Are GABAA receptors containing alpha5 subunits contributing to the sedative properties of benzodiazepine site agonists?

Comparative pharmacodynamic and pharmacokinetic study of MIDD0301 and its (S) enantiomer

MIDD0301 is being developed as an oral drug to relax airway smooth muscle (ASM) and reduce lung inflammation in asthma. We report a comparative study of MIDD0301 and its S isomer (MIDD0301S), and found that the compounds have equivalent affinity for γ-aminobutyric acid type A receptor (GABA_A R) expressed in rat brain, with half maximal inhibitory concentration values of 25.1 and 26.3 nM for the S and R enantiomers, respectively. Both compounds relaxed substance P contracted ASM within 30 min and neither enantiomer revealed affinity to 48 receptors in an off-target screen. Both enantiomers reduced airway hyperresponsiveness (AHR) with nebulized and oral dosing in two mouse models of bronchoconstriction. In A/J mice, which are very sensitive to methacholine-induced bronchoconstriction, we observed reduction of AHR at 10.8 mg/kg MIDD0301 and 15 mg/kg MIDD0301S. Using oral administration, 100 mg/kg/day for 3 days of either enantiomer was sufficient to reduce AHR. In a model of severe airway inflammation induced by interferon-γ and lipopolysaccharide (LPS), we observed reduction of AHR at 7.2 mg/kg for both enantiomers using nebulized administration, and at 100 mg/kg for oral administration. MIDD0301 and MIDD0301S did not undergo Phase I metabolism. Glucuronidation was observed for both compounds, whereas only MIDD0301 formed the corresponding glucoside in the presence of kidney microsomes. Pharmacokinetic analysis identified glucuronides as the major metabolite with concentrations up to 20-fold more than the parent compound. MIDD0301 glucuronide and MIDD0301 taurine bind GABA_A Rs, although 10-fold weaker than MIDD0301. In mouse blood, the taurine adduct was only observed for MIDD0301. Overall, both compounds exhibited similar receptor binding and pharmacodynamic properties with subtle differences in metabolism and greater oral availability and blood concentrations of MIDD0301S.

Modulation of relapse-like drinking in male Sprague-Dawley rats by ligands targeting the α5GABAA receptor

Preclinical evidence suggests a key role for GABA_A receptors containing the α5 subunit (i.e., α5GABA_A receptors) in the abuse-related effects of alcohol, including the reinforcing and discriminative stimulus effects, as well as cue-induced alcohol-seeking behavior. However, the contribution of this GABA_A receptor subtype to relapse-like drinking behavior remains unknown. The present study evaluated the capacity of ligands targeting α5GABA_A receptors to modulate the alcohol deprivation effect (ADE), a model of relapse-like drinking. Groups of Sprague-Dawley rats underwent repeated cycles of long-term access to alcohol solutions (5%, 10%, 20% v/v) and water in the home cage followed by water only deprivation periods. Upon evidence that the ADE could be reliably expressed across cycles, drug treatment was initiated. One group received the α5GABA_A receptor-preferring agonist QH-ii-066 and the other group received the α5GABA_A receptor-selective inverse agonist L-655,708. At the end of ADE testing, rats underwent testing in the elevated zero maze under vehicle or L-655,708 treatment for assessment of anxiety-like behavior. The ADE was reliably expressed across repeated cycles of alcohol access/deprivation in a subset of rats. Low doses of QH-ii-066 enhanced expression of the ADE; whereas, L-655,708 dose-dependently inhibited expression of the ADE. L-655,708 did not engender anxiogenic effects in the elevated zero maze under the conditions evaluated. These findings suggest a key role for α5GABA_A receptor mechanisms in relapse-like drinking. Moreover, they suggest that α5GABA_A receptors may represent a novel pharmacological target for the development of medications to prevent or reduce alcohol relapse.

Zolpidem reduces pain intensity postoperatively: a systematic review and meta-analysis of the effect of hypnotic medicines on post-operative pain intensity

BACKGROUND

This systematic review aimed to investigate whether the administration of hypnotic medicines, z-drugs, melatonin or benzodiazepines, reduced pain intensity postoperatively.

METHODS

Medline, Embase, Cinahl, Psych info, Central and PubMed databases were searched, from inception to February 2020 to identify relevant trials. The search was extended, post hoc, to include meta-Register of Controlled Trials, the Web of Science and the conference booklets for the 14th, 15th, and 16th International Association for the Study of Pain conferences. Two independent reviewers screened titles and abstracts and cross-checked the extracted data.

RESULTS

The search retrieved 5546 articles. After full-text screening, 15 trials were included, which had randomised 1252 participants. There is moderate-quality evidence that in the short-term [WMD - 1.06, CI - 1.48 to - 0.64, p ≤ .01] and low-quality evidence that in the medium-term [WMD - 0.90, CI - 1.43 to - 0.37, p ≤ .01] postoperative period oral zolpidem 5/10 mg with other analgesic medicines reduced pain intensity compared to the same analgesic medicines alone. There is low-quality evidence that melatonin was not effective on postoperative pain intensity compared to placebo. The results of benzodiazepines on pain intensity were mixed. The authors reported no significant adverse events.

CONCLUSIONS

There is promising evidence that the hypnotic medicine zolpidem, adjuvant to other analgesics, is effective at achieving a minimally clinically important difference in pain intensity postoperatively. There is no consistent effect of melatonin or benzodiazepines on postoperative pain intensity. Readers should interpret these results with some caution due to the lack of data on safety, the small number of trials included in the pooled effects and their sample sizes.

SYSTEMATIC REVIEW REGISTRATION

The protocol for this systematic review was registered with PROSPERO ID= CRD42015025327 .

A Structure-Activity Relationship Comparison of Imidazodiazepines Binding at Kappa, Mu, and Delta Opioid Receptors and the GABAA Receptor

Analgesic and anti-inflammatory properties mediated by the κ opioid receptor (KOR) have been reported for oxadiazole imidazodiazepines. Affinities determined by radioligand competition assays of more than seventy imidazodiazepines using cell homogenates from HEK293 cells that overexpress KOR, µ opioid receptor (MOR), and δ opioid receptor (DOR) are presented. Affinities to synaptic, benzodiazepine-sensitive receptors (BZR) were determined with rat brain extract. The highest affinity for KOR was recorded for GL-I-30 (Ki of 27 nM) and G-protein recruitment was observed with an EC50 of 32 nM. Affinities for MOR and DOR were weak for all compounds. Ester and amide imidazodiazepines were among the most active KOR ligands but also competed with 3H-flunitrazepam for brain extract binding, which is mediated predominately by gamma aminobutyric acid type A receptors (GABAAR) of the α1-3β2-3γ1-2 subtypes. Imidazodiazepines with carboxylic acid and primary amide groups did not bind KOR but interacted strongly with GABAARs. Pyridine substitution reduced KOR affinity. Oxadiazole imidazodiazepines exhibited good KOR binding and interacted weakly with BZR, whereas oxazole imidazodiazepines were more selective towards BZR. Compounds that lack the imidazole moiety, the pendent phenyl, or pyridine substitutions exhibited insignificant KOR affinities. It can be concluded that a subset of imidazodiazepines represents novel KOR ligands with high selectivity among opioid receptors.

