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H Lüddens

Publications and source records attributed to H Lüddens.

At least 37 records · Page 2Linked to original sources
PubMed1998

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.

Anesthetics↗
PubMed1997

[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.

Anti-Anxiety Agents↗
PubMed1997

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.

Animals↗
PubMed1997

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.

Benzodiazepines↗
PubMed1996

Effects of clozapine metabolites and chronic clozapine treatment on rat brain GABAA receptors.

Similarly to clozapine, a clozapine metabolite, N-desmethylclozapine, but not clozapine N-oxide, antagonized brain gamma-aminobutyric acid type A (GABAA) receptors at high micromolar concentrations. However, daily subcutaneous injections of clozapine (10 and 25 mg/kg) and haloperidol (0.5 mg/kg) for 14 days failed to alter the modulation by GABA of rat cerebrocortical and cerebellar benzodiazepine ([3H]flunitrazepam) or convulsant (t-[35S]bicyclophosphorothionate) binding sites of the GABAA receptor. The results thus suggest that the GABAA receptor antagonism exerted by chronic in vivo clozapine treatment is weak as compared to this treatment's actions on certain monoamine receptors and is unlikely to be involved in the therapeutic actions of clozapine.

Animals↗
PubMed1996

Blunted furosemide action on cerebellar GABAA receptors in ANT rats selectively bred for high alcohol sensitivity.

Furosemide specifically reverses the inhibition by gamma-aminobutyric acid (GABA) of t-[35S]-butylbicyclophosphorothionate ([35S]TBPS) binding and increases the basal [35S]TBPS binding to the cerebellar granule cell layer GABAA receptors. For the selectivity of furosemide, an interplay between GABAA receptor alpha 6 and beta 2 or beta 3 subunits is needed. We have now investigated the furosemide sensitivity of cerebellar [35S]TBPS binding in the alcohol-sensitive (ANT) rat line that harbors a pharmacologically critical point mutation in the alpha 6 subunit [alpha 6 (Q1000)], increasing benzodiazepine affinity of the normally insensitive alpha 6-containing receptors. ANT receptors were less efficiently affected by furosemide, while a normal GABAA receptor antagonism was observed with a specific GABAA receptor antagonist SR 95531. Reduced [3H]muscimol binding in ANT samples and small alterations in situ hybridization signals for alpha 1, alpha 6, beta 2, beta 3, gamma 2 and delta subunit mRNAs failed to correlate with impaired cerebellar furosemide efficacy in individual animals. The alpha 6 (q100) ANT mutation was not responsible for the reduced efficacy of furosemide in the ANT rat line, as judged from the potent furosemide antagonism in recombinant ANT-type alpha 6 (Q100)beta 3 gamma 2 receptors. This data suggest that presence of a novel abnormality in the structure and/or expression of alpha 6 subunit-containing GABAA receptors in the ANT rat line.

Animals↗
PubMed1996

Modulation of GABAA receptor tert-[35S]butylbicyclophosphorothionate binding by antagonists: relationship to patterns of subunit expression.

The multisubunit gamma-aminobutyric acid type A (GABAA) receptor is heterogeneous in molecular and pharmacological aspects. We used quantitative autoradiographic techniques to generate detailed pharmacological profiles for the binding of the GABAA-receptor ionophore ligand tert-[35S]butylbicyclophosphorothionate ([35S]TBPS) and its modulation by GABA and the GABAA antagonists bicuculline and 2'-(3'-carboxy-2',3'-propyl)-3-amino-6-p-methoxyphenylpyrazinium bromide (SR 95531). Regional differences in the actions of bicuculline and SR 95531 were correlated with the expression of 13 GABAA subunits in brain as reported previously. In some brain regions SR 95531 reduced [35S]TBPS binding much more than bicuculline, as illustrated by high ratios of bicuculline- to SR 95531-modulated [35S]TBPS binding. This ratio correlated positively with alpha 2-subunit mRNA levels. Binding that was equally affected by SR 95531 and bicuculline occurred prominently in regions with abundant alpha 1 mRNA expression. The present findings thus reveal a novel pharmacological heterogeneity based on differences between alpha 1 and alpha 2 subunit-containing GABAA receptors. The data aid in developing GABAA-receptor subtype-specific antagonists and in establishing receptor domains critical for the actions of GABAA antagonists.

Animals↗
PubMed1996

Pharmacological modulation of the diazepam-insensitive recombinant gamma-aminobutyric acidA receptors alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2.

