extent in the brain tissue of these animals. In the in vivo experiment, the percentage of the unchanged drug at 60 min postdose was 86% in the brain tissue of mice and 88% in the brain tissue of rats, while in the in vitro experiment, the corresponding percentage was 93% in mice, and 91% in rats.

Tolerance to diazepam-induced motor impairment: a study with GABAA receptor alpha6 subunit knockout mice

Development of tolerance to motor-impairing effects of repeated administration of moderate diazepam doses (5.0-7.5 mg/kg; three times daily PO 3 weeks) was compared between mice deficient in the cerebellar granule cell-restricted GABAA receptor alpha6 subunit and their wild-type controls. The alpha6 -/- mice were more impaired by the initial challenge doses of diazepam (5 or 10 mg/kg) than their controls, but acquired partial tolerance by the second tests with the same doses 4-7 days later. Chronic treatment produced complete tolerance in both mouse lines. Ligand autoradiography revealed a significant reduction in baseline benzodiazepine and chloride channel site-bindings in various regions of the alpha6 -/- brains, but the chronic diazepam treatment did not consistently alter baseline or benzodiazepine site agonist and inverse agonist-modulated binding in the alpha6 -/- and wildtype mice. The results indicate that tolerance to motor-impairing actions of diazepam is independent of the diazepam-insensitive alpha6 subunit-containing receptors, which rules out the possibility that tolerance emerges as an increase in structurally benzodiazepine-insensitive receptor population.

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.

GABA(A) receptors containing (alpha)5 subunits in the CA1 and CA3 hippocampal fields regulate ethanol-motivated behaviors: an extended ethanol reward circuitry

GABA receptors within the mesolimbic circuitry have been proposed to play a role in regulating alcohol-seeking behaviors in the alcohol-preferring (P) rat. However, the precise GABA(A) receptor subunit(s) mediating the reinforcing properties of EtOH remains unknown. We examined the capacity of intrahippocampal infusions of an alpha5 subunit-selective ( approximately 75-fold) benzodiazepine (BDZ) inverse agonist [i.e., RY 023 (RY) (tert-butyl 8-(trimethylsilyl) acetylene-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5a] [1,4] benzodiazepine-3-carboxylate)] to alter lever pressing maintained by concurrent presentation of EtOH (10% v/v) and a saccharin solution (0.05% w/v). Bilateral (1.5-20 microgram) and unilateral (0.01-40 microgram) RY dose-dependently reduced EtOH-maintained responding, with saccharin-maintained responding being reduced only with the highest doses (e.g., 20 and 40 microgram). The competitive BDZ antagonist ZK 93426 (ZK) (7 microgram) reversed the RY-induced suppression on EtOH-maintained responding, confirming that the effect was mediated via the BDZ site on the GABA(A) receptor complex. Intrahippocampal modulation of the EtOH-maintained responding was site-specific; no antagonism by RY after intra-accumbens [nucleus accumbens (NACC)] and intraventral tegmental [ventral tegmental area (VTA)] infusions was observed. Because the VTA and NACC contain very high densities of alpha1 and alpha2 subunits, respectively, we determined whether RY exhibited a "negative" or "neutral" pharmacological profile at recombinant alpha1beta3gamma2, alpha2beta3gamma2, and alpha5beta3gamma2 receptors expressed in Xenopus oocytes. RY produced "classic" inverse agonism at all alpha receptor subtypes; thus, a neutral efficacy was not sufficient to explain the failure of RY to alter EtOH responding in the NACC or VTA. The results provide the first demonstration that the alpha5-containing GABA(A) receptors in the hippocampus play an important role in regulating EtOH-seeking behaviors.

Small platform stress increases

Small platform stress was induced in male BALB/c mice by placing them on small platforms (d = 3.5 cm) surrounded by water for 24 or 72 h. This experimental model contains several factors of stress, like rapid eye movement (REM) sleep deprivation, isolation, immobilization and falling into the water. After 24 h small platform stress exposure latency to sleep was measured after the administration of the benzodiazepine receptor agonist diazepam (1.0 and 2.5 mg/kg, i.p.) and the benzodiazepine receptor inverse agonist Ro 15-4513 (1.0 mg/kg, i.p.). As could be expected, diazepam significantly shortened the latency to sleep. Surprisingly the administration of Ro 15-4513 also shortened the latency to sleep. In addition [3H]Ro 15-4513 binding was measured in the cerebellum of control and small platform stressed mice. Small platform stress for 24 h did not alter the maximal number of [3H]Ro 15-4513 binding sites (Bmax) and decreased their affinity (K(D)). Small platform stress for 72 h significantly increased the number of [3H]Ro 15-4513 binding sites and decreased their affinity. These effects were due to changes in diazepam-sensitive binding. In conclusion, it could be supposed that exposure of mice to small platform stress causes changes in the function of the [3H]Ro 15-4513 binding sites, probably a shift of binding sites toward agonist conformation, that leads to changes in the effects of Ro 15-4513.

