Synthesis and Pharmacological Evaluation of [11C]4-Methoxy-N-[2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-yl]benzamide as a Brain Penetrant PET Ligand Selective for the δ-Subunit-Containing γ-Aminobutyric Acid Type A Receptors

The α_4/6βδ-containing GABA_A receptors are involved in a number of brain diseases. Despite the potential of a δ-selective imaging agent, no PET radioligand is currently available for in vivo imaging. Here, we report the characterization of DS2OMe (1) as a candidate radiotracer, ¹¹C-labeling, and subsequent evaluation of [¹¹C]DS2OMe in a domestic pig as a PET radioligand for visualization of the δ-containing GABA_A receptors.

Nonacidic Farnesoid X Receptor Modulators

As a cellular bile acid sensor, farnesoid X receptor (FXR) participates in regulation of bile acid, lipid and glucose homeostasis, and liver protection. Clinical results have validated FXR as therapeutic target in hepatic and metabolic diseases. To date, potent FXR agonists share a negatively ionizable function that might compromise their pharmacokinetic distribution and behavior. Here we report the development and characterization of a high-affinity FXR modulator not comprising an acidic residue.

Increased Motor-Impairing Effects of the Neuroactive Steroid Pregnanolone in Mice with Targeted Inactivation of the GABAA Receptor γ2 Subunit in the Cerebellum

Endogenous neurosteroids and neuroactive steroids have potent and widespread actions on the brain via inhibitory GABA_A receptors. In recombinant receptors and genetic mouse models their actions depend on the α, β, and δ subunits of the receptor, especially on those that form extrasynaptic GABA_A receptors responsible for non-synaptic (tonic) inhibition, but they also act on synaptically enriched γ2 subunit-containing receptors and even on αβ binary receptors. Here we tested whether behavioral sensitivity to the neuroactive steroid agonist 5β-pregnan-3α-ol-20-one is altered in genetically engineered mouse models that have deficient GABA_A receptor-mediated synaptic inhibition in selected neuronal populations. Mouse lines with the GABA_A receptor γ2 subunit gene selectively deleted either in parvalbumin-containing cells (including cerebellar Purkinje cells), cerebellar granule cells, or just in cerebellar Purkinje cells were trained on the accelerated rotating rod and then tested for motor impairment after cumulative intraperitoneal dosing of 5β-pregnan-3α-ol-20-one. Motor-impairing effects of 5β-pregnan-3α-ol-20-one were strongly increased in all three mouse models in which γ2 subunit-dependent synaptic GABA_A responses in cerebellar neurons were genetically abolished. Furthermore, rescue of postsynaptic GABA_A receptors in Purkinje cells normalized the effect of the steroid. Anxiolytic/explorative effects of the steroid in elevated plus maze and light:dark exploration tests in mice with Purkinje cell γ2 subunit inactivation were similar to those in control mice. The results suggest that, when the deletion of γ2 subunit has removed synaptic GABA_A receptors from the specific cerebellar neuronal populations, the effects of neuroactive steroids solely on extrasynaptic αβ or αβδ receptors lead to enhanced changes in the cerebellum-generated behavior.

Early life stress is a risk factor for excessive alcohol drinking and impulsivity in adults and is mediated via a CRF/GABA(A) mechanism

Childhood stress and trauma are associated with substance use disorders in adulthood, but the neurological changes that confer increased vulnerability are largely unknown. In this study, maternal separation (MS) stress, restricted to the pre-weaning period, was used as a model to study mechanisms of protracted effects of childhood stress/traumatic experiences on binge drinking and impulsivity. Using an operant self-administration model of binge drinking and a delay discounting assay to measure impulsive-like behavior, we report that early life stress due to MS facilitated acquisition of binge drinking and impulsivity during adulthood in rats. Previous studies have shown heightened levels of corticotropin releasing factor (CRF) after MS, and here, we add that MS increased expression levels of GABA(A) α2 subunit in central stress circuits. To investigate the precise role of these circuits in regulating impulsivity and binge drinking, the CRF1 receptor antagonist antalarmin and the novel GABA(A) α2 subunit ligand 3-PBC were infused into the central amygdala (CeA) and medial prefrontal cortex (mPFC). Antalarmin and 3-PBC at each site markedly reduced impulsivity and produced profound reductions on binge-motivated alcohol drinking, without altering responding for sucrose. Furthermore, whole-cell patch-clamp studies showed that low concentrations of 3-PBC directly reversed the effect of relatively high concentrations of ethanol on α2β3γ2 GABA(A) receptors, by a benzodiazepine site-independent mechanism. Together, our data provide strong evidence that maternal separation, i.e. early life stress, is a risk factor for binge drinking, and is linked to impulsivity, another key risk factor for excessive alcohol drinking. We further show that pharmacological manipulation of CRF and GABA receptor signaling is effective to reverse binge drinking and impulsive-like behavior in MS rats. These results provide novel insights into the role of the brain stress systems in the development of impulsivity and excessive alcohol consumption.

Abstract 3872: Genotoxicity of zinc oxid nanoparticles and the activation of ATM-Chk2 DNA-damage-response pathway are caused by zinc-ions

Zinc oxide nanoparticles (ZnO-NP) are used in a variety of applications including cosmetics and pharmaceuticals. The common utilization of ZnO-NPs results in an increasing environmental release with unknown health effects. Recent studies have shown that ZnO-NP exposure cause cellular toxicity including DNA-damages. Discussed mechanisms are interactions of the ZnO-NP with intracellular structures and molecules after internalization, extracellular or intracellular dissociation of the particles, and the generation of ROS. In the present study we evaluated the genotoxic properties of ZnO-NP as well as the cellular DNA damage response in dependence of oxidative distress and ZnO-NP dissociation into Zn2+.

For this purpose, we exposed the human epithelial cell line A549 to defined ZnO-NP (100μg/ml). Genotoxic effects of the particles were evaluated by Comet assay and γH2AX-foci, measuring DNA double-strand breaks (DNA-DSBs). Dissociation of ZnO-NP and the intracellular increase of Zn2+ were performed with spectrofluorimetric and cLSM measurements by the use of FluoZin3. Oxidative distress was determined by the compound dihydrodichlorofluorescein diacetate (H2DCF-DA). The Activation of the DNA damage response pathway was proven by analyses of H2AX-, ATM-, Chk1- and Chk2 phosphorylation, and p21, p53 up-regulation after ZnO-NP exposure by western blot or ELISA. To figure out, if the genotoxic potential of ZnO-NP is dependent of released Zn2+ or the generation of ROS we inhibited oxidative distress by the antioxidant N-acetyl-L-cysteine (NAC) and Zn2+ were extracellular chelated by diethylene triamine pentaacetic acid (DTPA).

We found a fast extracellular dissociation of ZnO-NP and a quick intracellular increase of Zn2+. Furthermore, we ascertained an activation of H2AX induced by ZnO-NP; this phosphorylation is totally inhibited by DTPA. ZnO-NP activated the DSB-damage response indicated by ATM-, Chk2-phosphorylation as well as an up-regulation of p53 and p21. Pretreatment with NAC scavenging ROS leads to a reduced and Zn2+ complexation to no activation of ATM-Chk2-DNA damage pathway. The ATR/Chk1 pathway was not activated by ZnO-NP.