Photopotentiation of the GABAA receptor with caged diazepam

As the inhibitory γ-aminobutyric acid-ergic (GABAergic) transmission has a pivotal role in the central nervous system (CNS) and defective forms of its synapses are associated with serious neurological disorders, numerous versions of caged GABA and, more recently, photoswitchable ligands have been developed to investigate such transmission. While the complementary nature of these probes is evident, the mechanisms by which the GABA receptors can be photocontrolled have not been fully exploited. In fact, the ultimate need for specificity is critical for the proper synaptic exploration. No caged allosteric modulators of the GABA_A receptor have been reported so far; to introduce such an investigational approach, we exploited the structural motifs of the benzodiazepinic scaffold to develop a photocaged version of diazepam (CD) that was tested on basolateral amygdala (BLa) pyramidal cells in mouse brain slices. CD is devoid of any intrinsic activity toward the GABA_A receptor before irradiation. Importantly, CD is a photoreleasable GABA_A receptor-positive allosteric modulator that offers a different probing mechanism compared to caged GABA and photoswitchable ligands. CD potentiates the inhibitory signaling by prolonging the decay time of postsynaptic GABAergic currents upon photoactivation. Additionally, no effect on presynaptic GABA release was recorded. We developed a photochemical technology to individually study the GABA_A receptor, which specifically expands the toolbox available to study GABAergic synapses.

L-655,708 Does not Prevent Isoflurane-induced Memory Deficits in Old Mice

Background

General anesthesia and increasing age are two main risk factors for postoperative cognitive dysfunction (POCD). Effective agents for the prevention or treatment of POCD are urgently needed. L-655,708, an inverse agonist of α5 subunit-containing γ-aminobutyric acid subtype A (α5GABA_A) receptors, can prevent anesthesia-induced memory deficits in young animals. However, there is a lack of evidence of its efficacy in old animals.

Methodology

Young (3- to 5-month-old) and old (18- to 20-month-old) mice were given an inhalation of 1.33% isoflurane for 1 hour and their associative memory was evaluated 24 hours after anesthesia using fear-conditioning tests (FCTs). To evaluate the effect of L-655,708, mice received intraperitoneal injections of L-655,708 (0.7 mg/kg) or vehicle 30 minutes before anesthesia.

Results

Old mice exhibited impaired memory and lower hippocampal α5GABA_A levels than young mice under physiological conditions. Pre-injections of L-655,708 significantly alleviated isoflurane-induced memory decline in young mice, but not in old mice.

Conclusions

L-655,708 is not as effective for the prevention of POCD in old mice as it is in young mice. The use of inverse agonists of α5GABA_A in preventing POCD in old patients should be carefully considered.

Involvement of the GABAA receptor α subunit in the mode of action of etifoxine

Etifoxine (EFX) is a non-benzodiazepine psychoactive drug which exhibits anxiolytic effects through a dual mechanism, by directly binding to GABA_A receptors (GABA_ARs) and to the mitochondrial 18-kDa translocator protein, resulting in the potentiation of the GABAergic function. The β subunit subtype plays a key role in the EFX-GABA_AR interaction, however this does not explain the anxiolytic effects of this drug. Here, we combined behavioral and electrophysiological experiments to challenge the role of the GABA_AR α subunit in the EFX mode of action. After single administrations of anxiolytic doses (25-50 mg/kg, intraperitoneal), EFX did not induce any neurological nor locomotor impairments, unlike the benzodiazepine bromazepam (0.5-1 mg/kg, intraperitoneal). We established the EFX pharmacological profile on heteropentameric GABA_ARs constructed with α1 to α6 subunit expressed in Xenopus oocyte. Unlike what is known for benzodiazepines, neither the γ nor δ subunits influenced EFX-mediated potentiation of GABA-evoked currents. EFX acted first as a partial agonist on α2β3γ2S, α3β3γ2S, α6β3γ2S and α6β3δ GABA_ARs, but not on α1β3γ2S, α4β3γ2S, α4β3δ nor α5β3γ2S GABA_ARs. Moreover, EFX exhibited much higher positive allosteric modulation towards α2β3γ2S, α3β3γ2S and α6β3γ2S than for α1β3γ2S, α4β3γ2S and α5β3γ2S GABA_ARs. At 20 μM, corresponding to brain concentration at anxiolytic doses, EFX increased GABA potency to the highest extent for α3β3γ2S GABA_ARs. We built a docking model of EFX on α3β3γ2S GABA_ARs, which is consistent with a binding site located between α and β subunits in the extracellular domain. In conclusion, EFX preferentially potentiates α2β3γ2S and α3β3γ2S GABA_ARs, which might support its advantageous anxiolytic/sedative balance.

The α5-Containing GABAA Receptors-a Brief Summary

GABA_A receptors are the major inhibitory neurotransmitter receptor in the human brain. The receptors are assembled from combination of protein subunits in pentameric complex which may consist of α1-6, β1-3, γ1-3, ρ1-3, δ, ε, θ, or π subunits. There are a theoretical > 150,000 possible assemblies and arrangements of GABA_A subunits, although only a few combinations have been found in human with the most dominant consists of 2α1, 2β2, and 1γ2 in a counterclockwise arrangement as seen from the synaptic cleft. The receptors also possess binding sites for various unrelated substances including benzodiazepines, barbiturates, and anesthetics. The α5-containing GABA_ARs only make up ≤ 5% of the entire receptor population, but up to 25% of the receptor subtype is located in the crucial learning and memory-associated area of the brain-the hippocampus, which has ignited myriads of hypotheses and theories in regard to its role. As well as exhibiting synaptic phasic inhibition, the α5-containing receptors are also extrasynaptic and mediate tonic inhibition with continuously occurring smaller amplitude. Studies on negative-allosteric modulators for reducing this tonic inhibition have been shown to enhance learning and memory in neurological disorders such as schizophrenia, Down syndrome, and autism with a possible alternative benzodiazepine binding site. Therefore, a few α5 subunit-specific compounds have been developed to address these pharmacological needs. With its small population, the α5-containing receptors could be the key and also the answer for many untreated cognitive dysfunctions and disorders.

Design, Synthesis, and Pharmacological Evaluation of Novel β2/3 Subunit-Selective γ-Aminobutyric Acid Type A (GABAA) Receptor Modulators

Subunit-selective modulation of γ-aminobutyric acid type A receptors (GABA_AR) is considered to exert fewer side effects compared to unselective clinically used drugs. Here, the β2/3 subunit-selective GABA_AR modulators valerenic acid (VA) and loreclezole (LOR) guided the synthesis of novel subunit-selective ligands with simplified structures. We studied their effects on GABA_ARs expressed in Xenopus laevis oocytes using two-microelectrode voltage clamp technique. Five compounds showed significantly more efficacious modulation of GABA-evoked currents than VA and LOR with retained potency and selectivity. Compound 18 [( E)-2-Cyano-3-(2,4-dichlorophenyl)but-2-enamide] induced the highest maximal modulation of GABA-induced chloride currents ( E_max: 3114 ± 242%), while 12 [( Z)-3-(2,4-dichlorophenyl)but-2-enenitrile] displayed the highest potency (EC₅₀: 13 ± 2 μM). Furthermore, in hippocampal neurons 12 facilitated phasic and tonic GABAergic inhibition, and in vivo studies revealed significantly more potent protection against pentylenetetrazole (PTZ)-induced seizures compared to VA and LOR. Collectively, compound 12 constitutes a novel, simplified, and subunit-selective GABA_AR modulator with low-dose anticonvulsant activity.