Pharmacophore/receptor models for GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) via a comprehensive ligand-mapping approach

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

Dual action of the benzodiazepine receptor inverse agonist RU34347 on responses to exogenously applied GABA in the rat cerebellar slice

The benzodiazepine receptor inverse agonist has been shown to produce agonist-like effects at low concentrations. RU34347 has both inverse agonist (attenuation of GABA-responses) and agonist-like (reduction of spontaneous Purkinje cell firing rate) in the cerebellar slice preparation. The benzodiazepine antagonist flumazenil prevented the inverse agonist actions, but only partially reduced the agonist-like effects. Further, brief application of RU34347 to slices mimicked the response to GABA, and pharmacological investigation determined that this action was mediated through increased GABA through action at a site proximal to the parallel fiber-basket cell synapse, at an as yet undetermined receptor.

Synthesis and evaluation of [123I]labelled analogues of the partial inverse agonist Ro 15-4513 for the study of diazepam-insensitive benzodiazepine receptors

The imidazobenzodiazepines ethyl 8-iodo-5,6 dihydro-5-methyl-6-oxo-4H-imidazo[1,5a][1,4] benzodiazepine-3-carboxylate 1 and tert-butyl 8-iodo-5,6 dihydro-5-methyl-6-oxo-4H-imidazo [1,5a][1,4] benzodiazepine-3-carboxylate 2 were prepared to study the diazepam-insensitive (DI) benzodiazepine receptor (BZR) subtype. The [123I] analogues were prepared via iododestannylation reactions in radiochemical yields of 70-80% and a specific activity >2,500 Ci/mmol. The tert-butyl analogue [123I]-2 exhibited nanomolar affinity for BZRs in homogenate membranes of rat cerebellum with Kd values for the diazepam-sensitive (DS) and DI receptors of 3.18 +/- 0.58 and 13.55 +/- 2.72 nM, respectively. The Bmax for cerebellar DS and DI receptors were 1,276 +/- 195 and 518 +/- 26 fmol/mg protein, respectively.

Cerebellar granule-cell-specific GABAA receptors attenuate benzodiazepine-induced ataxia: evidence from alpha 6-subunit-deficient mice

Benzodiazepine- and alcohol-induced ataxias in rodents have been proposed to be affected by the gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit, which contributes to receptors specifically expressed in cerebellar granule cells. We have studied an alpha 6 -/- mouse line for motor performance and drug sensitivity. These mice, as a result of a specific genetic lesion, carry a precise impairment at their Golgi-granule cell synapses. On motor performance tests (rotarod, horizontal wire, pole descending, staircase and swimming tests) there were no robust baseline differences in motor function or motor learning between alpha 6 -/- and alpha 6 +/+ mice. On the rotarod test, however, the mutant mice were significantly more impaired by diazepam (5-20 mg/kg, i.p.), when compared with alpha 6 +/+ control and background C57BL/6J and 129/SvJ mouse lines. Ethanol (2.0-2.5 g/kg, i.p.) produced similar impairment in the alpha 6 -/- and alpha +/+ mice. Diazepam-induced ataxia in alpha 6 -/- mice could be reversed by the benzodiazepine site antagonist flumazenil, indicating the involvement of the remaining alpha 1 beta 2/3 gamma 2 GABAA receptors of the granule cells. The level of activity in this synapse is crucial in regulating the execution of motor tasks. We conclude that GABAA receptor alpha 6 subunit-dependent actions in the cerebellar cortex can be compensated by other receptor subtypes; but if not for the alpha 6 subunit, patients on benzodiazepine medication would suffer considerably from ataxic side-effects.

Predictive models for GABAA/benzodiazepine receptor subtypes: studies of quantitative structure-activity relationships for imidazobenzodiazepines at five recombinant GABAA/benzodiazepine receptor subtypes [alphaxbeta3gamma2 (x = 1-3, 5, and 6)] via comparative molecular field analysis