We demonstrated ZnO-NP induced DNA-damages and the activation of the ATM/Chk2 DNA damage response pathway but not the ATR/Chk1 pathway, indicating that ZnO-NP exposure induces DNA-DSBs. Scavanging of ROS leads to a slight reduction of DNA-DSBs but extracellular Zn2+ compelxation to a complete impairment suggesting that the mechanism of ZnO-NP genotoxicity is not mediated via ROS-generation but rather a direct interaction of dissociated Zn-ions with the DNA.

Citation Format: Julia Heim, Nawaz Muhammad Tahir, Anke Kaltbeitzel, Volker Mailänder, Hartmut Lüddens, Wolfgang Tremel, Jürgen Brieger. Genotoxicity of zinc oxid nanoparticles and the activation of ATM-Chk2 DNA-damage-response pathway are caused by zinc-ions. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3872. doi:10.1158/1538-7445.AM2015-3872

Proteomic identification of the heterogeneous nuclear ribonucleoprotein K as irradiation responsive protein related to migration

Irradiation resistance is a major obstacle of head and neck squamous cell carcinoma (HNSCC) therapy, limiting treatment success and patient survival. The aim of our experiments was to identify irradiation-regulated proteins as potential drug targets. Two established HNSCC cell lines (HNSCCUM-01T and HNSCCUM-02T) were treated with a single 8Gy (Gray) fraction of irradiation. Changes in cellular protein expression were studied after 24h by means of 2D-electrophoresis and MALDI-TOF-mass spectrometry. Ninety-four differentially expressed proteins were identified. The expression levels of four proteins were regulated similarly in both cell lines after irradiation treatment, i.e., GRP78, PRDX, ACTC, and the heterogeneous nuclear ribonucleoprotein K (hnRNPK), suggesting a relevant role during irradiation response. hnRNPK as a p53 interacting protein was verified by Western blotting and immunocytochemical staining as well as functionally analyzed. Knock-down by the use of siRNA resulted in only slightly reduced viability, however, migratory activity was strongly reduced. Combined application of siRNA against hnRNPK and irradiation reduced migration almost completely. We conclude that hnRNPK is potentially implicated in the radiogenic response of HNSCC. The inhibition of hnRNPK might reduce the metastasizing potential of HNSCC especially in combination with irradiation and suggest that this molecule should be further evaluated in this context.

BIOLOGICAL SIGNIFICANCE

We showed completely impaired migration of irradiated hnRNPK-knock-out HNSCC cells, suggesting this molecule as a potential drug target in combined treatment schedules.

Actions of two GABAA receptor benzodiazepine-site ligands that are mediated via non-γ2-dependent modulation

The potent sedative-hypnotic zolpidem and the convulsant methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) act primarily by binding to the benzodiazepine site of the main inhibitory neurotransmitter receptor, the pentameric γ-aminobutyric acid type A receptor (GABA(A)). This binding depends critically on the wild-type F77 residue of the GABA(A) receptor γ2 subunit. Mice with γ2 subunit F77I point mutation (γ2I77 mouse line) lose the high-affinity nanomolar binding of these ligands as well as their most robust behavioral actions at low doses. Interestingly, the γ2I77 mice offer a tool to study the actions of these substances mediated via other possible binding sites of the GABA(A) receptor. In ligand autoradiographic experiments, we discovered in γ2I77 mouse brain sections a significant amount of residual non-γ2 subunit-dependent benzodiazepine site binding enriched to the striatum and septum. Zolpidem only weakly affected this residual binding at micromolar concentrations, and only a high zolpidem dose (≥ 40 mg/kg) caused sedation and deficits in motor coordination in γ2I77 mice. DMCM had an agonistic action through a secondary, low-affinity non-benzodiazepine binding site of the GABA(A) receptor in the forebrain of γ2I77 mice, and this drug also fully displaced the residual benzodiazepine-site labeling. In behavioral tests, a high dose (20mg/kg) of DMCM was sedative and modulated fear learning. DMCM, but not zolpidem, acted as an agonist in recombinant GABA(A) α1/6β3 receptors studied using ligand binding and electrophysiological assays. Our results highlight the less well-known actions of high doses of DMCM and zolpidem that are not mediated via the γ2 subunit-containing benzodiazepine site of the GABA(A) receptor.

Ro 15-4513 Antagonizes Alcohol-Induced Sedation in Mice Through αβγ2-type GABA(A) Receptors

Ethyl alcohol (ethanol) has many molecular targets in the nervous system, its potency at these sites being low compared to those of sedative drugs. This has made it difficult to discover ethanol's binding site(s). There are two putative binding sites at γ-aminobutyric acid (GABA) type A receptor subtypes for the proposed ethanol antagonist Ro 15-4513, the established γ2 subunit-dependent benzodiazepine site and the recently reported δ subunit-dependent Ro 15-4513/ethanol binding site. Here, we aimed at clarifying the in vivo role of Ro 15-4513 at these two sites. We found that the antagonism of ethanol actions by Ro 15-4513 in wildtype mice was dependent on the test: an open field test showed that light sedation induced by 1.5–1.8 g/kg ethanol was sensitive to Ro 15-4513, whereas several tests for ethanol-induced anxiolytic effects showed that the ethanol-induced effects were insensitive to Ro 15-4513. Antagonism of ethanol-induced sedation by Ro 15-4513 was unaffected in GABA_A receptor δ subunit knockout mice. By contrast, when testing the GABA_A receptor γ2 subunit F77I knock-in mouse line (γ2I77 mice) with its strongly reduced affinity of the benzodiazepine sites for Ro 15-4513, we found that the ethanol-induced sedation was no longer antagonized by Ro 15-4513. Indeed, γ2I77 mice had only a small proportion of high-affinity binding of [³H]Ro 15-4513 left as compared to wildtype mice, especially in the caudate–putamen and septal areas, but these residual sites are apparently not involved in ethanol antagonism. In conclusion, we found that Ro 15-4513 abolished the sedative effect of ethanol by an action on γ2 subunit-dependent benzodiazepine sites.

A53T-alpha-synuclein overexpression impairs dopamine signaling and striatal synaptic plasticity in old mice

Background

Parkinson's disease (PD), the second most frequent neurodegenerative disorder at old age, can be caused by elevated expression or the A53T missense mutation of the presynaptic protein alpha-synuclein (SNCA). PD is characterized pathologically by the preferential vulnerability of the dopaminergic nigrostriatal projection neurons.

Methodology/Principal Findings

Here, we used two mouse lines overexpressing human A53T-SNCA and studied striatal dysfunction in the absence of neurodegeneration to understand early disease mechanisms. To characterize the progression, we employed young adult as well as old mice. Analysis of striatal neurotransmitter content demonstrated that dopamine (DA) levels correlated directly with the level of expression of SNCA, an observation also made in SNCA-deficient (knockout, KO) mice. However, the elevated DA levels in the striatum of old A53T-SNCA overexpressing mice may not be transmitted appropriately, in view of three observations. First, a transcriptional downregulation of the extraneural DA degradation enzyme catechol-ortho-methytransferase (COMT) was found. Second, an upregulation of DA receptors was detected by immunoblots and autoradiography. Third, extensive transcriptome studies via microarrays and quantitative real-time RT-PCR (qPCR) of altered transcript levels of the DA-inducible genes Atf2, Cb₁, Freq, Homer1 and Pde7b indicated a progressive and genotype-dependent reduction in the postsynaptic DA response. As a functional consequence, long term depression (LTD) was absent in corticostriatal slices from old transgenic mice.