Different Benzodiazepines Bind with Distinct Binding Modes to GABAA Receptors

Benzodiazepines are clinically relevant drugs that bind to GABAA neurotransmitter receptors at the α+/γ2- interfaces and thereby enhance GABA-induced chloride ion flux leading to neuronal hyperpolarization. However, the structural basis of benzodiazepine interactions with their high-affinity site at GABAA receptors is controversially debated in the literature, and in silico studies led to discrepant binding mode hypotheses. In this study, computational docking of diazepam into α+/γ2- homology models suggested that a chiral methyl group, which is known to promote preferred binding to α5-containing GABAA receptors (position 3 of the seven-membered diazepine ring), could possibly provide experimental evidence that supports or contradicts the proposed binding modes. Thus, we investigated three pairs of R and S isomers of structurally different chemotypes, namely, diazepam, imidazobenzodiazepine, and triazolam derivatives. We used radioligand displacement studies as well as two-electrode voltage clamp electrophysiology in α1β3γ2-, α2β3γ2-, α3β3γ2-, and α5β3γ2-containing GABAA receptors to determine the ligand binding and functional activity of the three chemotypes. Interestingly, both imidazobenzodiazepine isomers displayed comparable binding affinities, while for the other two chemotypes, a discrepancy in binding affinities of the different isomers was observed. Specifically, the R isomers displayed a loss of binding, whereas the S isomers remained active. These findings are in accordance with the results of our in silico studies suggesting the usage of a different binding mode of imidazobenzodiazepines compared to those of the other two tested chemotypes. Hence, we conclude that different chemically related benzodiazepine ligands interact via distinct binding modes rather than by using a common binding mode.

Attaining in vivo selectivity of positive modulation of α3βγ2 GABAA receptors in rats: A hard task!

It is unclear whether GABA_A receptors (GABA_ARs) that contain the α3-subunit are substantially involved in the anxiolytic effects of benzodiazepines (BDZs). In the present study, we tested YT-III-31, a newer BDZ ligand with functional preference for α3βγ2 GABA_ARs, in two paradigms of unconditioned anxiety, the open field and elevated plus maze in rats. The effective dose of YT-III-31 (2 mg/kg) displayed a clear anxiolytic-like profile, unhampered by sedative action, in both tests. At a higher dose (10 mg/kg), YT-III-31 induced ataxia in the rotarod and sedation in spontaneous locomotor activity test. The latter effect was preventable by flumazenil and βCCt, the non-selective and α1βγ2 GABA_AR affinity-selective antagonist, respectively, demonstrating that sedative properties of YT-III-31, when attained, are mediated by the α1γ2 site. To elucidate the receptor substrate of subtle behavioral differences between YT-III-31 and diazepam, we approximated in vivo receptor potentiation for both ligands, based on estimated unbound concentrations in rat brains. Far different from diazepam, YT-III-31 has significantly lower affinity for the α1γ2 over other BDZ-sensitive sites, and at lower doses (1-2 mg/kg) was devoid of potentiation at α1βγ2 GABA_ARs. The approximation approach revealed a modest selectivity of YT-III-31 for α3γ2- in comparison to α2γ2 and α5γ2 binding sites, suggesting that its anxiolytic-like activity may not necessarily or predominantly reflect potentiation at α3βγ2 GABA_ARs. Nonetheless, as the anxiolytic effects are achievable at a dose devoid of any sedative potential, and having favorable safety (cytotoxicity) and metabolic stability profile, YT-III-31 represents a valuable candidate for further translational research.

GABAergic inhibitory neurons as therapeutic targets for cognitive impairment in schizophrenia

Schizophrenia is considered primarily as a cognitive disorder. However, functional outcomes in schizophrenia are limited by the lack of effective pharmacological and psychosocial interventions for cognitive impairment. GABA (gamma-aminobutyric acid) interneurons are the main inhibitory neurons in the central nervous system (CNS), and they play a critical role in a variety of pathophysiological processes including modulation of cortical and hippocampal neural circuitry and activity, cognitive function-related neural oscillations (eg, gamma oscillations) and information integration and processing. Dysfunctional GABA interneuron activity can disrupt the excitatory/inhibitory (E/I) balance in the cortex, which could represent a core pathophysiological mechanism underlying cognitive dysfunction in schizophrenia. Recent research suggests that selective modulation of the GABAergic system is a promising intervention for the treatment of schizophrenia-associated cognitive defects. In this review, we summarized evidence from postmortem and animal studies for abnormal GABAergic neurotransmission in schizophrenia, and how altered GABA interneurons could disrupt neuronal oscillations. Next, we systemically reviewed a variety of up-to-date subtype-selective agonists, antagonists, positive and negative allosteric modulators (including dual allosteric modulators) for α5/α3/α2 GABA_A and GABA_B receptors, and summarized their pro-cognitive effects in animal behavioral tests and clinical trials. Finally, we also discuss various representative histone deacetylases (HDAC) inhibitors that target GABA system through epigenetic modulations, GABA prodrug and presynaptic GABA transporter inhibitors. This review provides important information on current potential GABA-associated therapies and future insights for development of more effective treatments.

Negative allosteric modulation of alpha 5-containing GABAA receptors engenders antidepressant-like effects and selectively prevents age-associated hyperactivity in tau-depositing mice

RATIONALE

Associated with frank neuropathology, patients with Alzheimer's disease suffer from a host of neuropsychiatric symptoms that include depression, apathy, agitation, and aggression. Negative allosteric modulators (NAMs) of α5-containing GABA_A receptors have been suggested to be a novel target for antidepressant action. We hypothesized that pharmacological modulation of this target would engender increased motivation in stressful environments.

METHODS

We utilized electrophysiological recordings from Xenopus oocytes and behavioral measures in mice to address this hypothesis.

RESULTS

In the forced-swim assay in mice that detects antidepressant drugs, the α5β3γ2 GABA_Α receptor NAM, RY-080 produced a marked antidepressant phenotype. Another compound, PWZ-029, was characterized as an α5β3γ2 receptor NAM of lower intrinsic efficacy in electrophysiological studies in Xenopus oocytes. In contrast to RY-080, PWZ-029 was only moderately active in the forced-swim assay and the α5β3γ2 receptor antagonist, Xli-093, was not active at all. The effects of RY-080 were prevented by the non-selective benzodiazepine receptor antagonist flumazenil as well as by the selective ligands, PWZ-029 and Xli-093. These findings demonstrate that this effect of RY-080 is driven by negative allosteric modulation of α5βγ2 GABA_A receptors. RY-080 was not active in the tail-suspension test. We also demonstrated a reduction in the age-dependent hyperactivity exhibited by transgenic mice that accumulate pathological tau (rTg4510 mice) by RY-080. The decrease in hyperactivity by RY-080 was selective for the hyperactivity of the rTg4510 mice since the locomotion of control strains of mice were not significantly affected by RY-080.

CONCLUSIONS

α5βγ2 GABA_A receptor NAMs might function as a pharmacological treatment for mood, amotivational syndromes, and psychomotor agitation in patients with Alzheimer's and other neurodegenerative disorders.