Affinities of a series of substituted imidazobenzodiazepines at recombinant alpha1beta3gamma2, alpha2beta3gamma2, alpha3beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2 GABAA/benzodiazepine receptor subtypes are reported. Many of these ligands displayed high affinities (low-nanomolar to subnanomolar scale) at all five receptor subtypes. Furthermore, a number of imidazobenzodiazepines exhibited relatively good selectivity at the alpha5-containing receptor isoform. For example, ligand 27 (RY-023) demonstrated a 55-fold higher selectivity at alpha5beta3gamma2 isoforms in comparison to other receptor subtypes. The affinity ratio of alpha1 (the most prevalent subtype in the brain) to alpha5 of this series of ligands ranged from 60- to 75-fold for the most selective ligands. Studies of quantitative structure-activity relationships (QSAR) by means of comparative molecular field analysis (CoMFA) were carried out. As a result, examination of CoMFA models for all five receptor subtypes demonstrated their predictability for affinities of imidazobenzodiazepines at the five receptor subtypes. Regions of molecular fields which would favor or disfavor the binding affinity of a ligand at a specific receptor subtype were examined via CoMFA for alpha1, alpha2, alpha3, alpha5, and alpha6 subtypes. A CoMFA regression analysis was applied to predict the ratio of Ki alpha1/Ki alpha5, an index for the selectivity of a ligand at the alpha5 subtype. All of the CoMFA models offered good cross-validated correlations for the ligands in the test set as well as the ratios of Ki alpha1/Ki alpha5, which demonstrated their potential for prediction.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

The effects of the novel benzodiazepine receptor inverse agonist Ru 34000 on ethanol-maintained behaviors

Ru 34000 [5-ethyl-7-methoxy-imidazo (1,2-a) pyrimidin-2-yl cyclopropyl methanone] is a novel imidazopyrimidine benzodiazepine inverse agonist that exhibits low affinity for central benzodiazepine receptors (Ki approximately 0.98 microM). The present study examined the in vivo actions of Ru 34000 (0.5-5 mg/kg) following intraperitoneal (i.p.), subcutaneous (s.c), oral (p.o.), and intraventral tegmental administration in alcohol-preferring (P) rats trained under a concurrent operant schedule (FR4-FR4) for ethanol (10% v/v) and a palatable saccharin (0.025% or 0.75% w/v) reinforcer. Ru 34000 (i.p., s.c., p.o.) markedly reduced ethanol responding by 28-96% of control levels without affecting saccharin responding, except for the highest dose level. Clear dose-dependent suppressant effects were observed with all routes of administration on ethanol responding. Flumazenil [ethyl-8-fluro-5, 6-dihydro-5-methyl-6-4H-imidazo [1,5-a]-[1,4]-benzodiazepine-3-carboxylate] (6 mg/kg; i.p.), a benzodiazepine receptor antagonist reversed the Ru 34000-reduction of ethanol responding, suggesting that the effects were mediated at the benzodiazepine receptor. Bilateral microinjections of Ru 34000 (50, 100, 200 ng) into the ventral tegmental area dose-dependently reduced ethanol responding by as much as 97% of control levels. The results suggest that the in vivo actions of Ru 34000 are determined not only by its binding affinity, but also by its bioavailability at active benzodiazepine sites and route of drug administration. Low affinity imidazopyrimidines may be useful pharmacological probes to further understand the role of the GABA(A)-benzodiazepine receptor complex in ethanol motivated behaviors.

Anomalous rectifying properties of 'diazepam-insensitive' GABA(A) receptors

Studies using recombinant systems indicate that 'diazepam-insensitive' GABA(A) receptors in the central nervous system contain alpha4 and alpha6 subunits while 'diazepam-sensitive' GABA(A) receptors contain alpha1, alpha2, alpha3 and alpha5 subunits. Both native and recombinant diazepam-sensitive GABA(A) receptors typically exhibit large, outwardly rectifying currents. For example, in patch clamp studies, Human Embryonic Kidney (HEK) 293 cells transfected with cDNAs encoding alpha1beta2gamma2 subunits exhibit a rectification ratio (I +60 mV/I -60 mV) of 1.95 +/- 0.21. However, anomalous rectification was observed in recombinant diazepam-insensitive GABA(A) receptors composed of either alpha4beta2gamma2 (rectification ratio, 0.74 +/- 0.09) or alpha6beta2gamma2 (rectification ratio, 0.67 +/- 0.11) subunits. Based on sequence differences between diazepam-sensitive and -insensitive GABA(A) receptor alpha subunits in the vicinity of the putative channel lining, a point mutation was introduced at His273 on the alpha4 subunit. The rectification ratio in cells expressing a mutated alpha4(Asn273)beta2gamma2 receptor increased to 1.92 +/- 0.17. Moreover, mutation of the homologous residue in the alpha1 subunit to histidine reduced the rectification ratio of alpha1(His274)beta2gamma2 to 1.02 +/- 0.12. The affinities of benzodiazepine site ligands at diazepam-sensitive and -insensitive GABA(A) receptors were unaffected by these mutations. Thus, the electrophysiological properties of diazepam-sensitive and -insensitive GABA(A) receptors may be as divergent as their pharmacological characteristics.

