The benzodiazepine triazolam: direct and GABA depressant effects on cultured mouse spinal cord neurons

Mexazolam: clinical efficacy and tolerability in the treatment of anxiety

Introduction

Mexazolam is indicated for the management of anxiety with or without psychoneurotic conditions. In adult patients, the recommended daily dosage of mexazolam is 1–3 mg, administered three times daily. The objective of this article is to review the available information on the benzodiazepine (BZD) mexazolam and its clinical utility in treating patients with anxiety.

Methods

The PubMed database was searched using the keyword “mexazolam” with no date or language restrictions applied to the search. As only 11 papers were retrieved, some previously published manuscripts of interest known by the authors (not indexed on PubMed) have been added for completeness. Relevant information was selected for inclusion by the authors.

Results

A number of early studies demonstrated the ability of mexazolam to reduce anxiety symptoms with few side effects in patients with disorders associated with anxiety. Following on from this preliminary evidence, controlled studies directly comparing mexazolam with other BZDs showed that the drug is more effective than bromazepam and oxazolam, and is at least as effective as alprazolam. A larger, multicenter, phase IV study also showed that mexazolam 2 or 3 mg/day rapidly improved Hamilton Anxiety Rating Scale scores and substantially reduced the frequency and severity of numerous somatic anxiety symptoms in patients with anxiety disorders. With regard to safety, the clinical evidence indicates that mexazolam is generally well tolerated, with a low incidence of drowsiness and sedation. Furthermore, the lack of psychomotor or cognitive performance impairment following mexazolam administration may lead to better treatment compliance.

Conclusion

The available clinical evidence suggests that mexazolam is an effective therapeutic option for the management of anxiety. However, larger, well-controlled clinical trials are needed to directly compare and contrast mexazolam’s efficacy and safety with other BZDs.

Electronic supplementary material

The online version of this article (doi:10.1007/s40120-014-0016-7) contains supplementary material, which is available to authorized users.

Insights into the structure and pharmacology of GABA(A) receptors

GABA is the major inhibitory neurotransmitter in the adult mammalian CNS. The ionotropic GABA type A receptors (GABA(A)Rs) belong to the Cys-loop family of receptors. Each member of the family is a large pentameric protein in which each subunit traverses the cell membrane four times. Within this family, the GABA type A receptors are particularly important for their rich pharmacology as they are targets for a range of therapeutically important drugs, including the benzodiazepines, barbiturates, neuroactive steroids and anesthetics. This review discusses new insights into receptor properties that allow us to begin to relate the structure of an individual receptor to its functional and pharmacological properties.

Triazolam suppresses the induction of hippocampal long-term potentiation

Benzodiazepines are sedative hypnotics that produce marked anterograde amnesia in humans. These pharmacological properties are thought to result from the potentiation of GABA-A receptor function and subsequent attenuation of long-term potentiation (LTP), however many reports have suggested this is not the case for triazolam. Using electrophysiological recordings in a cell line expressing recombinant GABA-A receptors, we confirm that triazolam is an efficacious positive allosteric modulator of GABA-A receptors. Triazolam also slowed the decay of spontaneous inhibitory synaptic currents, reduced the amplitude of fEPSPs elicited during a theta burst and reduced the magnitude of LTP in hippocampal CA1 neurones in vitro. These data show that triazolam modifies LTP induction consistent with an enhancement of GABA-A receptor function via activation of the allosteric benzodiazepine-site.

Effects of intravenous anaesthetic agents on fast inhibitory oscillations in the rat hippocampus in vitro