Further evaluation of the potential anxiolytic activity of imidazo[1,5-a][1,4]diazepin agents selective for α2/3-containing GABAA receptors

Positive allosteric modulators of GABAA receptors transduce a host of beneficial effects including anxiolytic actions. We have recently shown that bioavailability and anxiolytic-like activity can be improved by eliminating the ester functionality in imidazo[1,5-a][1,4]diazepines. In the present series of experiments, we further substantiate the value of heterocyle replacement of the ester for potential treatment of anxiety. None of three esters was active in a Vogel conflict test in rats that detects anxiolytic drugs like diazepam. Compounds 7 and 8, ester bioisosters, were selective for alpha 2 and 3 over alpha 1-containing GABAA receptors but also had modest efficacy at GABAA alpha 5-containing receptors. Compound 7 was efficacious and potent in this anxiolytic-detecting assay without affecting non-punished responding. The efficacies of the esters and of compound 7 were predicted from their efficacies as anticonvulsants against the GABAA antagonist pentylenetetrazole (PTZ). In contrast, the related structural analog, compound 8, did not produce anxiolytic-like effects in rats despite anticonvulsant efficacy. These data thus support the following conclusions: 1) ancillary pharmacological actions of compound 8 might be responsible for its lack of anxiolytic-like efficacy despite its efficacy as an anticonvulsant 2) esters of imidazo[1,5-a][1,4]diazepines do not demonstrate anxiolytic-like effects in rats due to their low bioavailability and 3) replacement of the ester function with suitable heterocycles markedly improves bioavailability and engenders molecules with the opportunity to have potent and efficacious effects in vivo that correspond to human anxiolytic actions.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Disinhibition, an emerging pharmacology of learning and memory

Learning and memory are dependent on interactive excitatory and inhibitory mechanisms. In this review, we discuss a mechanism called disinhibition, which is the release of an inhibitory constraint that effectively results in an increased activity in the target neurons (for example, principal or projection neurons). We focus on discussing the role of disinhibition in learning and memory at a basic level and in disease models with cognitive deficits and highlight a strategy to reverse cognitive deficits caused by excess inhibition, through disinhibition of α5-containing GABA _A receptors mediating tonic inhibition in the hippocampus, based on subtype-selective negative allosteric modulators as a novel class of drugs.

Sex-Dependent Anti-Stress Effect of an α5 Subunit Containing GABAA Receptor Positive Allosteric Modulator

Rationale: Current first-line treatments for stress-related disorders such as major depressive disorder (MDD) act on monoaminergic systems and take weeks to achieve a therapeutic effect with poor response and low remission rates. Recent research has implicated the GABAergic system in the pathophysiology of depression, including deficits in interneurons targeting the dendritic compartment of cortical pyramidal cells.

Objectives: The present study evaluates whether SH-053-2’F-R-CH3 (denoted “α5-PAM”), a positive allosteric modulator selective for α5-subunit containing GABA_A receptors found predominantly on cortical pyramidal cell dendrites, has anti-stress effects.

Methods: Female and male C57BL6/J mice were exposed to unpredictable chronic mild stress (UCMS) and treated with α5-PAM acutely (30 min prior to assessing behavior) or chronically before being assessed behaviorally.

Results: Acute and chronic α5-PAM treatments produce a pattern of decreased stress-induced behaviors (denoted as “behavioral emotionality”) across various tests in female, but not in male mice. Behavioral Z-scores calculated across a panel of tests designed to best model the range and heterogeneity of human symptomatology confirmed that acute and chronic α5-PAM treatments consistently produce significant decreases in behavioral emotionality in several independent cohorts of females. The behavioral responses to α5-PAM could not be completely accounted for by differences in drug brain disposition between female and male mice. In mice exposed to UCMS, expression of the Gabra5 gene was increased in the frontal cortex after acute treatment and in the hippocampus after chronic treatment with α5-PAM in females only, and these expression changes correlated with behavioral emotionality.

Conclusion: We showed that acute and chronic positive modulation of α5 subunit-containing GABA_A receptors elicit anti-stress effects in a sex-dependent manner, suggesting novel therapeutic modalities.

Ester to amide substitution improves selectivity, efficacy and kinetic behavior of a benzodiazepine positive modulator of GABAA receptors containing the α5 subunit

We have synthesized and characterized MP-III-022 ((R)-8-ethynyl-6-(2-fluorophenyl)-N,4-dimethyl-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxamide) in vitro and in vivo as a binding- and efficacy-selective positive allosteric modulator of GABA_A receptors containing the α5 subunit (α5GABA_ARs). By approximation of the electrophysiological responses which the estimated free rat brain concentrations can induce, we demonstrated that convenient systemic administration of MP-III-022 in the dose range 1-10mg/kg may result in a selective potentiation, over a wide range from mild to moderate to strong, of α5βγ2 GABA_A receptors. For eliciting a comparable range of potentiation, the widely studied parent ligand SH-053-2'F-R-CH3 containing an ester moiety needs to be administered over a much wider dose range (10-200mg/kg), but at the price of activating non-α5 GABA_ARs as well as the desired α5GABA_ARs at the highest dose. At the dose of 10mg/kg, which elicits a strong positive modulation of α5GABA_ARs, MP-III-022 caused mild, but significant muscle relaxation, while at doses 1-10mg/kg was devoid of ataxia, sedation or an influence on the anxiety level, characteristic for non-selective benzodiazepines. As an amide compound with improved stability and kinetic properties, MP-III-022 may represent an optimized tool to study the influence of α5GABA_ARs on the neuronal pathways related to CNS disorders such as schizophrenia, Alzheimer's disease, Down syndrome or autism.

Pharmacological Evaluation of Naproxen Metal Complexes on Antinociceptive, Anxiolytic, CNS Depressant, and Hypoglycemic Properties

Purpose. The present study was designed to investigate the antinociceptive, anxiolytic, CNS depressant, and hypoglycemic effects of the naproxen metal complexes. Methods. The antinociceptive activity was evaluated by acetic acid-induced writhing method and radiant heat tail-flick method while anxiolytic activity was evaluated by elevated plus maze model. The CNS depressant activity of naproxen metal complexes was assessed using phenobarbitone-induced sleeping time test and the hypoglycemic test was performed using oral glucose tolerance test. Results. Metal complexes significantly (P < 0.001) reduced the number of abdominal muscle contractions induced by 0.7% acetic acid solution in a dose dependent manner. At the dose of 25 mg/kg body weight p.o. copper, cobalt, and zinc complexes exhibited higher antinociceptive activity having 59.15%, 60.56%, and 57.75% of writhing inhibition, respectively, than the parent ligand naproxen (54.93%). In tail-flick test, at both doses of 25 and 50 mg/kg, the copper, cobalt, silver, and zinc complexes showed higher antinociceptive activity after 90 minutes than the parent drug naproxen. In elevated plus maze (EPM) model the cobalt and zinc complexes of naproxen showed significant anxiolytic effects in dose dependent manner, while the copper, cobalt, and zinc complexes showed significant CNS depressant and hypoglycemic activity. Conclusion. The present study demonstrated that copper, cobalt, and zinc complexes possess higher antinociceptive, anxiolytic, CNS depressant, and hypoglycemic properties than the parent ligand.

Effects of the 5-HT(1A) Receptor Agonist Tandospirone on ACTH-Induced Sleep Disturbance in Rats

The aim of this study was to compare the effect of the serotonin (5-HT)1A receptor agonist tandospirone versus that of the benzodiazepine hypnotic flunitrazepam in a rat model of long-term adrenocorticotropic hormone (ACTH)-induced sleep disturbance. Rats implanted with electrodes for recording electroencephalogram and electromyogram were injected with ACTH once daily at a dose of 100 µg/rat. Administration of ACTH for 10 d caused a significant increase in sleep latency, decrease in non-rapid eye movement (non-REM) sleep time, and increase in wake time. Tandospirone caused a significant decrease in sleep latency and increase in non-REM sleep time in rats treated with ACTH. The effect of tandospirone on sleep patterns was antagonized by the 5-HT1A receptor antagonist WAY-100635. In contrast, flunitrazepam had no significant effect on sleep parameters in ACTH-treated rats. These results clearly indicate that long-term administration of ACTH causes sleep disturbance, and stimulating the 5-HT1A receptor by tandospirone may be efficacious for improving sleep in cases in which benzodiazepine hypnotics are ineffective.