The novel benzodiazepine inverse agonist RO19-4603 antagonizes ethanol motivated behaviors: neuropharmacological studies

The novel imidazothienodiazepine inverse agonist RO19-4603 has been reported to attenuate EtOH intake in home cage drinking tests for at least 24 h post-drug administration after systemic administration. In the present study, selectively bred alcohol-preferring (P) rats were trained under a concurrent (FR4-FR4) operant schedule to press one lever for EtOH (10% v/v) and another lever for saccharin (0.05% or 0.75% g/v), then dose-response and timecourse effects of RO19-4603 were evaluated. Systemic RO19-4603 injections (0.0045-0.3 mg/kg; i.p.) profoundly reduced EtOH responding by as much as 97% of vehicle control on day 1. No effects were seen on saccharin responding except with the highest dose level (0.3 mg/kg). In a second experiment, microinjections of RO19-4603 (2-100 ng) directly into the nucleus accumbens (NA) suppressed EtOH responding on day 1 by as much as 53% of control: Control injections dorsal to the NA or ventral tegmental area did not significantly alter EtOH or saccharin responding. On day 2, rats in both experiments received no RO19-4603 treatments; however, all 7 of the i.p. doses, and all 3 of the intra-NA infusions continued to significantly suppress EtOH responding by 43-85% of vehicle control levels. In addition, i.p. injections of RO19-4603 produced a dose-dependent decrease in the slope of the cumulative record for EtOH responding, while concomitantly producing a dose-dependent increase in the slope for saccharin responding. RO19-4603's actions appear to be mediated via recognition sites at GABAA-BDZ receptors which regulate EtOH reinforcement, and not via mechanisms regulating ingestive behaviors. Based on recent in situ hybridization studies in our laboratory, we hypothesize that occupation of alpha4 containing GABAA diazepam insensitive (DI) receptors in the NA, may mediate in part, the RO19-4603 suppression of EtOH responding in EtOH-seeking P rats.

Benzodiazepine receptor antagonists modulate the actions of ethanol in alcohol-preferring and -nonpreferring rats

The pyrazoloquinoline CGS 8216 (2-phenylpyrazolo-[4,3-c]-quinolin-3 (5H)-one, 0.05-2 mg/kg) and the beta-carboline ZK 93426 (ethyl-5-isopropyl-4-methyl-beta-carboline-3-carboxylate, 1-10 mg/kg) benzodiazepine receptor antagonists were evaluated for their capacity to modulate the behavioral actions of ethanol in alcohol preferring and -nonpreferring rats. When alcohol-preferring rats were presented with a two-bottle choice test between ethanol (10% v/v) and a saccharin (0.0125% g/v) solution, both antagonists dose-dependently reduced intake of ethanol by 35-92% of control levels on day 1 at the initial 15 min interval of the 4 h limited access. Saccharin drinking was suppressed only with the highest doses. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) unmasked the anxiolytic effects of a hypnotic ethanol dose (1.5 g/kg ethanol) on the plus maze test in alcohol-preferring rats, but potentiated the ethanol-induced suppression in alcohol-nonpreferring rats. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) attenuated the ethanol (0.5 and 1.5 g/kg)-induced suppression in the open field in alcohol-nonpreferring rats; however, CGS 8216 potentiated the depressant effects of the lower ethanol dose (0.5 g/kg) in alcohol-preferring rats. These findings provide evidence that benzodiazepine receptor antagonists may differentially modulate the behavioral actions of ethanol in alcohol-preferring and-nonpreferring rats. It is possible that the qualitative pharmacodynamic differences seen in the present study may be related to selective breeding for alcohol preference. The findings indicate the potential for development of receptor specific ligands devoid of toxic effects which may be useful in the treatment of alcohol abuse and alcoholism.

Localization and pharmacological characterization of pigeon diazepam-insensitive GABAA receptors