Conclusions/Significance

Taken together, the dysfunctional neurotransmission and impaired synaptic plasticity seen in the A53T-SNCA overexpressing mice reflect early changes within the basal ganglia prior to frank neurodegeneration. As a model of preclinical stages of PD, such insights may help to develop neuroprotective therapeutic approaches.

Preliminary in vivo and ex vivo evaluation of the 5-HT2A imaging probe [(18)F]MH.MZ

INTRODUCTION

The 5-HT(2A) receptor is one of the most interesting targets within the serotonergic system because it is involved in a number of important physiological processes and diseases.

METHODS

[(18)F]MH.MZ, a 5-HT(2A) antagonistic receptor ligand, is labeled by (18)F-fluoroalkylation of the corresponding desmethyl analogue MDL 105725 with 2-[(18)F]fluoroethyltosylate ([(18)F]FETos). In vitro binding experiments were performed to test selectivity toward a broad spectrum of neuroreceptors by radioligand binding assays. Moreover, first micro-positron emission tomography (microPET) experiments, ex vivo organ biodistribution, blood cell and protein binding and brain metabolism studies of [(18)F]MH.MZ were carried out in rats.

RESULTS

[(18)F]MH.MZ showed a K(i) of 3 nM toward the 5-HT(2A) receptor and no appreciable affinity for a variety of receptors and transporters. Ex vivo biodistribution as well as microPET showed highest brain uptake at approximately 5 min p.i. and steady state after approximately 30 min p.i. While [(18)F]MH.MZ undergoes extensive first-pass metabolism which significantly reduces its bioavailability, it is insignificantly metabolized within the brain. The binding potential in the rat frontal cortex is 1.45, whereas the cortex to cerebellum ratio was determined to be 2.7 after approximately 30 min.

CONCLUSION

Results from microPET measurements of [(18)F]MH.MZ are in no way inferior to data known for [(11)C]MDL 100907 at least in rats. [(18)F]MH.MZ appears to be a highly potent and selective serotonergic PET ligand in small animals.

Synthesis of GABAA receptor agonists and evaluation of their alpha-subunit selectivity and orientation in the GABA binding site

Drugs used to treat various disorders target GABA A receptors. To develop alpha subunit selective compounds, we synthesized 5-(4-piperidyl)-3-isoxazolol (4-PIOL) derivatives. The 3-isoxazolol moiety was substituted by 1,3,5-oxadiazol-2-one, 1,3,5-oxadiazol-2-thione, and substituted 1,2,4-triazol-3-ol heterocycles with modifications to the basic piperidine substituent as well as substituents without basic nitrogen. Compounds were screened by [(3)H]muscimol binding and in patch-clamp experiments with heterologously expressed GABA A alpha ibeta 3gamma 2 receptors (i = 1-6). The effects of 5-aminomethyl-3 H-[1,3,4]oxadiazol-2-one 5d were comparable to GABA for all alpha subunit isoforms. 5-piperidin-4-yl-3 H-[1,3,4]oxadiazol-2-one 5a and 5-piperidin-4-yl-3 H-[1,3,4]oxadiazol-2-thione 6a were weak agonists at alpha 2-, alpha 3-, and alpha 5-containing receptors. When coapplied with GABA, they were antagonistic in alpha 2-, alpha 4-, and alpha 6-containing receptors and potentiated alpha 3-containing receptors. 6a protected GABA binding site cysteine-substitution mutants alpha 1F64C and alpha 1S68C from reacting with methanethiosulfonate-ethylsulfonate. 6a specifically covalently modified the alpha 1R66C thiol, in the GABA binding site, through its oxadiazolethione sulfur. These results demonstrate the feasibility of synthesizing alpha subtype selective GABA mimetic drugs.

Enhanced behavioral sensitivity to the competitive GABA agonist, gaboxadol, in transgenic mice over-expressing hippocampal extrasynaptic alpha6beta GABA(A) receptors

The behavioral and functional significance of the extrasynaptic inhibitory GABA(A) receptors in the brain is still poorly known. We used a transgenic mouse line expressing the GABA(A) receptor alpha6 subunit gene in the forebrain under the Thy-1.2 promoter (Thy1alpha6) mice ectopically expressing alpha6 subunits especially in the hippocampus to study how extrasynaptically enriched alphabeta(gamma2)-type receptors alter animal behavior and receptor responses. In these mice extrasynaptic alpha6beta receptors make up about 10% of the hippocampal GABA(A) receptors resulting in imbalance between synaptic and extrasynaptic inhibition. The synthetic GABA-site competitive agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; 3 mg/kg) induced remarkable anxiolytic-like response in the light : dark exploration and elevated plus-maze tests in Thy1alpha6 mice, while being almost inactive in wild-type mice. The transgenic mice also lost quicker and for longer time their righting reflex after 25 mg/kg gaboxadol than wild-type mice. In hippocampal sections of Thy1alpha6 mice, the alpha6beta receptors could be visualized autoradiographically by interactions between gaboxadol and GABA via [(35)S]TBPS binding to the GABA(A) receptor ionophore. Gaboxadol inhibition of the binding could be partially prevented by GABA. Electrophysiology of recombinant GABA(A) receptors revealed that GABA was a partial agonist at alpha6beta3 and alpha6beta3delta receptors, but a full agonist at alpha6beta3gamma2 receptors when compared with gaboxadol. The results suggest strong behavioral effects via selective pharmacological activation of enriched extrasynaptic alphabeta GABA(A) receptors, and the mouse model represents an example of the functional consequences of altered balance between extrasynaptic and synaptic inhibition.

Does ethanol act preferentially via selected brain GABAA receptor subtypes? the current evidence is ambiguous

In rodent models, gamma-aminobutyric acid A (GABAA) receptors with the alpha6 and delta subunits, expressed in the cerebellar and cochlear nucleus granule cells, have been linked to ethanol sensitivity and voluntary ethanol drinking. Here, we review the findings. When considering both in vivo contributions and data on cloned receptors, the evidence for direct participation of the alpha6-containing receptors to increased ethanol sensitivity is poor. The alpha6 subunit-knockout mouse lines do not have any changed sensitivity to ethanol, although these mice do display increased benzodiazepine sensitivity. However, in general the compensations occurring in knockout mice (regardless of which particular gene is knocked out) tend to fog interpretations of drug actions at the systems level. For example, the alpha6 knockout mice have increased TASK-1 channel expression in their cerebellar granule cells, which could influence sensitivity to ethanol in the opposite direction to that obtained with the alpha6 knockouts. Indeed, TASK-1 knockout mice are more impaired than wild types in motor skills when given ethanol; this might explain why GABAA receptor alpha6 knockout mice have unchanged ethanol sensitivities. As an alternative to studying knockout mice, we examined the claimed delta subunit-dependent/gamma2 subunit-independent ethanol/[3H]Ro 15-4513 binding sites on GABAA receptors. We looked at [3H]Ro 15-4513 binding in HEK 293 cell membrane homogenates containing rat recombinant alpha6/4beta3delta receptors and in mouse brain sections. Specific high-affinity [3H]Ro 15-4513 binding could not be detected under any conditions to the recombinant receptors or to the cerebellar sections of gamma2(F77I) knockin mice, nor was this binding to brain sections of wild-type C57BL/6 inhibited by 1-100 mM ethanol. Since ethanol may act on many receptor and channel protein targets in neuronal membranes, we consider the alpha6 (and alpha4) subunit-containing GABAA receptors unlikely to be directly responsible for any major part of ethanol's actions. Therefore, we finish the review by discussing more generally alcohol and GABAA receptors and by suggesting potential future directions for this research.