A Review of the Updated Pharmacophore for the Alpha 5 GABA(A) Benzodiazepine Receptor Model

An updated model of the GABA(A) benzodiazepine receptor pharmacophore of the α5-BzR/GABA(A) subtype has been constructed prompted by the synthesis of subtype selective ligands in light of the recent developments in both ligand synthesis, behavioral studies, and molecular modeling studies of the binding site itself. A number of BzR/GABA(A) α5 subtype selective compounds were synthesized, notably α5-subtype selective inverse agonist PWZ-029 (1) which is active in enhancing cognition in both rodents and primates. In addition, a chiral positive allosteric modulator (PAM), SH-053-2′F-R-CH₃ (2), has been shown to reverse the deleterious effects in the MAM-model of schizophrenia as well as alleviate constriction in airway smooth muscle. Presented here is an updated model of the pharmacophore for α5β2γ2 Bz/GABA(A) receptors, including a rendering of PWZ-029 docked within the α5-binding pocket showing specific interactions of the molecule with the receptor. Differences in the included volume as compared to α1β2γ2, α2β2γ2, and α3β2γ2 will be illustrated for clarity. These new models enhance the ability to understand structural characteristics of ligands which act as agonists, antagonists, or inverse agonists at the Bz BS of GABA(A) receptors.

Selective targeting of the α5-subunit of GABAA receptors relaxes airway smooth muscle and inhibits cellular calcium handling

The clinical need for novel bronchodilators for the treatment of bronchoconstrictive diseases remains a major medical issue. Modulation of airway smooth muscle (ASM) chloride via GABAA receptor activation to achieve relaxation of precontracted ASM represents a potentially beneficial therapeutic option. Since human ASM GABAA receptors express only the α4- and α5-subunits, there is an opportunity to selectively target ASM GABAA receptors to improve drug efficacy and minimize side effects. Recently, a novel compound (R)-ethyl8-ethynyl-6-(2-fluorophenyl)-4-methyl-4H-benzo[f]imidazo[1,5-a][1,4] diazepine-3-carboxylate (SH-053-2'F-R-CH3) with allosteric selectivity for α5-subunit containing GABAA receptors has become available. We questioned whether this novel GABAA α5-selective ligand relaxes ASM and affects intracellular calcium concentration ([Ca(2+)]i) regulation. Immunohistochemical staining localized the GABAA α5-subunit to human ASM. The selective GABAA α5 ligand SH-053-2'F-R-CH3 relaxes precontracted intact ASM; increases GABA-activated chloride currents in human ASM cells in voltage-clamp electrophysiology studies; and attenuates bradykinin-induced increases in [Ca(2+)]i, store-operated Ca(2+) entry, and methacholine-induced Ca(2+) oscillations in peripheral murine lung slices. In conclusion, selective subunit targeting of endogenous α5-subunit containing GABAA receptors on ASM may represent a novel therapeutic option to treat severe bronchospasm.

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

Characterization of a novel vasopressin V1b receptor antagonist, V1B-30N, in animal models of anxiety-like and depression-like behavior

Overactivity of the hypothalamic-pituitary-adrenal (HPA) axis has been linked to affective disorders such as anxiety and depression. Dampening HPA activity has, therefore, been considered as a possible means of treating affective disorders. Given the important role of vasopressin in modulating the HPA axis, one strategy has focused on inhibiting activity of the vasopressin 1b (V1b) receptor. In animals, V1b receptor antagonists reduce plasma stress hormone levels and have been shown to have an anxiolytic-like effect. Recently, V1B-30N was identified as a highly potent V1b receptor antagonist with selectivity over other vasopressin receptors, which is evaluated here in rodent models of anxiety-like and depression-like behaviors. V1B-30N (1-30mg/kg, IP) dose-dependently reduced separation-induced vocalizations in rat pups without producing any sedative effects in the animals. Similarly, V1B-30N (3-30mg/kg, IP) dose-dependently reduced separation-induced vocalizations in guinea pig pups. In a conflict assay, conditioned lick suppression, V1B-30N (3-30mg/kg, IP) increased punished licking. To assess antidepressive-like properties, V1B-30N (1-30mg/kg) was tested in the mouse and rat forced-swim tests but was found to be inactive. These results are consistent with previous findings with other V1b antagonists, which suggest that acute pharmacological antagonism of the V1b receptor has anxiolytic-like but not antidepressant-like properties.

Experimental design in formulation of diazepam nanoemulsions: physicochemical and pharmacokinetic performances

With the aid of experimental design, we developed and characterized nanoemulsions for parenteral drug delivery. Formulations containing a mixture of medium-chain triglycerides and soybean oil as oil phase, lecithin (soybean/egg) and polysorbate 80 as emulsifiers, and 0.1 M phosphate buffer solution (pH 8) as aqueous phase were prepared by cold high-pressure homogenization. To study the effects of the oil content, lecithin type, and the presence of diazepam as a model drug and their interactions on physicochemical characteristics of nanoemulsions, a three factor two-level full factorial design was applied. The nanoemulsions were evaluated concerning droplet size and size distribution, surface charge, viscosity, morphology, drug-excipient interactions, and physical stability. The characterization revealed the small spherical droplets in the range 195 -220 nm with polydispersity index below 0.15 and zeta potential between -30 and - 60 mV. Interactions among the investigated factors, rather than factors alone, were shown to more profoundly affect nanoemulsion characteristics. In vivo pharmacokinetic study of selected diazepam nanoemulsions with different oil content (20%, 30%, and 40%, w/w) demonstrated fast and intense initial distribution into rat brain of diazepam from nanoemulsions with 20% and 30% (w/w) oil content, suggesting their applicability in urgent situations.

Enhancing the function of alpha5-subunit-containing GABAA receptors promotes action potential firing of neocortical neurons during up-states

Neocortical neurons mediate the sedative and anticonvulsant properties of benzodiazepines. These agents enhance synaptic inhibition via positive modulation of γ-aminobutyric acid (GABAA) receptors harboring α1-, α2-, α3- or α5-protein subunits. Benzodiazepine-sensitive GABAA receptors containing the α5-subunit are abundant in the neocortex, but their impact in controlling neuronal firing patterns is unknown. Here we studied how the discharge rates of cortical neurons are modified by a positive (SH-053-2'F-R-CH3) and a negative (L 655,708) α5-subunit-preferring allosteric modulator in comparison to diazepam, the classical non-selective benzodiazepine. Drug actions were characterized in slice cultures from wild-type and α5(H105R) knock-in mice by performing extracellular multi-unit-recordings. In knock-in mice, receptors containing the α5 subunit are insensitive to benzodiazepines. The non-selective positive allosteric modulator diazepam decreased the discharge rates of neocortical neurons during episodes of ongoing neuronal activity (up states). In contrast to diazepam, the α5-preferring positive modulator SH-053-2'F-R-CH3 accelerated action potential firing during up states. This promoting action was absent in slices from α5(H105R) mice, confirming that it is mediated by the α5-subunit. Consistent with these observations, the negative α5-selective modulator L 655,708 inhibited up state action potential activity in slices from wild-type mice. The opposing actions of diazepam and SH-053-2'F-R-CH3, which both enhance GABAA receptor function but differ in subtype-selectivity, uncovers contrasting roles of GABAA receptor subtypes in controlling the firing rates of cortical neurons. These findings may have important implications for the design of novel anaesthetic and anticonvulsant benzodiazepines displaying an improved efficacy and fewer side effects.