We characterized modulation of the gamma-aminobutyric acid (GABA)-evoked responses of the diazepam-insensitive alpha 4 beta 2 gamma2 and alpha 6 beta 2 gamma 2 recombinant GABAA receptors. The partial agonist bretazenil potentiated the responses of both receptors with similar dose dependence but with a higher maximal enhancement at the alpha 4 beta 2 gamma 2 receptor. The bretazenil-induced potentiation was reduced by the benzodiazepine antagonist flumazenil. At a high concentration (10 microM), flumazenil was a weak potentiator of the GABA response. The partial agonist imidazenil was inactive. The imidazobenzodiazepine inverse agonist Ro 15-4513, which is known to bind with high affinity to the alpha 6 beta 2 gamma 2 receptor, potentiated the GABA responses of the alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor subtypes with similar dose dependence over the concentration range of 0.1-10 microM. Methyl-6, 7-dimethoxy-4-ethyl-beta-carboline, a beta-carboline inverse agonist, had a similar potentiating effect when tested at a concentration of 10 microM. The alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor-mediated currents had equal sensitivities to furosemide and Zn2+ ions, both of which reduced the GABA-evoked responses. The alpha 6 beta 2 gamma 2 receptor but not the alpha 4 beta 2 gamma 2 receptor exhibited a low level of spontaneous activity in the absence of GABA; this resting current could be directly potentiated by Ro 15-4513, methyl-6,7-dimethoxy-4-ethyl-beta-carboline, bretazenil and flumazenil and was blocked by picrotoxin. Thus, although the alpha 4 beta 2 gamma 2 receptors are insensitive to benzodiazepine binding site full agonists, such as diazepam, they can be modulated by certain ligands acting as partial and inverse agonists at diazepam-sensitive receptors and thereby contribute to the respective pharmacological profiles.

Animals↗
PubMed1995

Subtype specificity of gamma-aminobutyric acid type A receptor antagonism by clozapine.

Clozapine, an atypical neuroleptic, functionally antagonizes the gamma-aminobutyric acid-induced chloride uptake via the main central inhibitory receptor, gamma-aminobutyric acid type A (GABAA) receptor, in brain vesicles. GABAA antagonism by micromolar concentrations of clozapine is more efficient in rat cerebrocortical and hippocampal membranes than in cerebellar membranes, as evidenced by clozapine reversal GABA-inhibition of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding. A typical neuroleptic, haloperidol, failed to antagonize GABA in any of these brain regions, while the specific GABAA antagonist 2'-(3'-carboxy-2',3'-propyl)-3-amino-6-p -methoxyphenylpyrazinium bromide (SR 95531) was efficient in all three brain regions. Clozapine action on [35S]TBPS binding was unaffected by the benzodiazepine receptor antagonist flumazenil. Clozapine inhibited the binding of [3H]muscimol and [3H]SR 95531 to the GABA recognition site, but this effect only partially correlated with the regional differences in and the potency of clozapine antagonism of GABA-inhibition of [35S]TBPS binding, suggesting that also other than GABA sites may mediate clozapine actions. Autoradiography of [35S]TBPS binding revealed GABA antagonism by clozapine in most brain regions. Main exceptions were cerebellar granule cell and molecular layers, olfactory bulb external plexiform and glomerular layers and primary olfactory cortex, where clozapine antagonized GABA inhibition less than average, and lateral hypothalamic and preoptic areas where its antagonism was greater than average. Recombinant alpha 6 beta 2 gamma 2 receptors, the predominant alpha 6 subunit-containing receptor subtype in cerebellar granule cells, failed to show GABA antagonism by clozapine up to 100 microM. In contrast, recombinant alpha 1 beta 2 gamma 2 receptors, forming the predominant receptor subtype in the brain, were clozapine sensitive. Recombinant alpha 6 beta 2 gamma 2 and alpha 6 beta 3 gamma 2 receptors resulted in clozapine-insensitive receptors, whereas alpha 6 beta 1 gamma 2 receptors were clozapine sensitive. The efficacy of clozapine to antagonize GABA in alpha 1 beta x gamma 2 receptors decreased in the order of alpha 1 beta 1 gamma 2 > alpha 1 beta 2 gamma 2> alpha 1 beta 3 gamma 2. The results indicate that clozapine antagonizes the function of most GABAA receptor subtypes, and that the interaction is determined by the interaction of the alpha and beta subunit variants. GABA antagonism is a unique property of clozapine, not shared by haloperidol, which might be involved in the pharmacological mechanism for the increased seizure susceptibility associated with clozapine treatment.

Animals↗
PubMed1995

Phenotypic and genotypic analysis of rats with cerebellar GABAA receptors composed from mutant and wild-type alpha 6 subunits.