Transduction mechanisms associated with ligand binding at diazepam-insensitive subtypes of GABAA receptors remain largely unknown, but unique behavioral effects of ligands binding at these sites have been reported in pigeons. The present study further evaluated the pharmacological characteristics of diazepam-insensitive GABAA receptors in pigeon brain, using [3H]Ro 15-4513. Autoradiography detected diazepam-insensitive benzodiazepine sites on GABAA receptors in a number of brain regions, with the highest densities present in the olfactory bulb, hippocampus, thalamic nuclei and cerebellar granule cell layers, with densities of approximately 10-20% of total benzodiazepine receptor binding. Saturation analysis revealed significant densities (approximately 10% of total benzodiazepine receptor binding) of extracerebellar diazepam-insensitive benzodiazepine receptors in optic lobe, hippocampus, and brainstem compared to 27% in cerebellum. As reported for mammalian diazepam-sensitive benzodiazepine receptors, GABA (50 microM) generally increased the affinities of agonists and partial agonists, had little effect on the affinities of antagonists, and decreased the affinity of an inverse agonist for pigeon cerebellar diazepam-sensitive benzodiazepine receptors. GABA modulation of ligand binding to diazepam-insensitive benzodiazepine receptors was less than that observed for diazepam-sensitive sites, and no positive modulation was observed. These results demonstrate the presence of cerebellar and extracerebellar diazepam-insensitive benzodiazepine receptors in pigeon brain, with distribution patterns and pharmacology similar to those reported in mammals. The comparable central localization and pharmacological properties of drugs at diazepam-sensitive and -insensitive benzodiazepine receptors in pigeons and rats attests to the evolutionary conservation of GABAA systems.

Blockade of behavioral effects of bretazenil by flumazenil and ZK 93,426 in pigeons

Benzodiazepine receptor partial agonists manifest full efficacy in preclinical tests of anxiolytic drug action but do not fully reproduce the discriminative stimulus effects of benzodiazepine receptor full agonists in pigeons. The partial agonist, bretazenil, binds to both diazepam-sensitive and diazepam-insensitive GABAA receptors. Previous studies have suggested a role for each of these receptor populations in some behavioral effects of bretazenil in pigeons. A possible role for these receptor subtypes in the behavioral effects of bretazenil was further investigated through drug interaction studies with the benzodiazepine receptor antagonists, flumazenil and ZK 93,426. Whereas flumazenil binds with high affinity to both receptor isoforms, ZK 93,426 binds preferentially to diazepam-sensitive binding sites. Bretazenil markedly increased punished responding of pigeons without significantly affecting nonpunished responding. In pigeons discriminating the full benzodiazepine receptor agonist, midazolam, from saline, bretazenil produced only 60-75% maximal effect. Flumazenil and ZK 93,426 neither increased punished responding nor substituted for midazolam, but dose-dependently blocked the effects of bretazenil on punished responding. Flumazenil also dose-dependently blocked the effects of bretazenil in midazolam-discriminating pigeons, whereas ZK 93,426 only attenuated this effect. These results indicate that bretazenil's actions as a partial agonist at diazepam-sensitive benzodiazepine receptors mediate increases in punished responding and substitution for the discriminative stimulus effects of midazolam in pigeons. The differences in the effects of flumazenil and ZK 93,426 on the discriminative stimulus effects of bretazenil suggest a potential contribution of diazepam-insensitive sites to this behavioral effect.

Ro 15-4513 potentiates, instead of antagonizes, ethanol-induced sleep in mice exposed to small platform stress

The effects of ethanol and the benzodiazepine receptor ligand ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-[1,5a] [1,4] benzodiazepine-3-carboxylate (Ro 15-4513), were examined in NMRI mice exposed to small platform stress. This model contains several factors of stress, like rapid eye movement (REM) sleep deprivation, isolation, immobilization, falling into water and soaking. In control mice, ethanol exerted an anxiolytic effect in the plus-maze, but did not further enhance the anxiolytic-like effects induced by small platform stress. Ro 15-4513 antagonized ethanol-induced sleep in control animals, but enhanced the hypnotic and lethal actions of ethanol in small platform stressed mice. Small platform stress did not alter the characteristics (KD and Bmax) of [3H]Ro 15-4513 binding to cerebellar membranes. Muscimol-stimulated 36Cl- uptake into brain microsacs was significantly reduced in cortex from small platform stressed animals. Ethanol had no effect on 36Cl- uptake into brain microsacs from cortex or cerebellum. It is proposed that small platform stress alters the activity of the gamma-aminobutyric acid (GABA)A receptor-chloride ionophore complex, causing changes in the interaction between ethanol and Ro 15-4513.

Astroglia-released factor with negative allosteric modulatory properties at the GABA A receptor

We have previously shown, using whole-cell patch-clamp techniques, that astrocytes release a negative allosteric modulator of the gamma-aminobutyric acid type A receptor (GABAA receptor) with beta-carboline-like properties, thus, likely to act at the benzodiazepine site. Here, using patch-clamp and binding techniques, we confirm that the low-molecular-weight fraction of astroglia-conditioned medium (ACM lmf) contains a factor(s) that negatively modulates GABAA-receptor function. This factor, like beta-carbolines, enhances the specific binding of [35S]t-butyl bicyclophosphorothionate (TBPS) to adult rat cortical membranes in the presence of GABA. However, it fails to interact with various ligands of the benzodiazepine (BZD) site of the GABAA receptor ([3H]flunitrazepam, [3H]Ro 15-1788 and [3H]Ro 15-4513). The question of the actual binding site of the astroglia-derived factor on the GABAA receptor, thus, remains open and can be addressed only after the purification of the active molecule(s) of ACM Imf has been completed, and a labeled form of the endogenous ligand becomes available. Taken together, however, the data suggest that type 1 astrocytes are able to modulate the effects of the main inhibitory neurotransmission in the central nervous system.