PWZ-029, an inverse agonist selective for α₅ GABAA receptors, improves object recognition, but not water-maze memory in normal and scopolamine-treated rats

Inverse agonism at the benzodiazepine site of α(5) subunit-containing GABA(A) receptors is an attractive approach for the development of putative cognition-enhancing compounds, which are still far from clinical application. Several ligands with binding and/or functional selectivity for α(5) GABA(A) receptors have been synthesized and tested in a few animal models. PWZ-029 is an α(5) GABA(A) selective inverse agonist whose memory enhancing effects were demonstrated in the passive avoidance task in rats and in Pavlovian fear conditioning in mice. In the present study we investigated the effects of PWZ-029 administration in novel object recognition test and Morris water maze, in normal and scopolamine-treated rats. All the three doses of PWZ-029 (2, 5 and 10 mg/kg) improved object recognition after the 24-h delay period, as shown by significant differences between the exploration times of the novel and old object, and the respective discrimination indices. PWZ-029 (2 mg/kg) also successfully reversed the 0.3 mg/kg scopolamine-induced deficit in recognition memory after the 1-h delay. In the Morris water maze test, PWZ-029 (5, 10 and 15 mg/kg) did not significantly influence swim patterns, either during five acquisition days or during the treatment-free probe trial. PWZ-029 (2, 5 and 10 mg/kg) also proved to be ineffective in the reversal of the 1mg/kg scopolamine-induced memory impairment in the water maze. The present mixed results encourage use of a variety of tests and experimental conditions in order to increase the predictability of preclinical testing of selective α(5) GABA(A) inverse agonists.

GABA(A) receptor α subunits differentially contribute to diazepam tolerance after chronic treatment

Background

Within the GABA_A-receptor field, two important questions are what molecular mechanisms underlie benzodiazepine tolerance, and whether tolerance can be ascribed to certain GABA_A-receptor subtypes.

Methods

We investigated tolerance to acute anxiolytic, hypothermic and sedative effects of diazepam in mice exposed for 28-days to non-selective/selective GABA_A-receptor positive allosteric modulators: diazepam (non-selective), bretazenil (partial non-selective), zolpidem (α₁ selective) and TPA023 (α_2/3 selective). In-vivo binding studies with [³H]flumazenil confirmed compounds occupied CNS GABA_A receptors.

Results

Chronic diazepam treatment resulted in tolerance to diazepam's acute anxiolytic, hypothermic and sedative effects. In mice treated chronically with bretazenil, tolerance to diazepam's anxiolytic and hypothermic, but not sedative, effects was seen. Chronic zolpidem treatment resulted in tolerance to diazepam's hypothermic effect, but partial anxiolytic tolerance and no sedative tolerance. Chronic TPA023 treatment did not result in tolerance to diazepam's hypothermic, anxiolytic or sedative effects.

Conclusions

Our data indicate that: (i) GABA_A-α₂/α₃ subtype selective drugs might not induce tolerance; (ii) in rodents quantitative and temporal variations in tolerance development occur dependent on the endpoint assessed, consistent with clinical experience with benzodiazepines (e.g., differential tolerance to antiepileptic and anxiolytic actions); (iii) tolerance to diazepam's sedative actions needs concomitant activation of GABA_A-α₁/GABA_A-α₅ receptors. Regarding mechanism, in-situ hybridization studies indicated no gross changes in expression levels of GABA_A α₁, α₂ or α₅ subunit mRNA in hippocampus or cortex. Since selective chronic activation of either GABA_A α₂, or α₃ receptors does not engender tolerance development, subtype-selective GABA_A drugs might constitute a promising class of novel drugs.

The role of α1 and α5 subunit-containing GABAA receptors in motor impairment induced by benzodiazepines in rats

Benzodiazepines negatively affect motor coordination and balance and produce myorelaxation. The aim of the present study was to examine the extent to which populations of γ-aminobutyric acid A (GABAA) receptors containing α1 and α5 subunits contribute to these motor-impairing effects in rats. We used the nonselective agonist diazepam and the α1-selective agonist zolpidem, as well as nonselective, α1-subunit and α5-subunit-selective antagonists flumazenil, βCCt, and XLi093, respectively. Ataxia and muscle relaxation were assessed by rotarod and grip strength tests performed 20 min after intraperitoneal treatment. Diazepam (2 mg/kg) induced significant ataxia and muscle relaxation, which were completely prevented by pretreatment with flumazenil (10 mg/kg) and βCCt (20 mg/kg). XLi093 antagonized the myorelaxant, but not the ataxic actions of diazepam. All three doses of zolpidem (1, 2, and 5 mg/kg) produced ataxia, but only the highest dose (5 mg/kg) significantly decreased the grip strength. These effects of zolpidem were reversed by βCCt at doses of 5 and 10 mg/kg, respectively. The present study demonstrates that α1 GABAA receptors mediate ataxia and indirectly contribute to myorelaxation in rats, whereas α5 GABAA receptors contribute significantly, although not dominantly, to muscle relaxation but not ataxia.

GABA(A) receptor physiology and its relationship to the mechanism of action of the 1,5-benzodiazepine clobazam

Clobazam was initially developed in the early 1970s as a nonsedative anxiolytic agent, and is currently available as adjunctive therapy for epilepsy and anxiety disorders in more than 100 countries. In October 2011, clobazam (Onfi™; Lundbeck Inc., Deerfield, IL, USA) was approved by the US FDA for use as adjunctive therapy for the treatment of seizures associated with Lennox-Gastaut syndrome in patients aged 2 years and older. It is a long-acting 1,5-benzodiazepine whose structure distinguishes it from the classic 1,4-benzodiazepines, such as diazepam, lorazepam and clonazepam. Clobazam is well absorbed, with peak concentrations occurring linearly 1-4 hours after administration. Both clobazam and its active metabolite, N-desmethylclobazam, are metabolized in the liver via the cytochrome P450 pathway. The mean half-life of N-desmethylclobazam (67.5 hours) is nearly double the mean half-life of clobazam (37.5 hours). Clobazam was synthesized with the anticipation that its distinct chemical structure would provide greater efficacy with fewer benzodiazepine-associated adverse effects. Frequently reported adverse effects of clobazam therapy include dizziness, sedation, drowsiness and ataxia. Evidence gathered from approximately 50 epilepsy clinical trials in adults and children indicated that the sedative effects observed with clobazam treatment were less severe than those reported with 1,4-benzodiazepines. In several studies of healthy volunteers and patients with anxiety, clobazam appeared to enhance participants' performance in cognitive tests, further distinguishing it from the 1,4-benzodiazepines. The anxiolytic and anticonvulsant effects of clobazam are associated with allosteric activation of the ligand-gated GABA(A) receptor. GABA(A) receptors are found extensively throughout the CNS, occurring synaptically and extrasynaptically. GABA(A) receptors are composed of five protein subunits, two copies of a single type of α subunit, two copies of one type of β subunit and a γ subunit. This arrangement results in a diverse assortment of receptor subtypes. As benzodiazepine pharmacology is influenced by differences in affinity for particular GABA(A) subtypes, characterizing the selectivity of different benzodiazepines is a promising avenue for establishing appropriate use of these agents in neurological disorders. Molecular techniques have significantly advanced since the inception of clobazam as a clinical agent, adding to the understanding of the GABA(A) receptor, its subunits and benzodiazepine pharmacology. Transgenic mouse models have been particularly useful in this regard. Comparative studies between transgenic and wild-type mice have further defined relationships between GABA(A) receptor composition and drug effects. From such studies, we have learned that sedating and amnesic effects are mediated by the GABA(A) α(1) subunit, α(2) receptors mediate anxiolytic effects, α subunits are involved with anticonvulsant activity, α(5) may be implicated in learning and memory, and β(3) subunit deficiency decreases GABA inhibition. Despite progress in determining the role of various subunits to specific benzodiazepine pharmacological actions, the precise mechanism of action of clobazam, and more importantly, how that mechanism of action translates into clinical consequences (i.e. efficacy, tolerability and safety) remain unknown. Testing clobazam and 1,4-benzodiazepines using a range of recombinant GABA(A) receptor subtypes would hopefully elucidate the subunits involved and strengthen our understanding of clobazam and its mechanism of action.