1. General anaesthetic agents prevent awareness of sensory input and subsequent recall of sensory events after administration. The mechanisms involved in higher sensory processing, including awareness and recall, are not fully elucidated. However, fast oscillations in neuronal activity in the 20-80 Hz (gamma) range have been strongly implicated. Here we have investigated the effects of two anaesthetic agents and a sedative/hypnotic drug on these oscillations. 2. Trains of fast oscillations, shown previously to be shaped by gamma-aminobutyric acid (GABAA) receptor activation, were evoked by pressure ejection of L-glutamate (10 nM) onto the perisomatic region of hippocampal area CAI in the presence of 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (R-CPP), 50 microM, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX), 50 microM and 2-hydroxysaclofen, 0.2 mM. 3. Thiopentone (10-200 microM) and propofol (0.5-10 microM) dose-dependently decreased both the maximum oscillation frequency, by approx. 90%, and the incidence of evoked rhythmic oscillations by approx. 60%. Diazepam (0.05-1 microM) decreased maximum oscillation frequency by about 40% but did not affect the incidence of evoked oscillations. 4. The similar effects of thiopentone and propofol were mediated by both a large (about 600%) increase in the decay constant (tau D) of GABAA receptor-mediated inhibitory postsynaptic currents (i.p.s.cs) and a bicuculline-sensitive leak current. The two drugs had differing effects on i.p.s.c. amplitude. Diazepam caused a small increase in tau D (about 170%) and did not alter leak currents at the doses used. 5. Effects of the anaesthetic agents were seen on the above measurements at similar concentrations to those estimated in the CNS during clinical and veterinary anaesthesia. We suggest that the effects on fast oscillations associated with cognition may contribute to the mechanism by which these agents produce general anaesthesia.

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

Benzodiazepine-induced modifications of GABA (gamma-aminobutyric acid) activated Cl- currents were studied in native GABAA receptors expressed in neonatal rat brain cortical neurons in primary cultures and in recombinant GABAA receptors expressed in transformed human embryonic kidney cells (293) after a transient transfection with cDNAs encoding for different molecular forms of alpha, beta, and gamma subunits of GABAA 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 alpha 1 beta 1 gamma 1, alpha 1 beta 1 gamma 2, alpha 1 beta 1 gamma 3, alpha 6 beta 1 gamma 2 and alpha 1 beta 3 gamma 2 receptors than diazepam, except in alpha 3 beta 1 gamma 2 receptors where diazepam was more efficacious. When triazolam and diazepam were applied together to GABAA receptors assembled by transfecting cDNAs encoding for alpha 1 beta 1 gamma 1 subunits, the action of triazolam was curtailed in a manner related to the dose of diazepam. In recombinant receptors assembled with alpha 1 beta 1 gamma 1 receptors, maximally active doses of triazolam were more efficacious than those of clonazepam, alpidem, zolpidem, diazepam or bretazenil.

Benzodiazepines block long-term potentiation in slices of hippocampus and piriform cortex

The effects of two benzodiazepines, diazepam and triazolam, on long-term potentiation were tested in slices of hippocampus and piriform cortex. The drugs had little influence on baseline synaptic responses but both were very effective in blocking LTP elicited by theta pattern stimulation. The effects were fully reversible upon washout. Diazepam reduced the increase in burst responses that occurs during theta stimulation and thus appears to interfere with the initial triggering events for long-term potentiation. This may reflect the enhancing action of the drug on GABA-mediated inhibitory potentials. Triazolam did not detectably change the burst responses elicited by theta pattern stimulation. Experiments with slices of piriform cortex indicated that triazolam also failed to disrupt the development of long-term potentiation but instead caused the potentiation to decay back to baseline in 15-30 min. Triazolam thus seems to act on the mechanisms that stabilize long-term potentiation. These results provide a possible explanation for the amnestic effects of benzodiazepines in humans and animals and support the hypothesis that long-term potentiation contributes to memory encoding.

Triazolam as a discriminative stimulus in humans

Seven healthy normal male and female volunteers (19-42 years) were trained to discriminate between the benzodiazepine triazolam (0.32 mg/70 kg; e.g. drug A) and placebo (e.g. drug B). During the first four daily sessions, drug A and drug B were administered orally in capsules 60 min prior to the session on alternate days and subjects were informed of the drug label at the time of drug administration. Subsequently, drug A and drug B were administered in a randomized-block fashion and subjects identified the drug code they thought they received. Subjects were informed of the drug code post-session. Once the criterion for discrimination was met (i.e. correct drug code identification on four consecutive sessions), the dose-effect curve for triazolam (0.1-0.75 mg/70 kg) was determined. The discrimination was acquired in all subjects; triazolam (0.32 mg/70 kg) and placebo produced approximately 85-95% correct responding. During the dose-effect curve determination, triazolam produced dose-related increases in triazolam-appropriate responding and self-reported sedation and drug strength. These results indicate that a triazolam-placebo discrimination can be acquired and that the triazolam discriminative stimulus effect is related to dose and to self-reported sedation.

