Triazolam is more efficacious than diazepam in a broad spectrum of recombinant GABAA receptors

doi:10.1016/0922-4106(93)90056-F

European Journal of Pharmacology: Molecular Pharmacology

Volume 244, Issue 1, 4 January 1993, Pages 29-35

https://doi.org/10.1016/0922-4106(93)90056-F Get rights and content

Abstract

Benzodiazepine-induced modifications of GABA (γ-aminobutyric acid) activated Cl⁻ currents were studied in native GABA_A receptors expressed in neonatal rat brain cortical neurons in primary cultures and in recombinant GABA_A receptors expressed in transformed human embryonic kidney cells (293) after a transient transfection with cDNAs encoding for different molecular forms of α, β, and γ subunits of GABA_A receptors. The efficacy of triazolam in cortical neurons was higher than that of diazepam. In transfected cells, triazolam showed a greater efficacy as a positive modulator of GABA-elicited Cl⁻ currents in α1β1γ1, α1β1γ2, α1β1γ3, α6β1γ2 and α1β3γ2 receptors than diazepam, except in α3β1γ2 receptors where diazepam was more efficacious. When triazolam and diazepam were applied together to GABA_A receptors assembled by transfecting cDNAs encoding for α1β1γ1 subunits, the action of triazolam was curtailed in a manner related to the dose of diazepam. In recombinant receptors assembled with α1β1γ1 receptors, maximally active doses of triazolam were more efficacious than those of clonazepam, alpidem, zolpidem, diazepam or bretazenil.