GABAA-positive modulator selective discriminative stimulus effects of 1,1,1-trichloroethane vapor

BACKGROUND

The abuse-related behavioral effects of inhalant vapors are poorly understood but probably involve multiple neurotransmitter receptor mechanisms. The present study examined the receptor systems responsible for transducing the discriminative stimulus of the abused chlorinated hydrocarbon 1,1,1-trichloroethane (TCE) in mice.

METHODS

Thirty mice were trained to discriminate 10 min of 12,000 ppm TCE vapor exposure from air using an operant procedure. Substitution tests were then conduced with positive GABA(A) receptor modulators and/or NMDA receptor antagonists.

RESULTS

The nonselective benzodiazepines midazolam and diazepam produced 62% and 61% and the barbiturate pentobarbital produced 68% TCE-lever selection. Zaleplon, an alpha1 subunit-preferring positive GABA(A) receptor benzodiazepine-site positive modulator resulted in 29% TCE-lever selection. The direct extrasynaptic GABA(A) agonist gaboxodol (THIP) and the GABA reuptake inhibitor tiagabine failed to substitute for TCE. No substitution was elicited by a competitive (CGS-19755), noncompetitive (dizocilpine) or glycine-site (L701,324) NMDA antagonist. The mixed benzodiazepine/noncompetitive NMDA antagonist anesthetic Telazol and the anticonvulsant valproic acid exhibited low levels of partial substitution for TCE (38% and 39%, respectively). Ethanol and nitrous oxide failed to substitute for TCE.

CONCLUSIONS

The results suggest that the discriminative stimulus effects of TCE are fairly selectively mediated by positive modulation of GABA(A) receptors. The failure of gaboxadol to substitute and the poor substitution by zaleplon suggests that extrasynaptic GABA(A) receptors as well as GABA(A) receptors containing alpha1 subunits and are not involved in transducing the discriminative stimulus of TCE. Studies with additional GABA(A) benzodiazepine-site positive modulators will be necessary to confirm and extend these findings.

Behavioural characterization of four endemic Stachys taxa

We performed a basic behavioral characterization of methanol extracts of four Balkan endemic Stachys taxa: S. anisochila (SA), S. beckeana (SB), S. plumosa (SP) and S. alpina subsp. dinarica (SAD). The behavioral activity of extracts dosed intraperitoneally in the range 100-400 mg/kg was examined in adult male Wistar rats, in the elevated plus maze, spontaneous locomotor activity, and grip strength tests, mainly predictive of anxiolytic, sedative and myorelaxant actions, respectively. All investigated Stachys extracts lacked anxiolytic or myorelaxant activities, while SB at 400 mg/kg exerted an anxiogenic-like effect. The study with the selective antagonist beta-CCt showed that the sedative effect of SAD was only partially mediated by GABAA receptors containing the alpha1-subunit. While discernible, the behavioral effects of SA and SP were not distinct. In all extracts, chlorogenic acid and verbascoside were identified. In SA, SB, and SAD the flavonoid fraction was constituted of isoscutellarein and hypolaetine glycosides, while in SP chrysoeriol and apigenin glycosides were present. The results reveal the psychotropic potential of four endemic Stachys taxa, of which SAD appeared most promising as a natural sedative.

MRK-409 (MK-0343), a GABAA receptor subtype-selective partial agonist, is a non-sedating anxiolytic in preclinical species but causes sedation in humans

MRK-409 binds to α1-, α2-, α3- and α5-containing human recombinant GABA(A) receptors with comparable high affinity (0.21-0.40 nM). However, MRK-409 has greater agonist efficacy at the α3 compared with α1 subtypes (respective efficacies relative to the full agonist chlordiazepoxide of 0.45 and 0.18). This compound readily penetrates the brain in rats and occupies the benzodiazepine site of GABA(A) receptors, measured using an in vivo [(3)H]flumazenil binding assay, with an Occ(50) of 2.2 mg/kg p.o. and a corresponding plasma EC(50) of 115 ng/mL. Behaviourally, the α3-preferring agonist efficacy profile of MRK-409 produced anxiolytic-like activity in rodent and primate unconditioned and conditioned models of anxiety with minimum effective doses corresponding to occupancies, depending on the particular model, ranging from ∼35% to 65% yet there were minimal overt signs of sedation at occupancies greater than 90%. In humans, however, safety and tolerability studies showed that there was pronounced sedation at a dose of 2 mg, resulting in a maximal tolerated dose of 1 mg. This 2 mg dose corresponded to a C(max) plasma concentration of 28 ng/mL, which, based on the rodent plasma EC(50) for occupancy of 115 ng/mL, suggested that sedation in humans occurs at low levels of occupancy. This was confirmed in human positron emission tomography studies, in which [(11)C]flumazenil uptake following a single dose of 1 mg MRK-409 was comparable to that of placebo, indicating that occupancy of GABA(A) receptor benzodiazepine binding sites by MRK-409 was below the limits of detection (i.e. <10%). Taken together, these data show that MRK-409 causes sedation in humans at a dose (2 mg) corresponding to levels of occupancy considerably less than those predicted from rodent models to be required for anxiolytic efficacy (∼35-65%). Thus, the preclinical non-sedating anxiolytic profile of MRK-409 did not translate into humans and further development of this compound was halted.

The inhibitory GABA system as a therapeutic target for cognitive symptoms in schizophrenia: investigational agents in the pipeline

IMPORTANCE OF THE FIELD

Cognitive impairments associated with schizophrenia include neuropsychological deficits in attention, working memory, learning and executive function. Because these cognitive deficits precede the onset of psychosis, are present in non-affected relatives and constitute the best predictor of functional outcome, they are a cardinal clinical feature in schizophrenia. Currently, no effective treatment for the cognitive symptoms in schizophrenia exists.

AREAS COVERED IN THIS REVIEW

There is evidence that the inhibitory GABA system is affected in schizophrenia, suggesting that cognitive impairments associated with schizophrenia may be effectively treated by drugs that modulate the GABA(A) receptor. However, classical benzodiazepines produce cognitive impairments and are associated with numerous side effects. The recent development of compounds with selective efficacy for different α subunits at the benzodiazepine site of the GABA(A) receptor has renewed interest for the therapeutic potential of GABAergic drugs.

WHAT THE READER WILL GAIN

This review summarizes the involvement of the inhibitory GABA system in the cognitive abnormalities of schizophrenia and discusses putative (selective) GABAergic cognition-enhancing drugs for schizophrenia.

TAKE HOME MESSAGE

If cognitive abnormalities in schizophrenic individuals are the result of GABAergic dysfunction, selectively modulating the GABA system could comprise a promising therapeutic intervention for cognitive symptoms in schizophrenia.

Searching for perfect sleep: the continuing evolution of GABAA receptor modulators as hypnotics

The non-benzodiazepine GABA(A) receptor modulators ('Z-drugs') - zaleplon, zolpidem, zopiclone and eszopiclone - have become the accepted treatments for insomnia where they are available. However, recent randomized, placebo-controlled trials suggest that, for these drugs, there may be particular efficacy and tolerability profiles and distinct clinical outcomes in specific patient populations. This is particularly apparent when hypnotic/ selective serotonin reuptake inhibitor co-therapy is used to treat patients with co-morbid insomnia and psychiatric disorders, as patient recovery appears to be accelerated and enhanced by some drugs but not others. Emerging evidence of why this should be the case is that these hypnotic drugs may differ significantly from each other in their pharmacodynamic and pharmacokinetic profiles. Functional selectivity for specific GABA(A) receptor subtypes may determine each drug's clinical attributes, while the pharmacokinetic characteristics of Z-drugs also determine to a large extent how they perform in the clinic. For example, activity at GABA(A) alpha 1 receptor subtypes may be associated with sedative effects, whereas activity at alpha 2 and alpha 3 receptor subtypes may be associated with anxiolytic and antidepressant effects. In summary, the distinct clinical outcomes of zaleplon, zolpidem, zopiclone and eszopiclone may be explained by each drug's unique GABA(A) receptor subunit selectivity and pharmacokinetic profile. Further investigation of GABA( A) receptor subtype effects would help to increase understanding of current hypnotic drug effects, while knowledge of each drug's specific binding profile should enable clinicians to tailor treatment to individual patient's needs.