Inhibition of GABAA ligand-gated Cl- channels by zinc in adult rat brain: a regional study

Zinc (Zn2+) was shown to invariably inhibit muscimol-stimulated 36Cl- uptake by synaptoneurosomes in the cerebral cortex, hippocampus and cerebellum. The Zn2+ sensitivity of the GABAA receptor-gated 36Cl- uptake in the cerebral cortex was comparable to that in the hippocampus, whereas the uptake in the cerebellum was less sensitive to Zn2+. Although diazepam-potentiation of muscimol-stimulated 36Cl- uptake was unaltered by 100 microM Zn2+ in the cerebral cortex and hippocampus, diazepam caused no enhancement in the presence of Zn2+ in the cerebellum. Zn2+ inhibited [3H]diazepam binding significantly at 1 mM in the cerebral cortex and cerebellum, whereas Ni2+ increased the binding in a concentration-dependent manner in both regions. Although lower concentrations of Zn2+ did not affect [3H]Ro 15-4513 binding to diazepam-sensitive sites, higher concentrations of ZN2+ increased the binding in both regions. Unlike the diazepam-sensitive sites, the diazepam-insensitive [3H]Ro 15-4513 binding was not affected by Zn2+ or Ni2+ at any of the tested concentrations. These results suggest that the GABAA ligand-gated Cl- flux and its diazepam-potentiation are heterogeneously modulated in various brain regions. It is also suggested that cerebellar diazepam-insensitive [3H]Ro 15-4513 binding sites are insensitive to Zn2+ and Ni2+.

Biphasic actions of the benzodiazepine receptor inverse agonist RU34347 in the rat cerebellar slice

We have characterised the biphasic response of the benzodiazepine receptor inverse agonist RU34347 in the rat cerebellar slice preparation using extracellular electrophysiological recordings from Purkinje cells. RU34347 (10 fM-10 nM) produced an increase in GABA(A)-mediated inhibition at between 10 fM and 10 nM, a response normally associated with benzodiazepine agonists. This response was biphasic, being dose dependent between 10 fM and the peak effect at 10 pM, and inversely related to concentration between 100 pM and 10 nM. Associated with this increase in inhibition was a decrease in firing rate, also showing a biphasic concentration-response relationship. The agonist-like response was composed of two elements, an initial increase occurring after 5 min followed by a slow decline over the next 20 min. This second, declining, phase was more evident at higher concentrations of RU34347. The peak effect seen at 10 pM was fully antagonised by 1 microM flumazenil, a benzodiazepine receptor antagonist. This concentration of flumazenil also antagonised the decrease in firing rate induced by 10 pM RU34347. The response to 10 nM RU34347 was further characterised since the largest second phase decline was demonstrated at this concentration. 10 nM flumazenil fully antagonised the second, declining, phase of the response, but not the magnitude of the initial increase. A partial antagonism of the peak effect was seen with 1 microM flumazenil, and a full antagonism at 10 microM flumazenil. This effect was similar to that observed with Ro15-4513 in a previous study. Therefore, we investigated the binding of RU34347 to diazepam-insensitive benzodiazepine receptors. [3H]Ro15-4513 was displaced by Ro15-4513, flumazenil and Ro19-4603, but not by RU34347. Therefore, although the electrophysiological data correlate well with that previously reported for Ro15-4513, RU34347 does not displace Ro15-4513 binding at the diazepam-insensitive benzodiazepine receptor. Therefore, either both ligands bind to complimentary sites on the same receptor complex, or both induce a similar physiological response through an action on different receptors.