Effects of prolonged administration of triazolam on adenosine A1 and A2 receptors in the brain of rats

Continuous subcutaneous administration of triazolam, a benzodiazepine with short plasma half-life, for 10 days either decreased (31%, 2 mg/day) or increased (15%, 0.5 mg/day) radioligand binding to adenosine A2 receptors in the rat striatum. In a similar manner, we have shown previously that diazepam (5-10 mg/day), a benzodiazepine with a long plasma half-life attenuated radioligand binding to adenosine A2 receptors in the rat striatum by 45-25%.

Membrane channels activated by taurine in cultured mouse spinal cord neurons

Patch-clamp methods were used to compare biophysical properties of anion channels activated by taurine and gamma-aminobutyric acid in the membrane of cultured mouse spinal neurons. Outside-out patches were voltage clamped at -80 mV at a temperature of 21-23 degrees C. Bath application of GABA (1.5-2 microM) or taurine (5-40 microM) induced chloride-dependent single-channel currents in 14/20 patches tested. Amplitude distributions of these currents showed peaks corresponding to conductance levels of 8, 16, 27 and 46 pS. Only a few percent of GABA-induced events reached the 46 pS level, while 30% of taurine-induced currents were of this size. The average lifetime of taurine-activated channels in the open state was 1.0 +/- 0.07 ms, significantly shorter than the corresponding value for GABA (1.6 +/- 0.08 ms). Taurine-induced currents were abolished by 10 microM strychnine, but persisted in the presence of 50 microM bicuculline.

Potentiation of adenosine-evoked depression of rat cerebral cortical neurons by triazolam

The triazolobenzodiazepine triazolam, applied iontophoretically onto rat cerebral cortical neurons, potentiated the magnitude and duration of adenosine-elicited depressions of spontaneous activity. Triazolam did not enhance the depressions evoked by adenosine 5'-N-ethylcarboxamide, an uptake resistant analog of adenosine, suggesting that potentiation of adenosine resulted from an inhibition of adenosine uptake. With larger application currents, triazolam depressed the firing of cortical neurons. This action was blocked by the adenosine antagonist caffeine (20 mg/kg, i.v.) implying that the depression resulted from an accumulation of endogenously released adenosine.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of adenosine in the central actions of the benzodiazepines

1. Evidence is presented which indicates that the central actions of the benzodiazepines cannot be fully accounted for by assuming an action only at the GABAA-Cl- channel supramolecular complex. 2. The hypothesis is presented, together with supporting evidence, that inhibition of adenosine uptake can account for many of the actions of the benzodiazepines. 3. New findings showing that Ro 15-1788 and Ro 5-4864 have both potentiative and antagonistic interactions with adenosine are discussed. 4. The proconvulsant beta-carbolines are shown to be adenosine antagonists. 5. The concept that benzodiazepine action may involve several mechanisms is presented.

Effect of agonist concentration on the lifetime of GABA-activated membrane channels in spinal cord neurons

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) hyperpolarizes spinal neurons by activating bicuculline sensitive GABAA receptors coupled to chloride permeable ionic channels in the cell membrane. Single channel measurements using outside-out membrane patches from cultured mouse spinal neurons showed that there is a large excess of brief openings of these channels, compared to the number predicted from a simple, two-state model of channel function. The cumulative open time distributions for channel openings were well fit by the sum of two exponential terms, a fast component with amplitude Nf and time constant TAUf, and a slow term with amplitude Ns and time constant TAUs. These kinetics could arise if monoliganded GABAA receptors triggered an open state less stable than that induced by the biliganded receptor. A prediction of this hypothesis is that the ratio Nf/Ns should decline in an approximately linear fashion with increasing doses of GABA. This was found to be the case, as Nf/Ns declined by a factor of 4.8 on increasing the GABA dose from 0.5 microM to 5 microM. Application of 1.25 microM GABA produced an intermediate value of Nf/Ns, as predicted. In contrast the parameters TAUf and TAUs were not significantly influenced by GABA concentration. Spontaneous, bicuculline sensitive currents were seen in some patches. These events resembled currents triggered by low doses of exogenous GABA, with regard to the values of Nf/Ns, TAUf and TAUs. These results suggest that the spontaneous currents are triggered by endogenous GABA molecules, present in the culture environment at a concentration of 0.5 microM or less.