The novel anxiolytic ELB139 displays selectivity to recombinant GABAA receptors different from diazepam

A chemically heterogeneous group of compounds acts at the benzodiazepine (BZ) recognition site of the diverse gamma-aminobutyric acid type A (GABA(A)) receptor complexes which can assemble from more than 16 known subunits. Most 1,4-BZs like diazepam recognize all GABA(A)/BZ receptors containing the alpha1-3 or alpha5 together with any beta and the gamma2 subunit. Other compounds differentiate less, e.g. Ro15-4513, that additionally recognizes alpha4- and a6-containing receptors, or differentiate more, e.g. zolpidem, that recognizes preferentially alpha1-containing receptors. Here we describe the functional properties of 1-(4-chloro-phenyl)-4-piperidin-1-yl-1,5-dihydro-imidazol-2-on (ELB139) in the presence and absence of the BZ receptor antagonist flumazenil (Ro15-1788) on recombinant alphaibeta2gamma2 (i=1-5) receptor subtypes expressed in HEK 293 cells. The properties were measured with the whole-cell variation of the patch-clamp technique and compared to those of diazepam. Like the latter, ELB139 did not potentiate GABA-induced currents in alpha4-containing receptors, but it displays functional subtype specificity between alpha1, alpha2, alpha3, and alpha5beta2gamma2 receptors with highest potency in alpha3-containing receptors but highest efficacy in alpha1- or alpha2-containing receptors, respectively. ELB139 acted as a partial agonist on these receptor subtypes reaching 40-50% of the efficacy of diazepam.

Evidence for a reduction of coupling between GABAA receptor agonist and ionophore binding sites by inorganic phosphate

[35S]TBPS binding to the GABAA receptor ionophore binding site is anion dependent. Using autoradiography on rat brain sections, we show that permeabilities of anions through the receptor channel correlate with their efficiencies to promote basal [35S]TBPS binding. Phosphate made an exception as it induced more binding than expected from its permeability. Well-permeable anions (chloride, nitrate, formate) allowed [35S]TBPS binding to be effectively displaced by 1 mM GABA, whereas low-permeable anions (acetate, phosphate, propionate) markedly prevented this GABA effect, especially in the thalamus, the transition from the high to the low GABA effect being between formate and acetate. In the presence of phosphate, GABA enhanced [3H]flunitrazepam binding to benzodiazepine site of recombinant alpha1beta2gamma2 receptors with the same efficacy but lower potency as compared to the presence of chloride, whereas [35S]TBPS binding was abnormally modulated by GABA. These results suggest that inorganic phosphate affects coupling between agonist and ionophore sites in GABAA receptors.

Autoradiographic imaging of altered synaptic alphabetagamma2 and extrasynaptic alphabeta GABAA receptors in a genetic mouse model of anxiety

To image the possible alterations in brain regional GABAA receptor subtype properties in a genetic animal model of human anxiety, mice heterozygous for the deletion of GABAA receptor gamma2 subunit (gamma2+/-) were studied using ligand autoradiographic assays on brain cryostat sections. The [35S]TBPS binding assay was designed to reveal impaired GABA and channel site coupling shown to be more prominent in recombinant alpha1/6beta3 than in alpha1/2beta3gamma2 or beta2 subunit-containing GABAA receptors expressed in HEK 293 cells. Increased GABA-insensitive [35 S]TBPS binding in the gamma2+/- mouse brains was evident in the cerebral cortex and in subcortical regions, the alterations being regionally similar to the loss of gamma2 subnunit-dependent benzodiazepine (BZ) sites as revealed by [3H]Ro 15-4513 autoradiography. As the gamma2 subunit protein is needed for synaptic clustering of GABAA receptors, these results indicate that the extrasynaptic alphabeta3 receptors can be visualized in vitro as atypical GABA-insensitive [35S]TBPS binding sites. The results suggest that GABAAergic synaptic inhibition is widely decreased in the brains of anxiety-prone gamma2+/- mice, while extrasynaptic GABAA receptors are increased. These autoradiographic imaging findings further demonstrate the need to develop GABAA receptor subtype-selective in vivo ligands to aid in assessing the contributions of various subcellular receptor populations in anxious and other patient groups.

Four amino acids in the alpha subunits determine the gamma-aminobutyric acid sensitivities of GABAA receptor subtypes

GABA(A) receptors, mediators of fast inhibitory neurotransmission, are heteropentameric assemblies from a large array of subunits. Differences in the sensitivity of receptor subtypes to endogenous GABA may permit subunit-dependent finely tuned responsiveness to the same GABAergic inputs. Using both radioligand binding and electrophysiology combined with mutagenesis, we identified a domain of four amino acids within the alpha subunits that mediates the distinct sensitivities to GABA allowing their selective switch between alphabeta3gamma2 combinations. Replacing this domain in alpha3 by the corresponding segments of alpha1-alpha5 resulted in mutant receptors displaying the GABA EC(50) values of the respective wild-type receptors. Vice versa, the alpha3 motif forced the low sensitivity to GABA of alpha3 upon alpha1beta3gamma2, alpha4beta3gamma2, and alpha5beta3gamma2. Binding of the GABA agonist [(3)H]muscimol was not affected by the exchange of the motif between alpha1 and alpha3 subunits. Thus, the equilibrium binding pocket is maintained upon replacement of the four amino acids. Taken together our data suggest that the identified motifs contribute to a structure involved in the transduction of the binding signal rather than to the binding itself.

INCREASED CEREBELLAR PET GLUCOSE METABOLISM CORRESPONDS TO ATAXIA IN WERNICKE-KORSAKOFF SYNDROME

AIMS

To investigate a possible relationship between cerebellar glucose metabolism and recovery from ataxia in the first months of acute Wernicke-Korsakoff syndrome.

METHODS

Two cases of alcoholic Wernicke-Korsakoff syndrome were followed up with the clinical status and cerebral glucose metabolism over a 4- and 9-month period.

RESULTS

Initially both patients showed severe ataxia and elevated cerebellar glucose metabolism that decreased corresponding to the restitution of stance and gait.

CONCLUSION

Increased cerebellar glucose metabolism at the onset of the illness may reflect the reorganization process of disturbed motor skills and may indicate cerebellar plasticity.

Receptor subtype-dependent positive and negative modulation of GABA(A) receptor function by niflumic acid, a nonsteroidal anti-inflammatory drug

In addition to blocking cyclooxygenases, members of the fenamate group of nonsteroidal anti-inflammatory drugs have been proposed to affect brain GABAA receptors. Using quantitative autoradiography with GABAA receptor-associated ionophore ligand [35S]t-butylbicyclophosphorothionate (TBPS) on rat brain sections, one of the fenamates, niflumate, at micromolar concentration was found to potentiate GABA actions in most brain areas, whereas being in the cerebellar granule cell layer an efficient antagonist similar to furosemide. With recombinant GABAA receptors expressed in Xenopus laevis oocytes, we found that niflumate potentiated 3 microM GABA responses up to 160% and shifted the GABA concentration-response curve to the left in alpha1beta2gamma2 receptors, the predominant GABAA receptor subtype in the brain. This effect needed the gamma2 subunit, because on alpha1beta2 receptors, niflumate exhibited solely an antagonistic effect at high concentrations. The potentiation was not abolished by the specific benzodiazepine site antagonist flumazenil. Niflumate acted as a potent antagonist of alpha6beta2 receptors (with or without gamma2 subunit) and of alphaXbeta2gamma2 receptors containing a chimeric alpha1 to alpha6 subunit, which suggests that niflumate antagonism is dependent on the same transmembrane domain 1- and 2-including fragment of the alpha6 subunit as furosemide antagonism. This antagonism was noncompetitive because the maximal GABA response, but not the potency, was reduced by niflumate. These data show receptor subtype-dependent positive and negative modulatory actions of niflumate on GABAA receptors at clinically relevant concentrations, and they suggest the existence of a novel positive modulatory site on alpha1beta2gamma2 receptors that is dependent on the gamma2 subunit but not associated with the benzodiazepine binding site.