Pharmacologic actions of subtype-selective and novel GABAergic ligands in rat lines with differential sensitivity to ethanol

Alcohol-nontolerant (ANT) rats, produced by selective breeding for high sensitivity to motor-impairing effects of ethanol, have a point mutation in the cerebellar gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit, which has been proposed to underlie enhanced sensitivity to benzodiazepine agonists as well. We compared ANT and alcohol-tolerant (AT) rats using behavioral and neurochemical methods to assess the significance of alpha 6- and non alpha 6-containing GABAA receptor subtypes. Motor performance in a tilting plane test was largely unaffected by a type I benzodiazepine receptor-preferring agonist, zolpidem [1-10 mg/kg, intraperitoneally (IP)], partial benzodiazepine agonists bretazenil and ZG-63 (both at 40 mg/kg, IP), and a novel broad-spectrum anticonvulsant loreclezole (40 mg/kg, IP) in both ANT and AT rats. In contrast, diazepam (10 mg/kg, IP) impaired performance of the ANT but not AT animals. These data, supported by results from brain regional autoradiography of [3H]Ro15-4513 and membrane binding of [3H]ZG-63 and [35S]TBPS as influenced by these ligands, strongly suggest that only ligands with full agonist actions on mutant (ANT) but not wild-type (AT) alpha 6-containing GABAA receptors are able to produce motor impairment in the ANT rats.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

GabaA Receptors: Pharmacology, Behavioral Roles, and Motor Disorders

τ-Aminobutyric acid (GABA), the most prevalent inhibitory neurotransmitter in the mammalian brain, exerts its main action through GABAA receptors. They belong to the superfamily of ligand-gated ion channels and respond to GABA by the opening of an intrinsic anion channel. Multiple GABAA receptor subtypes in the brain show differential regional and developmental expression patterns. The receptors have a pentameric structure and are formed from members of at least three different subunit families (α1–6, β1–3, and τ1–3). The regulation of functional properties by GABA and its analogs and by benzodiazepine (BZ) receptor ligands differs dramatically with the type of α variant in the receptor complex. Additional variations of GABAA receptors result from substitution of γ subunits. The role of the β subunits, which are essential for receptor assembly, is less well defined on a functional basis. Besides their involvement in anxiolysis and sedation, GABAA receptors clearly have an impact on motor coordination. However, with the possible exception of the alcohol-and BZ-sensitive alcohol non-tolerant (ANT) rat line, it is not well documented whether a genetic alteration in this receptor system is directly involved in the impairment of animal or human motor activity.

Pharmacological properties of recombinant "diazepam-insensitive" GABAA receptors

Both native and recombinant "diazepam-insensitive" GABAA receptors (DI) are characterized by the very low affinities of prototypic 1,4-benzodiazepines such as diazepam and the high affinity of an imidazobenzodiazepine, Ro 15-4513. The presence of either an alpha 4 or alpha 6 subunit imparts this unusual pharmacological profile to DI. Based on the affinities of these compounds at recombinant DI, the pharmacological properties of alpha 4- and alpha 6-bearing receptor isoforms appear to be very similar if not identical. Using a larger sample of structurally diverse compounds, we now demonstrate distinct but related ligand binding profiles of recombinant alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 DI. Comparison of 18 ligands drawn from three principal structural groups (beta-carbolines, imidazobenzodiazepines and pyrazoloquinolinones) revealed that the affinity of at least one representative from each group differed by > 5-fold between alpha 4- and alpha 6 beta 2 gamma 2 receptors. While the high correlation (r2 = 0.926; p < 0.001) obtained between the affinities of these ligands at alpha 4- and alpha 6-containing receptors underscores the similarity between these receptor isoforms, a significant deviation of the slope of this correlation (0.792; 95% C.I. 0.673-0.911) from unity is substantive evidence that these DI possess distinct pharmacological profiles. These findings indicate that it is feasible to develop selective ligands for these DI isoforms.

The cerebellum: a model system for studying GABAA receptor diversity

The basic unsolved questions concerning GABAA receptors are: "How many receptor subtypes exist?", "What subtypes are used by which types of neuron and where are they located on the cell?", and "What are the functions of the different subtypes?" As described in this Review, the cerebellum is an ideal vertebrate brain region for investigating these issues.

Contribution of "diazepam-insensitive" GABAA receptors to the alcohol antagonist properties of Ro 15-4513 and related imidazobenzodiazepines

Both in vivo and in vitro studies have shown that Ro 15-4513 can antagonize many of the pharmacologic actions of ethanol. In contrast to many benzodiazepine receptor (BzR) ligands, Ro 15-4513 binds with high affinity to a novel GABAA receptor subtype, referred to as "diazepam-insensitive" (DI). This study examined the contribution of DI GABAA receptors to the modulation of ethanol-induced sleep time by Ro 15-4513 and related imidazobenzodiazepines [e.g., Ro 19-4603, Ro 16-6028, and ZG-63 (t-butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-imidazo[1,5,a] [1,4]benzodiazepine-3-carboxylate)] that possess high affinities for this GABAA receptor subtype. Ro 15-4513 (0.6-5 mg/kg) significantly reduced ethanol (3.5 g/kg, i.p.) sleep time in mice (p < 0.001, analysis of variance). This effect was not blocked by BzR antagonists ZK 93426 (5 mg/kg) and Ro 14-7437 (5 mg/kg), which possess low affinities for DI but bind with high affinities to other "diazepam-sensitive" (DS) GABAA receptor isoforms. Although Ro 19-4603 (2.5 mg/kg) also reduced ethanol sleep time (p < 0.01), this effect was attenuated by coadministration of ZK 93426 (2.5 mg/kg). Ro 16-6028 (2.5 mg/kg) prolonged (p < 0.01) ethanol sleep time. However, in the presence of either Ro 19-7437 (5 mg/kg) or ZK 93426 (2.5 mg/kg) ethanol-induced sleep time was reduced to values approaching those obtained with ethanol in the presence of Ro 15-4513. A low dose (2.5 mg/kg) of ZG-63 did not significantly affect alcohol sleep time. However, in the presence of ZK 93426, ZG-63 increased sleep time (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of 1,4-benzodiazepines to a novel [3H]Ro15-4513 binding site in the rat spinal cord