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Using reconstituted GABAA receptor systems, he and his collaborators were among those demonstrating that not all pentameric combinations of GABAA receptor subunits display the same pharmacological selectivity (Puia et al., 1989, 1992b). This finding was extended to show that differences in the pharmacological responses to various anxiolytics and hypnotics, even within the benzodiazepine chemical class, are due, to some extent, to differences in the subunit composition of the particular GABAA receptor with which each interacts (Massotti et al., 1991a; Puia et al., 1991; Ducic et al., 1993). They also reported that tolerance to some actions of benzodiazepines, such as the amnesic and anticonvulsant effects, may be due to drug-induced changes in the expression of certain GABAA receptor subunits (Longone et al., 1996; Zheng et al., 1996; Impagnatiello et al., 1996; Pesold et al., 1997).
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This article is part of a Special Issue entitled ‘Trends in Neuropharmacology: In Memory of Erminio Costa’.
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Flumazenil (FLU), a specific benzodiazepine (BZ) receptor antagonist has been used in the treatment of acute BZ intoxication or the alleviation of BZ-induced withdrawal syndrome on the basis of its weak partial agonist action at GABA_A receptors. However, given to patients, FLU can worsen diazepam-induced withdrawal syndrome by lowering seizure threshold. We therefore investigated whether imidazenil, a selective positive allosteric modulator of GABA action at GABA_A receptors containing α₅ subunit, can antagonize diazepam-induced sedative action and suppression of locomotor activity without affecting diazepam anti-bicuculline action. We report here that while FLU (16.5 μmol/kg) showed no effect on locomotor activity and bicuculline-induced convulsion, it completely antagonized diazepam (10.5 μmol/kg) anti-bicuculline action and the suppression of locomotor activity. However, imidazenil (0.76 μmol/kg) elicited anti-bicuculline action and was dose-dependently antagonized by FLU (16.5 and 33 μmol/kg). Furthermore, imidazenil showed no effect on path length traveled but slightly decreased (40%) horizontal activity when compared to diazepam (85%), and maintained the anti-bicuculline action of diazepam to a threshold level similar to that observed with diazepam. Whereas cross-tolerance between BZs has been reported in animals and humans, we previously reported the absence of cross-tolerance between imidazenil and diazepam. Thus, we suggest that imidazenil might be more effective than FLU at alleviating the withdrawal syndrome associated with long-term BZ administration.
GABA<inf>A</inf> receptors and benzodiazepines: A role for dendritic resident subunit mRNAs
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This review is designed to describe the evolution of the seminal observation made simultaneously in 1975 by Dr. W. Haefely’s laboratory (Hoffman La Roche, Basel, Switzerland) and in the Laboratory of Preclinical Pharmacology (NIH, St. Elizabeths Hospital, Washington DC), that benzodiazepine action was mediated by a modulation of GABA action at GABA_A receptors. In fact, our suggestion was that the benzodiazepine receptor was “a receptor on a receptor” and that this receptor was GABA_A. Needless to say, this suggestion created opposition, but we did not abandon the original idea, in fact, as shown in this review, there is now universal agreement with our hypothesis on the mode of action of benzodiazepines. Hence, this review deals with the allosteric modulation of GABA_A receptors by benzodiazepines, the role of GABA_A receptors and benzodiazepine structure diversities in this modulation, and describes the results of our attempts to establish a benzodiazepine (imidazenil) devoid of tolerance, withdrawal symptoms, and changes in the expression of GABA_A receptor subunits during tolerance. It also deals with the idea that the synthesis of GABA_A receptor subunits triggered by tolerance resides in dendrites and spines where mRNAs and the apparatus for this translation is located. New analytic procedures may foster progress in the understanding of tolerance to and withdrawal from benzodiazepines.
Functional analysis of GABA<inf>A</inf> receptors in nucleus tractus solitarius neurons from neonatal rats
2001, Brain Research
Citation Excerpt :
The efficacy of diazepam is also instructive regarding α subunit isoform. Ducic et al. [6] reported a 350% potentiation by diazepam of the GABA response in α3β1γ2 receptors, whereas α1β1γ2 receptors were potentiated approximately 125%. In agreement with these previous studies, we have recorded a 311% maximal response to diazepam in recombinant rat α3β2γ2 GABAA receptors, a response roughly three times greater than observed in rat α1β2γ2 receptors (data not shown).
To gain insight into specific GABA_A receptor configurations functionally expressed in the nucleus tractus solitarius (NTS), we conducted several physiological and pharmacological assessments. NTS neurons were characterized in thin brain slices from 1–14 day old rats using whole-cell patch clamp recordings. GABA_A− receptor-mediated currents were detected in all neurons tested, with an average EC₅₀ of 22.2 μM. GABA currents were consistently stimulated by diazepam (EC₅₀=63 nM), zolpidem (EC₅₀=85 nM), loreclezole (EC₅₀=10.1 μM) and the neurosteroid 5α-pregnan-3α-hydroxy-20-one (3α-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn²⁺ (IC₅₀=33.6 μM) picrotoxin (IC₅₀=2.4 μM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10–1000 μM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant α subunit isoform present in GABA_A receptors of the NTS appears to be the α1 and/or α2 subunit. β2 and/or β3 subunits are the major β isoform, while the predominant γ subunit is likely γ2. Our data suggest the contribution to NTS GABA currents by α3–α6, β1, γ1 and δ subunits, if present, is minor by comparison.
Expression of GABA<inf>A</inf> receptor subunits in the rat central nucleus of the inferior colliculus
2001, Molecular Brain Research
Expression of GABA_A receptor (GABA_AR) α₁, α₂, β₂, γ₁, γ_2L and γ_2S subunit mRNA was examined in three cell classes in the central nucleus of the rat inferior colliculus (CNIC). GABA_AR α₁ and γ_2L subunit mRNA expression was greatest in large cells (over 25 μm long diameter), intermediate in medium sized cells (15 to 25 μm long diameter) and lowest in small cells (10 to 15 μm long diameter). GABA_AR γ_2S and α₂ subunits had the opposite pattern, highest in the small cells, intermediate in medium cells and lowest in large cells. GABA_AR β₂ was significantly lower in small cells than the two other classes, while differences between large and medium cells were not significant. GABA_AR γ₁ subunit mRNAs expression was not above background in any of the three cell types assessed. The expression of GABA_AR subunits suggests that cell classes in the rat CNIC may differ in their response to GABA and GABAergic drugs.

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Regular paperTriazolam is more efficacious than diazepam in a broad spectrum of recombinant GABAA receptors

Abstract

Dev. Brain Res.

Trends Pharmacol. Sci.

Life Sci.

Trends Pharmacol. Sci.

Neurosci. Lett.

Neuropharmacology

Eur. J. Pharmacol.

Lancet

Brain Res.

Neuron

Eur. J. Pharmacol.

Can a rapidly-eliminated hypnotic cause daytime anxiety

Pharmacopsychiatry

High-efficiency transformation of mammalian cells by plasmid DNA

Mol. Cell. Biol.

Regular paper
Triazolam is more efficacious than diazepam in a broad spectrum of recombinant GABA_A receptors