Oxytocin regulates neurosteroid modulation of GABA(A) receptors in supraoptic nucleus around parturition

In this study, we investigate how neurosteroid sensitivity of GABA(A) receptors (GABA(A)Rs) is regulated. We examined this issue in neurons of the supraoptic nucleus (SON) of the rat and found that, during parturition, the GABA(A)Rs become insensitive to the neurosteroid allopregnanolone attributable to a shift in the balance between the activities of endogenous Ser/Thr phosphatase and PKC. In particular, a constitutive endogenous tone of oxytocin within the SON after parturition suppressed neurosteroid sensitivity of GABA(A)Rs via activation of PKC. Vice versa before parturition, during late pregnancy, application of exogenous oxytocin brings the GABA(A)Rs from a neurosteroid-sensitive mode toward a condition in which the receptors are not sensitive. This indicates that there may be an inverse causal relationship between the extent to which the GABA(A)R or one of its interacting proteins is phosphorylated and the neurosteroid sensitivity of the GABA(A)R. Neurosteroid sensitivity was not affected by changes in subunit composition of GABA(A)Rs known to occur concurrently in these cells.

Organotypic rat cerebellar slice culture as a model to analyze the molecular pharmacology of GABAA receptors

The preservation of the neuronal circuitry in rat cerebellar slice cultures provides an advantage in monitoring the development and characterizing the pharmacology of GABA(A) receptor subtypes. Sprague-Dawley rats, 8-11 days of age, were decapitated, their cerebella were cut into 400-microm slices and transferred into culture dishes. Cell viability and organotypic cerebellar organization of the culture remained well preserved up to 3 weeks. Autoradiographic procedures were introduced in these advanced culture technique and employed [(3)H]Ro 15-4513 in the absence and presence of 10 microM diazepam to visualize all benzodiazepine (BZD) and diazepam-insensitive (DIS) binding sites, respectively. Since expression of the alpha6 subunit variant of the GABA(A)/BZD receptor is restricted to the cerebellar granule cells and the BZD receptor agonist diazepam has very low affinity for this subunit, changes in DIS [(3)H]Ro 15-4513 binding sites during cultivation time can be attributed to changes in alpha6 subunit expression. A time-dependent development of total and DIS [(3)H]Ro 15-4513 binding sites were observed in the culture with a trend towards an increase in GABA(A) receptor alpha6 subunit levels during the first week. These findings suggest that explant preparations can be used to examine morphological changes in rat cerebellar slices. In addition, these preparations can be utilized to study the pharmacological effects of GABA(A)/BZD selective drugs on postnatal development of GABA(A) receptors in rat cerebellum.

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.

Altered receptor subtypes in the forebrain of GABA(A) receptor delta subunit-deficient mice: recruitment of gamma 2 subunits

A GABA(A) receptor delta subunit-deficient mouse line was created by homologous recombination in embryonic stem cells to investigate the role of the subunit in the brain GABA(A) receptors. High-affinity [(3)H]muscimol binding to GABA sites as studied by ligand autoradiography was reduced in various brain regions of delta(-/-) animals. [(3)H]Ro 15-4513 binding to benzodiazepine sites was increased in delta(-/-) animals, partly due to an increment of diazepam-insensitive receptors, indicating an augmented forebrain assembly of gamma 2 subunits with alpha 4 subunits. In the western blots of forebrain membranes of delta(-/-) animals, the level of gamma 2 subunit was increased and that of alpha 4 decreased, while the level of alpha1 subunits remained unchanged. In the delta(-/-) forebrains, the remaining alpha 4 subunits were associated more often with gamma 2 subunits, since there was an increase in the alpha 4 subunit level immunoprecipitated by the gamma 2 subunit antibody. The pharmacological properties of t-butylbicyclophosphoro[(35)S]thionate binding to the integral ion-channel sites were slightly altered in the forebrain and cerebellum, consistent with elevated levels of alpha 4 gamma 2 and alpha 6 gamma 2 subunit-containing receptors, respectively.The altered pharmacology of forebrain GABA(A) receptors and the decrease of the alpha 4 subunit level in delta subunit-deficient mice suggest that the delta subunit preferentially assembles with the alpha 4 subunit. The delta subunit seems to interfere with the co-assembly of alpha 4 and gamma 2 subunits and, therefore, in its absence, the gamma 2 subunit is recruited into a larger population of alpha 4 subunit-containing functional receptors. These results support the idea of subunit competition during the assembly of native GABA(A) receptors.

Anxiolytic-like effects of acute and chronic GABA transporter inhibition in rats

Acute GABA transporter inhibition can induce anxiolytic-like behaviors. The present analysis addressed whether chronic treatment (23 days via drinking water) with a GABA transporter inhibitor affects rat behavior similar to acute treatment and interferes with additional benzodiazepine-receptor agonistic treatment. Seventy-one rats divided into seven groups were acutely treated with either vehicle, diazepam (2 mg/kg), zolpidem (0.05 mg/kg), tiagabine (19 mg/kg) or chronically with tiagabine with or without acute diazepam or zolpidem. Animals were behaviorally characterized in an elevated plus-maze. None of the treatments induced changes in the activity of the animals. Acute and chronic treatment with tiagabine induced anxiolytic-like effects, similar to acute doses of diazepam. Acute diazepam did not enhance chronic tiagabine effects, whereas acute zolpidem attenuated the anxiolytic-like effects of chronic tiagabine. It is concluded that anxiolytic effects of acute GABA-uptake inhibition by tiagabine persist under chronic treatment and are sensitive to concomitant use of benzodiazepine receptor ligands.

Pharmacological heterogeneity of gamma-aminobutyric acid receptors during development suggests distinct classes of rat cerebellar granule cells in situ

The gamma-aminobutyric acid receptor (GABA(A)R) represents a ligand-gated Cl(-)-channel assembling as heteropentamere from 19 known subunits. Cerebellar granule cells contain a unique subset, namely the alpha1-, alpha6-, beta2-, gamma2- and delta-subunits. We studied their GABAergic pharmacology in situ using whole-cell patch-clamp recordings in brain slices and a modified Y-tube application system. The distribution of the EC50s for GABA in young (P8-P14) and medium aged animals (P15-P28) could be fitted with the sum of two Gaussian distributions with means of 60 and 185 microM and 27 and 214 microM, respectively. In older animals (P29-P48) the observed homogeneous range of sensitivities fitted a single Gaussian distribution (11 microM). In young animals (< or =P14) GABA-responses were largely insensitive towards 300 microM of the alpha6-specific inhibitor furosemide (82% of control response). The sensitivity increased in older animals at the EC5-20 of GABA (31% of control responses), supporting an increased expression of alpha6-subunits as molecular basis for the observed developmental changes. Approximately 50% of cells in the age range P15-P48 were potentiated by 1 microM diazepam and by 3 microM methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), suggesting the concurrent presence of alpha1- and alpha6-containing receptors, whereas the remaining of cells were neither potentiated by diazepam nor did they show the alpha6-typical DMCM potentiation, though they were potentiated by loreclezole. These properties indicate unknown pharmacological characteristics of cerebellar receptor-subunit combinations in approximately 50% of granule cells in situ.