The alcohol-sensitive (ANT) rat line, developed for high behavioral sensitivity to ethanol, also exhibits enhanced sensitivity to benzodiazepines, such as diazepam. The rat line carries a point mutation in the cerebellum-specific gamma-aminobutyric acid type A (GABAA) receptor subunit alpha 6, making their diazepam-insensitive (DIS) receptors sensitive to diazepam. We now report that phenotypes of individual ANT and alcohol-insensitive rats, classified on diazepam sensitivity of cerebellar [3H]Ro 15-4513 binding, correlated well with homozygous wild-type, homozygous mutant, and heterozygous genotypes, although some heterozygotes were biased toward the parental phenotypes. GABA down-modulated DIS [3H]Ro 15-4513 binding in mutant homozygotes but tended to up-modulate it in heterozygotes and wild-type homozygotes. Slopes for GABA inhibition of cerebellar t-butyl- bicyclophosphoro[35S]thionate binding were larger in mutant than in wild-type homozygotes, with heterozygotes being intermediate. Diazepam displacement of [3H]Ro 15-4513 binding in heterozygotes revealed three components, with their affinities indistinguishable from those in combined wild-type and mutant homozygotes. This lack of interaction in DIS binding between wild-type and mutant alpha 6 subunits was substantiated by experiments on recombinant receptors. The data suggest that the alpha 6 subunit-containing GABAA receptors in the heterozygotes are formed from individual mutant and wild-type subunits with their relative expression differing from animal to animal.

Animals↗
PubMed1995

Effects of ethanol on recombinant rat GABAA receptors: [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding study.

To determine the roles of the alternatively spliced short and long forms of the gamma 2 subunit in the effect of ethanol on the GABAA receptor function, picrotoxin-sensitive [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding was studied in recombinant rat alpha 1 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptors expressed in human embryonic kidney 293 cells. Ethanol (10-500 mM) in the absence of added GABA had only minor effects on [35S]TBPS binding irrespective of the gamma 2 splice variant, its effects being greater in alpha 6 beta 2 gamma 2 than in alpha 1 beta 2 gamma 2 receptors. Ethanol (100 mM) decreased the binding in all four subunit combinations at various concentrations of GABA, again an effect independent of the gamma 2 variant. The two gamma 2 variants had different effects on GABA modulation of the binding, with the long gamma 2 variant decreasing the efficiency of GABA inhibition in alpha 6 beta 2 gamma 2 receptors and enhancing the biphasic GABA stimulation and inhibition in alpha 1 beta 2 gamma 2 receptors. The findings confirm the importance of the alpha subunits in the allosteric interactions between the convulsant binding site and other effector sites, which can be modified only to a minor extent by the type of the gamma 2 splice variant.

Animals↗
PubMed1995

GABA antagonists differentiate between recombinant GABAA/benzodiazepine receptor subtypes.

Seventeen rat GABAA receptor subtypes were transiently expressed in the human embryonic kidney 293 cell line from alpha 1, alpha 2, alpha 3, alpha 5, or alpha 6 variants with any of the three beta subunits and gamma 2S or gamma 3. We obtained fingerprints in the form of subtype characteristic concentration-response curves of 35S-TBPS binding to GABA and the GABAA antagonists SR 95531 and bicuculline. alpha 3 beta 3 gamma 2S/3 and alpha 5 beta 3 gamma 2S/3 containing receptors effectively recognized 35S-TBPS but not when beta 3 was replaced by the beta 1 or beta 2 subunit. This indicates a specific interaction of alpha and beta variants to form high-affinity 35S-TBPS binding sites. At low levels GABA allosterically increased 35S-TBPS binding to all receptors with the concentration and magnitude depending on the subunit combination. Exchange of the beta variant did not alter the concentration-response curves for alpha 1 and alpha 6 containing receptors but did so for alpha 2 containing receptors. alpha 2 beta 3 gamma 3 receptors displayed strong GABA-induced stimulation of 35S-TBPS binding, whereas binding to alpha 2 beta 3 gamma 3 receptors was marginally increased. SR 95531 and bicuculline decreased 35S-TBPS binding to all gamma 3 containing receptors. In addition, bicuculline was effective on alpha 1 beta x gamma 2 receptors. SR 95531 was threefold more potent than bicuculline in reversing GABA-induced modulation of 35S-TBPS binding in most receptor types, but was 30-fold more potent on alpha 2 beta 1 gamma 3 and alpha 6 beta 1 gamma 2S receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Selective antagonist for the cerebellar granule cell-specific gamma-aminobutyric acid type A receptor.