An alpidem-insensitive benzodiazepine binding site in the rat spinal cord has recently been identified in our laboratory. We report here the binding of 23 1,4-benzodiazepines to this site using [3H]Ro15-4513 (ethyl-8-azido-6-dihydro-5-methyl-4H-imidazo[1,2- a][1,4]benzodiazepine-3-carboxylate) in the presence of 65 microM alpidem (6-chloro-2-(4-chlorophenyl)-N,N- dipropylimidazo[1,2-a]pyridine-3-acetamide). This binding site displays a wide affinity for 1,4-benzodiazepines, most of which show much higher affinity for benzodiazepine receptors in various brain regions and transfected cell systems. The highest affinity ligands are: brotizolam (1-bromo-4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2- f][1,2,4]triazolo[4,3-a][1,4]diazepine) (4.3 nM), Ro15-4513 (5.0 nM), Ro42-8773 (7-chloro-3-[3-(cyclopropylmethoxy)-1-propynyl]-4,5-dihydro- 5-methyl-6H-imidazo[1,5-a][1,4]benzodiazepine-6-one) (5.7 nM), Ro16-6028 (t-butyl (s)-8-bromo-11,12,13,13a-tetrahydro-9-oxo-9H- imidazo[1,5-a][1,4]benzodiazepine-1-carboxylate) (5.9 nM) and triazolam (8-chloro-6-(2-chlorophenyl)-1-methyl-4H- [1,2,4]triazolo[4,3-a][1,4]benzodiazepine) (7.9 nM). The structural feature common to these compounds is an imidazo- or triazolo-ring on the 1- and 2-position of the benzodiazepine. However, the presence of this feature does not guarantee high affinity binding as Ro15-1788 (8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5- a][1,4]benzodiazepine-3-carboxylic acid ethyl ester) (100 nM) and Ro23-0364 (6-[2-chlorophenyl]-4H- imidazo[1,5-a][1,4]benzodiazepine-3-carboxamide) (360 nM) display much lower affinity for this site. Studies are currently underway to investigate the functional significance of this unusual benzodiazepine binding site.

Stereospecific transduction of behavioral effects via diazepam-insensitive GABAA receptors

Previous studies reported a positive correlation between ligand affinities at diazepam-insensitive GABAA receptors and substitution for the discriminative stimulus effects of the benzodiazepine receptor antagonist, flumazenil, in pigeons. In the present experiments, bretazenil and Ro 14-5974 (ethyl-(S)-11,12,13,13 a-tetrahydro-9-oxo-9H-imidazo[1,5-a]-pyrrolo-[2,1-c] [1,4]benzodiazepine-1-carboxylate) partially substituted for, and blocked the discriminative stimulus effects of midazolam, congruent with their actions at diazepam-sensitive GABAA receptors in vitro. In addition, bretazenil and Ro 14-5974, but not their R-enantiomers, had high affinity for diazepam-insensitive receptors and fully substituted for the discriminative stimulus effects of flumazenil. The R-enantiomers of these compounds had low affinity (Ki > 1 microM) for diazepam-sensitive and diazepam-insensitive receptors, and did not share discriminative stimulus effects with flumazenil or midazolam. Ro 19-0528 (7-chloro-3-(3-cyclopropyl-1,2,4-oxadiazol-5-yl)-4,5-dihydro-5-met hyl-6H- imidazo[1,5-a][1,4]benzodiazepin-6-one), a structurally related compound with full agonist actions at diazepam-sensitive GABAA receptors, had high diazepam-insensitive receptor affinity (Ki = 96 nM) and partially substituted for the discriminative stimulus effects of flumazenil. These results are consistent with stereospecific mediation of the discriminative stimulus effects of flumazenil through high affinity binding to diazepam-insensitive receptors in pigeons.