[18F]Fluoroethylflumazenil: a novel tracer for PET imaging of human benzodiazepine receptors

5-(2'-[18F]Fluoroethyl)flumazenil ([18F]FEF) is a fluorine-18 labelled positron emission tomography (PET) tracer for central benzodiazepine receptors. Compared with the established [11C]flumazenil, it has the advantage of the longer half-life of the fluorine-18 label. After optimisation of its synthesis and determination of its in vitro receptor affinities, we performed first PET studies in humans. PET studies in seven healthy human volunteers were performed on a Siemens ECAT EXACT whole-body scanner after injection of 100-280 MBq [L8F]FEF. In two subjects, a second PET scan was conducted after pretreatment with unlabelled flumazenil (1 mg or 2.5 mg i.v., 3 min before tracer injection). A third subject was studied both with [18F]FEF and with [11C]flumazenil. Brain radioactivity was measured for 60-90 min p.i. and analysed with a region of interest-oriented approach and on a voxelwise basis with spectral analysis. Plasma radioactivity was determined from arterial blood samples and metabolites were determined by high-performance liquid chromatography. In human brain, maximum radioactivity accumulation was observed 4 +/- 2 min p.i., with a fast clearance kinetics resulting in 50% and 20% of maximal activities at about 10 and 30 min, respectively. [18F]FEF uptake followed the known central benzodiazepine receptor distribution in the human brain (occipital cortex >temporal cortex >cerebellum >thalamus >pons). Pretreatment with unlabelled flumazenil resulted in reduced tracer uptake in all brain areas except for receptor-free reference regions like the pons. Parametric images of distribution volume and binding potential generated on a voxelwise basis revealed two- to three-fold lower in vivo receptor binding of [18F]FEF compared with [11C]flumazenil, while relative uptake of [18F]FEF was higher in the cerebellum, most likely owing to its relatively higher affinity for benzodiazepine receptors containing the alpha6 subunit. Metabolism of [18F]FEF was very rapid. Polar metabolites represented about 50%-60% of total plasma radioactivity at 5 min and 80%-90% at 20 min p.i. Although [11C]flumazenil has some advantages over [18F]FEF (higher affinity, slower metabolism, slower kinetics), our results indicate that [18F]FEF is a suitable PET ligand for quantitative assessment of central benzodiazepine receptors, which can be used independently of an on-site cyclotron.

The inhibitory neural circuitry as target of antiepileptic drugs

Impairments and defects in the inhibitory neurotransmission in the CNS can contribute to various seizure disorders, i.e., gamma-aminobutyric acid (GABA) and glycine as the main inhibitory neurotransmitters in the brain play a crucial role in some forms of epilepsy. Recent advances in deciphering the molecular basis of the GABAergic and glycinergic systems has been achieved by means of cloning techniques and gene targeting strategies in animals, contributing to the understanding of drug action. As well, several anticonvulsive substances emerged which target key molecules of the inhibitory systems. Employment of recombinant expression systems, including, but not restricted to the inhibitory circuitry, will further facilitate drug screening and rational approaches to design novel specific antiepileptic drugs, which act highly efficiently to prevent or reduce generation and spread of seizures.

Magnesium potentiation of the function of native and recombinant GABA(A) receptors

Mg2+ decreased basal and GABA-inhibited t-butylbicyclophosphoro[35S]thionate binding to GABAA receptor ion channels in rat brain sections up to 1 mM, but increased the binding at 10 mM. The Mg2+-effect was detectable in the presence of a specific GABA site competitive antagonist. Two-electrode voltage clamp recordings of recombinant alpha1beta2gamma2S, alpha1beta2, alpha2beta2gamma2S and alpha2beta2 GABAA receptors revealed a potentiation by 0.1-1 mM Mg2+ of EC20 GABA-evoked ion currents. At 10 mM, Mg2+ decreased the currents. In the absence of GABA, Mg2+ did not evoke any currents. The results show that physiologically relevant Mg2+ concentrations affect the GABA responses on GABAA receptors in native and the main recombinant receptor subtypes, suggesting putative Mg2+ binding sites on the receptor complex.

Behavioral analysis indicates benzodiazepine-tolerance mediated by the benzodiazepine binding-site at the GABA(A)-receptor

1. GABA(A)-receptor induced changes in locomotion and anxiety-like behaviors were studied in rats using an open-field and an elevated plus-maze. Acute and chronic doses of the benzodiazepine diazepam without and in combination with the GABA uptake inhibitor SKF-89976A were investigated. 2. Fifty-six male rats of the strain PVG/OlaHsd (PVG; 180-200 g body wt) were used to assess the influence of the benzodiazepine binding-site to the development of tolerance. Rats were divided into six groups: The first receiving saline (0.9%), the second and third diazepam (10.0 mg/kg) daily for 23 days with or without an acute challenge of 2.0 mg/kg diazepam. The fourth group received diazepam (10.0 mg/kg) daily and acutely SKF-89976A (15.0 mg/kg) plus diazepam and the fifth and sixth group received acute treatment with diazepam (2.0 mg/kg) or SKF-89976A (15.0 mg/kg). 3. Under chronic treatment with diazepam the animals became tolerant to acute doses of diazepam in activity and anxiety-related behaviors. Acute treatment with SKF-89976A increased exploration. Parameters expressing anxiolytic-like behaviors were increased, too, but not all of them significantly. In diazepam tolerant animals SKF-89976A produced anxiolytic-like behaviors 4. We conclude that the BZ- and not the GABA-binding site at the GABA(A)-receptor is involved in the development of BZ-tolerance.

Characterization of gamma-aminobutyrate type A receptors with atypical coupling between agonist and convulsant binding sites in discrete brain regions

Gamma-ainobutyric acid type A (GABA(A)) receptor ionophore ligand t-[35S]butylbicyclophosphorothionate ([35S]TBPS) was used in an autoradiographic assay on brain cryostat sections to visualize and characterize atypical GABA-insensitive [35S]TBPS binding previously described in certain recombinant GABA(A) receptors and the cerebellar granule cell layer. Picrotoxinin-sensitive but 1-mM GABA-insensitive [35S]TBPS binding was present in the rat cerebellar granule cell layer, many thalamic nuclei, subiculum and the internal rim of the cerebral cortex, amounting in these regions up to 6% of the basal binding determined in the absence of exogenous GABA. Similar binding properties were detected also in human and chicken brain sections. Like the GABA-sensitive [35S]TBPS binding, GABA-insensitive binding was profoundly decreased by pentobarbital, pregnanolone, loreclezole and Mg2+. The binding was reversible and apparently dependent on Cl- ions. Localization of the GABA-insensitive [35S]TBPS binding was not identical to that of high-affinity [3H]muscimol binding and diazepam-insensitive [3H]Ro 15-4513 binding, two previously established receptor subtype-dependent binding heterogeneities in the rat brain. The present study reveals a component of the GABA-ionophore enriched in the thalamus and cerebellar granule cells, possibly representing poorly desensitized or desensitizing receptors.