Numerous ligands affect inhibitory gamma-aminobutyric acid (GABA)A receptors, none of them showing strict receptor subtype specificity. We report here that a cerebellar GABAA receptor subtype can be uniquely modulated by furosemide but not by bumetanide, another Cl-/cation transport blocker. Furosemide specifically reversed the inhibition by GABA of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding in the cerebellar granule cell layer, as detected by autoradiography of rat brain sections. With recombinant receptors expressed in Xenopus oocytes, furosemide antagonized potently (IC50, about 10 microM), rapidly, and reversibly GABA-evoked currents of cerebellar granule cell-specific alpha 6 beta 2 gamma 2 receptors but not alpha 1 beta 2 gamma 2 receptors (IC50, > 3 mM). Furosemide reversed GABA inhibition of [35S]TBPS binding and elevated basal [35S]TBPS binding only with alpha 6 beta 2 gamma 2 and alpha 6 beta 3 gamma 2 receptors and not with alpha 6 beta 1 gamma 2 or alpha 1 beta 1/2/3 gamma 2 receptors. It appeared to interact with the receptor complex via a novel recognition site that allosterically regulates the Cl- ionophore. Furosemide is the first subtype-selective GABAA receptor (alpha 6 beta 2/3 gamma 2) antagonist and should facilitate studies on cerebellar physiology. It might serve as a prototypic structure for the development of additional subtype-selective GABAA ligands.

Animals↗
PubMed1994

Four amino acid exchanges convert a diazepam-insensitive, inverse agonist-preferring GABAA receptor into a diazepam-preferring GABAA receptor.

Benzodiazepines (BZ) exert their effects through GABAA receptors, which belong to the superfamily of ligand-gated ion channels. Coexpression of recombinant alpha, beta, and gamma subunits in a cell culture system mimics the BZ binding sites. The alpha variants largely determine the nature of the BZ binding site in such alpha i beta j gamma k heteromultimers (i = 1-6; j = 1-3; k = 1-3). Notably, the alpha 1 and alpha 6 variants confer high and low affinity for BZ agonists to the resulting receptor subtype, respectively. Glycine/glutamate and histidine/arginine positions in the alpha subunits of alpha x beta 2 gamma 2 receptors are involved in BZ I versus BZ II type selectivity. We now identify four amino acids in alpha 6 which together increase the affinity of the mutant alpha x beta 2 gamma 2 receptor for classical BZ receptor agonists above the level seen for any wild-type GABAA/BZ receptor. The most pronounced effect was due to an isoleucine to valine exchange. It simultaneously decreased the affinity for the BZ partial inverse agonist Ro 15-4513 20-fold and increased the affinity for diazepam 4-fold. The four amino acid residues stretch over most part of the N-terminal extracellular domain of the alpha subunit, suggesting that amino acids distant in the primary sequence form the BZ binding pocket.

Amino Acid Sequence↗
PubMed1994

NMDA receptor channels: subunit-specific potentiation by reducing agents.

Sulfhydryl redox agents affect NMDA receptor activity. We investigated a putative redox site in four recombinant NMDA receptors. In 293 cells expressing NR1-NR2A channels dithiothreitol (DTT) rapidly potentiated L-glutamate-activated whole-cell currents and decreased the time course of desensitization and deactivation. Part of the current potentiation (reversible component) and all kinetic changes reversed upon washout. The remaining potentiation (persistent component) was abolished by an oxidizing agent. The N-terminal 370 residues of NR2A mediate the reversible component in chimeric NR2 subunits. In cells expressing the NR1-NR2B, -NR2C, and -NR2D channels DTT elicited only a slowly developing, persistent potentiation and increased the deactivation time course. In these, but not in NR1-NR2A, the DTT effect was rendered insensitive to reoxidation by alkylation. Reduced glutathione mimicked the DTT effects only in the NR1-NR2A receptor. Hence, molecularly distinct NMDA receptors differ profoundly in their responses to sulfhydryl redox agents.

Alanine↗
PubMed1994

Small N-terminal deletion by splicing in cerebellar alpha 6 subunit abolishes GABAA receptor function.

Sequence variation was found in cDNA coding for the extracellular domain of the rat gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit. About 20% of polymerase chain reaction (PCR)-amplified alpha 6 cDNA prepared from rat cerebellar mRNA lacked nucleotides 226-255 as estimated by counting single-stranded phage plaques hybridized specifically to the short (alpha 6S) and long (wild-type) forms of the alpha 6 mRNA. Genomic PCR revealed an intron located upstream of the 30-nucleotide sequence. Both splice forms were detected in the cerebellum by in situ hybridization. Recombinant receptors, resulting from coexpression of the alpha 6S subunit with the GABAA receptor beta 2 and gamma 2 subunits in human embryonic kidney 293 cells, were inactive at binding [3H]muscimol and [3H]Ro 15-4513. In agreement, injection of complementary RNAs encoding the same subunits into Xenopus oocytes produced only weak GABA-induced currents, indistinguishable from those produced by beta 2 gamma 2 receptors. Therefore, the 10 amino acids encoded by the 30-nucleotide fragment may be essential for the correct assembly or folding of the alpha 6 subunit-containing receptors.

Amino Acid Sequence↗