Altered atypical coupling of gamma-aminobutyrate type A receptor agonist and convulsant binding sites in subunit-deficient mouse lines

We searched for subunit correlations for GABA(A) receptor-associated atypically GABA-insensitive [35S]TBPS binding. The homomeric beta3 subunit receptors could be excluded, as GABA-insensitive [35S]TBPS binding was present in beta3-/- mice. Localization of GABA-insensitive [35S]TBPS binding correlated best with those of delta, alpha4 and alpha6 subunit mRNAs. The amounts of GABA-insensitive [35S]TBPS binding components were increased in delta-/- mice, but dramatically reduced in alpha6-/- mice, suggesting a role for alpha6 but excluding delta subunits.

Assembly of functional alpha6beta3gamma2delta GABA(A) receptors in vitro

Transgenic mice deficient in the alpha6 subunit of the GABA(A) receptor show reduced levels of the delta subunit protein and an altered GABA(A) receptor pharmacology, suggesting selective assembly mechanisms. Delta reduced the binding of [3H]Ro15-4513 or t-butylbicyclophosphoro[35S]thionate and, to a lesser extent, [3H]muscimol to recombinant alpha1beta1gamma2(delta), alpha4beta1gamma2(delta) and alpha6beta1gamma2(delta) receptors, paralleled by diminished GABA-evoked maximal currents in electrophysiological recordings for the latter one. The delta subunit gave rise to a lower EC50 for GABA and a slowed desensitization indicating its assembly in alpha6beta2delta, alpha6beta1gamma2delta and alpha6beta2gamma2delta receptors. The data show that the delta subunits assemble in various functional GABA(A) receptor subtypes in vitro to reduce GABA-evoked maximal currents and ligand binding, but increase the potency for GABA.

Coupling between agonist and chloride ionophore sites of the GABA(A) receptor: agonist/antagonist efficacy of 4-PIOL

Eight gamma-aminobutyric acid (GABA) mimetics were tested on their ability to differentiate native GABA(A) receptor subtypes present in various rat brain regions. In rat brain cryostat sections, little regional variations by the agonistic actions of muscimol, thiomuscimol, 4,5,6,7-tetrahydroisoazolo(5,4-c)pyridin-3-ol, piperidine-4-sulphonic acid, taurine and beta-alanine on [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to GABA(A) receptor channels were found. They were very similar to those found for GABA itself and indicated no direct correlation with single subunit distributions for any of these compounds. Only the low-efficacy GABA mimetic 5-(4-piperidyl)isoxazol-3-ol (4-PIOL) acted like a weak partial agonist or antagonist depending on the brain area. As the cerebellar granule cell layer was relatively insensitive to both modes of action, we tested 4-PIOL in recombinant alpha1beta2gamma2 (widespread major subtype) and alpha6beta2gamma2 (cerebellar granule cell restricted) receptors where it had different effects on GABA-modulated [35S]TBPS binding and on electrophysiological responses. 4-PIOL may thus serve as a potential lead for receptor subtype selective compounds.

Neonatal 6-hydroxydopamine treatment affects GABA(A) receptor subunit expression in the frontal cortex but not the hippocampus of rats during postnatal development

The influence of neonatal administration of 6-hydroxydopamine (6-OHDA) on the maturation of GABA(A) receptors in the frontal cortex and hippocampus was studied using 5- to 40-day-old rats. In situ hybridization with antisense oligonucleotide probes was performed for alpha(1), alpha(2), alpha(5), beta(2), beta(3) and gamma(2) subunit mRNAs of the GABA(A) receptor. We demonstrated that neonatal treatment with 6-OHDA temporarily delays the postnatal transcription of the alpha(1) and gamma(2) subunits in the rat prefrontal cortex, as assessed by in situ hybridization histochemistry. The effect was selective for these subunits (the alpha(2), alpha(5), beta(2), and beta(3) subunit mRNAs remained unchanged) and for this region (the mRNA levels in the hippocampus were not changed). The reduction in mRNA levels at early postnatal stages (postnatal day 5, PD5, and PD10) also affected the subunit protein levels, as shown by immunohistochemistry for the alpha(1) subunit, and the formation of GABA(A) receptor-associated picrotoxinin-insensitive TBPS binding sites, as shown by autoradiography. Our findings indicate that without a noradrenergic influence, the maturation of GABAergic interneurons in the frontal cortex is transiently delayed (from PD5 to PD40). However, it is possible that this transient reduction of the expression of certain GABA subunits - caused by depletion of noradrenergic innervation - cannot cause a lasting alteration to the GABAergic function in the prefrontal cortex.

Neonatal 6-hydroxydopamine treatment affects GABA(A) receptor subunit expression during postnatal development of the rat cerebellum

Neurotoxic elimination of noradrenergic terminals by 6-hydroxydopamine (6-OHDA) leads to alteration of the granule cell layer formation. We have studied the developmental expression of GABA(A) receptor subunits in rat cerebellum after neonatal administration of 6-OHDA during the first postnatal month of life. 6-OHDA was injected subcutaneously. The expression of GABA(A) receptor subunits was studied by in situ hybridization and immunohistochemistry. The alterations were observed in the neocerebellum - the part of the cerebellum which starts development postnatally. The migration of granule cells was delayed, and the total area of the granule cell layer in the neocerebellum from 6-OHDA-treated rats was reduced to 22.6+/-5% of the corresponding area from control rats. In situ hybridization with subunit-specific antisense oligonucleotide probes was performed for alpha1, alpha2, alpha3, alpha5, alpha6, beta1, beta2, gamma1 and gamma2 subunits of the GABA(A) receptor. In neocerebellum, 6-OHDA treatment caused a significant reduction in the alpha1, alpha6 and gamma2 subunit mRNA levels. The expression of the other subunits was not changed. It has been shown that in the postnatal cerebellum alpha1 and alpha6 subunits can be detected in granule cells only when the cells had migrated to their final destination. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of cerebellar granule cells.

Behavioral effects of GABA(A) receptor stimulation and GABA-transporter inhibition

The present analysis addressed behavioral changes after treatment with 4.5 mg/kg or 18.5 mg/kg of the GABA-uptake inhibitor tiagabine combined with either the benzodiazepine diazepam (1.5 mg/kg) or the imidazopyridine zolpidem (0.05 mg/kg), the latter two acting differentially on GABA(A) receptor subtypes. The study included 97 male PVG/OIaHsd rats. A standard open field, an enriched open field, and an elevated plus-maze was used to study rat behavior. Treatment with the low dose of tiagabine alone induced no specific behavioral effects, whereas the high dose had an anxiolytic-like potential. Furthermore, diazepam but not zolpidem displayed anxiolytic-like effects. Combination of each benzodiazepine receptor agonist with tiagabine at the low dose decreased explorative activity. Diazepam plus the high dose of tiagabine increased the activity in the open-field test. Zolpidem together with 18.5 mg/kg tiagabine had an angiogenic-like effect compared to pure tiagabine treatment. These results provide evidence for a pharmacodynamic interaction between the GABA-uptake inhibitor tiagabine and diazepam or zolpidem. The interaction might be relevant in the clinic when combining the anticonvulsant tiagabine and a benzodiazepine receptor agonist.

Quantification of GABA(A) receptor subunit mRNAs by non-radioisotopic competitive RT-PCR utilizing plate-based EIA methodology

We developed a non-radioisotopic quantitative competitive RT-PCR method for the measurement of gamma-aminobutyric acid (GABA) type A receptor subunit mRNA levels. The specificity of the method was optimized by the use of four subunit-specific oligonucleotides in the sequential steps: reverse transcription, polymerase chain reaction (PCR), and detection. The biotinylated PCR products were bound on streptavidin-coated microtiter plates allowing detection of the products using dinitrophenyl (DNP)-labeled probes and anti-DNP alkaline phosphatase conjugate. The method was set up for the six major cerebellar GABA(A) receptor subunits: alpha1; alpha6; beta2; beta3; gamma2 and delta. The method is quantitative and rapid. With a large dynamic range from 10 fg to 1 ng of subunit mRNA, the accuracy was 12 and 19% (intra- and interassay coefficients of variation, respectively), which might be improved by using a smaller range of standards. The use of a double logarithmic standard curve [log (standard to competitor signal) vs. log (standard mRNA originally present)] requires only one reaction from each sample, allowing the analysis of a large number of samples in one experiment.

Furosemide action on cerebellar GABA(A) receptors in alcohol-sensitive ANT rats

Furosemide increases the basal tert-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding and reverses the inhibition of the binding by gamma-aminobutyric acid (GABA) in the cerebellar GABA(A) receptors containing the alpha6 and beta2/beta3 subunits. These effects are less pronounced in the alcohol-sensitive (ANT) than in the alcohol-insensitive (AT) rat line. The difference between the rat lines in the increase of basal [35S]TBPS binding was removed after a longer preincubation with ethylendiaminetetraacetic acid (EDTA) containing buffer, but long preincubation did not reduce the GABA content of the incubation fluid or remove the difference in GABA antagonism by furosemide. The GABA sensitivity of the [35S]TBPS binding did not differ between the rat lines. There was no nucleotide sequence difference in the beta2 or beta3 subunits between the rat lines and similar beta2/3 subunit-dependent agonistic actions by methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) in the rat lines were detected. The data suggest that there are still unknown structural alterations in the cerebellar GABA(A) receptors between the AT and ANT rat lines, possibly associated with differential alcohol sensitivity.

The main determinant of furosemide inhibition on GABA(A) receptors is located close to the first transmembrane domain

Inhibitory GABA(A) receptors are regulated by numerous allosteric modulators, the most receptor-subtype specific of which is furosemide. It recognises receptors of the subunit composition alpha6beta2/3gamma2, restricted to cerebellar granule cells. To locate furosemide's site of action we constructed chimeras of the furosemide-sensitive alpha6 and the furosemide-insensitive alpha1 subunit, and expressed and studied them together with the beta3 and gamma2 subunits in Xenopus oocytes by the two-electrode voltage clamp technique. The inhibition of GABA-induced currents by furosemide mainly depended on a short domain proximal to the first transmembrane region of the alpha6 subunit.

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.

Structure-activity relationship of furosemide-derived compounds as antagonists of cerebellum-specific GABA(A) receptors

The Na+-K+-2Cl- cotransporter blocker furosemide inhibits gamma-aminobutyric acid (GABA)-gated chloride currents and reverses GABA-mediated inhibition of [35S]-t-butylbicyclophosphorothionate ([35S]TBPS) binding of the cerebellar alpha6 subunit-containing GABA(A) receptors much more potently than the cerebrocortical non-alpha6 subunit-containing receptors. Of the 44 compounds studied, all precursors or derivatives of diuretics, one compound [hydrazinosulfonyl-furosemide (PF 1885)] reversed 5-microM GABA-induced inhibition of [35S]TBPS binding to cerebellar and cerebrocortical receptors. Three other compounds, all of which are structurally closely related to furosemide, were selective antagonists for the cerebellar receptors comparable to the lead compound. Still, the diuretic and GABAergic structure-activity relationships differ, since we found potent diuretic structures lacking GABA antagonistic activity. Further development of the GABAergic potency of furosemide derivatives can now focus on the modification of the carboxyl group, replaceable by tetrazole but not by sulfonic or phosphinic acids and the furanyl moiety which could be substituted by thienyl and benzyl groups.

[Better cure for anxiety, sleep disorders, epilepsy?]

Gamma-aminobutyric acid (GABA), the most important inhibitory neurotransmitter, is characterised by rapid effects that are mediated via GABAA, receptors. These receptors are also targets for many drugs including benzodiazepines, barbiturates and general anaesthetics. Recognition of the heterogeneity of GABAA receptors has opened up new possibilities for the development of more selective therapeutic agents. In particular, subtype-specific receptor ligands varying in intrinsic activity are likely to give rise to fewer side effects than do currently available drugs.

GABA(A)-receptor subtypes: clinical efficacy and selectivity of benzodiazepine site ligands

The main inhibitory neurotransmitter receptor of the brain, the gamma-aminobutyric acid type A receptor (GABA[A]), mediates the actions of several classes of clinically important drugs, such as benzodiazepines, barbiturates and general anaesthetics. This review summarizes the current knowledge on how classical benzodiazepines and novel nonbenzodiazepine compounds act on the benzodiazepine site of GABA(A) receptors and on their clinical pharmacology related to anxiolytic, sedative, hypnotic and cognitive effects or side-effects. Partial agonism, receptor subtype selectivity and novel binding sites are discussed as possible strategies to develop new drugs with fewer adverse effects than are seen in the clinical use of benzodiazepines.

Furosemide interactions with brain GABAA receptors

1. The loop diuretic furosemide is known to antagonize the function of gamma-aminobutyric acid type A (GABAA) receptors. The purpose of the present study was to examine the direct interaction of furosemide with the GABAA receptors by autoradiography and ligand binding studies with native rat and human receptors and with recombinant receptors composed of rat subunits. 2. Autoradiography with [35S]-t-butylbicyclophosphorothionate ([35S]-TBPS) as a ligand indicated that furosemide (0.1-1 mM) reversed the 5 microM GABA-induced inhibition of binding only in the cerebellar granule cell layer of rat brain sections. In all other regions studied, notably also in the hippocampal and thalamic areas, furosemide failed to antagonize GABA. Furosemide 1 mM decreased [35S]-TBPS binding only in a limited number of brain regions, but facilitation of the GABA-inhibition of the binding was much more widespread. 3. In well-washed rat cerebellar, but not cerebrocortical, membranes, furosemide enhanced the [35S]-TBPS binding over basal level in the absence of added GABA. The GABAA antagonist, SR 95531, and the convulsant, Ro 5-4864, blocked this furosemide-induced increase. Both interactions with the furosemide enhancement are likely to be allosteric, since furosemide affected the binding of [3H]-SR 95531 and [3H]-Ro 5-4864 identically in the cerebellar and cerebrocortical membranes. Maximal GABA-antagonism induced by furosemide in cerebellar membranes was further increased by SR 95531 but not by Ro 5-4864, indicating additive antagonism only for SR 95531. In human cerebellar receptors, only GABA antagonism by furosemide, but not the enhancement without added GABA, was observed. 4. In recombinant GABAA receptors, furosemide antagonism of GABA-inhibition of [35S]-TBPS binding depended only on the presence of alpha 6 and beta 2/3 subunits, irrespective of the presence or absence of gamma 2 or delta subunits. 5. In alpha 6 beta 3 gamma 2 receptors, clozapine reversed the enhancement of [35S]-TBPS binding by furosemide in the absence of GABA. However, it failed to affect the GABA-antagonism of furosemide, suggesting that the enhancement of basal binding and the GABA antagonism might represent two different allosteric actions of furosemide. 6. In conclusion, the present results indicate that furosemide is a subtype-selective GABAA antagonist with a mode of action not shared by several other antagonists, which makes furosemide a unique compound for development of potential GABAA receptor subtype-specific and -selective ligands.