The point mutation gamma 2F77I changes the potency and efficacy of benzodiazepine site ligands in different GABAA receptor subtypes

Benzodiazepine site agonists or inverse agonists enhance or reduce gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated inhibition of neurons, respectively. Recently, it was demonstrated that the point mutation gamma 2F77I causes a drastic change in the affinity of a variety of benzodiazepine agonists or inverse agonists in receptor binding studies. Here we investigated the potency and efficacy of 10 benzodiazepine site ligands from 6 structural classes in wild-type and gamma 2F77I point mutated recombinant GABA(A) receptors composed of alpha 1 beta 3 gamma 2, alpha 2 beta 3 gamma 2, alpha 3 beta 3 gamma 2, alpha 4 beta 3 gamma 2, alpha 5 beta 3 gamma 2, and alpha 6 beta 3 gamma 2 subunits. Results indicate that the effects of the benzodiazepine site ligands zolpidem, zopiclone, Cl218872, L-655,708 and DMCM were nearly completely eliminated in all mutated receptors up to a 1 microM concentration. The effects of bretazenil, Ro15-1788 or abecarnil were eliminated in some, but not all mutated receptors, suggesting that the gamma 2F77I mutation differentially influences the actions of these ligands in different receptor subtypes. In addition, this point mutation also influences the efficacy of diazepam for enhancing GABA-induced chloride flux, suggesting that the amino acid residue gamma 2F77 might also be involved in the transduction of the effect of benzodiazepines from binding to gating. The application of these drugs in a novel mouse model is discussed.

Novel positive allosteric modulators of GABAA receptors: do subtle differences in activity at alpha1 plus alpha5 versus alpha2 plus alpha3 subunits account for dissimilarities in behavioral effects in rats?

Over the last years, genetic studies have greatly improved our knowledge on the receptor subtypes mediating various pharmacological effects of positive allosteric modulators at GABA(A) receptors. This stimulated the development of new benzodiazepine (BZ)-like ligands, especially those inactive/low-active at GABA(A) receptors containing the alpha(1) subunit, with the aim of generating more selective drugs. Hereby, the affinity and efficacy of four recently synthesized BZ site ligands: SH-053-2'N, SH-053-S-CH3-2'F, SH-053-R-CH3-2'F and JY-XHe-053 were assessed. They were also studied in behavioral tests of spontaneous locomotor activity, elevated plus maze, and water maze in rats, which are considered predictive of, respectively, the sedative, anxiolytic, and amnesic influence of BZs. The novel ligands had moderately low to low affinity and mild to partial agonistic efficacy at GABA(A) receptors containing the alpha(1) subunit, with variable, but more pronounced efficacy at other BZ-sensitive binding sites. While presumably alpha(1) receptor-mediated sedative effects of GABA(A) modulation were not fully eliminated with any of the ligands tested, only SH-053-2'N and SH-053-S-CH3-2'F, both dosed at 30 mg/kg, exerted anxiolytic effects. The lack of clear anxiolytic-like activity of JY-XHe-053, despite its efficacy at alpha(2)- and alpha(3)-GABA(A) receptors, may have been partly connected with its preferential affinity at alpha(5)-GABA(A) receptors coupled with weak agonist activity at alpha(1)-containing subtypes. The memory impairment in water-maze experiments, generally reported with BZ site agonists, was completely circumvented with all four ligands. The results suggest that a substantial amount of activity at alpha(1) GABA(A) receptors is needed for affecting spatial learning and memory impairments, while much weaker activity at alpha(1)- and alpha(5)-GABA(A) receptors is sufficient for eliciting sedation.

The differential role of alpha1- and alpha5-containing GABA(A) receptors in mediating diazepam effects on spontaneous locomotor activity and water-maze learning and memory in rats

The clinical use of benzodiazepines (BZs) is hampered by sedation and cognitive deterioration. Although genetic and pharmacological studies suggest that alpha1- and alpha5-containing GABA(A) receptors mediate and/or modulate these effects, their molecular substrate is not fully elucidated. By the use of two selective ligands: the alpha1-subunit affinity-selective antagonist beta-CCt, and the alpha5-subunit affinity- and efficacy-selective antagonist XLi093, we examined the mechanisms of behavioural effects of diazepam in the tests of spontaneous locomotor activity and water-maze acquisition and recall, the two paradigms indicative of sedative- and cognition-impairing effects of BZs, respectively. The locomotor-activity decreasing propensity of diazepam (significant at 1.5 and 5 mg/kg) was antagonized by beta-CCt (5 and 15 mg/kg), while it tended to be potentiated by XLi093 in doses of 10 mg/kg, and especially 20 mg/kg. Diazepam decreased acquisition and recall in the water maze, with a minimum effective dose of 1.5 mg/kg. Both antagonists reversed the thigmotaxis induced by 2 mg/kg diazepam throughout the test, suggesting that both GABA(A) receptor subtypes participate in BZ effects on the procedural component of the task. Diazepam-induced impairment in the declarative component of the task, as assessed by path efficiency, the latency and distance before finding the platform across acquisition trials, and also by the spatial parameters in the probe trial, was partially prevented by both, 15 mg/kg beta-CCt and 10 mg/kg XLi093. Combining a BZ with beta-CCt results in the near to control level of performance of a cognitive task, without sedation, and may be worth testing on human subjects.

Rosewood oil induces sedation and inhibits compound action potential in rodents

AIM OF THE STUDY

Aniba rosaeodora is an aromatic plant which has been used in Brazil folk medicine due to its sedative effect. Therefore, the purpose of the present study was to evaluate the sedative effect of linalool-rich rosewood oil in mice. In addition we sought to investigate the linalool-rich oil effects on the isolated nerve using the single sucrose-gap technique.

MATERIALS AND METHODS

Sedative effect was determined by measuring the potentiation of the pentobarbital-induced sleeping time. The compound action potential amplitude was evaluated as a way to detect changes in excitability of the isolated nerve.

RESULTS

The results showed that administration of rosewood oil at the doses of 200 and 300 mg/kg significantly decreased latency and increased the duration of sleeping time. On the other hand, the dose of 100 mg/kg potentiated significantly the pentobarbital action decreasing pentobarbital latency time and increasing pentobarbital sleeping time. In addition, the effect of linalool-rich rosewood oil on the isolated nerve of the rat was also investigated through the single sucrose-gap technique. The amplitude of the action potential decreased almost 100% when it was incubated for 30 min at 100 microg/ml.

CONCLUSIONS

From this study, it is suggested a sedative effect of linalool-rich rosewood oil that could, at least in part, be explained by the reduction in action potential amplitude that provokes a decrease in neuronal excitability.

Glutamatergic and GABAergic modulations of ultrasonic vocalizations during maternal separation distress in mouse pups

INTRODUCTION

Dysregulation of GABAergic inhibition and glutamatergic excitation has been implicated in exaggerated anxiety. Mouse pups emit distress-like ultrasonic vocalizations (USVs) when they are separated from their dam/siblings, and this behavior is reduced by benzodiazepines (BZs) which modulate GABAergic inhibition. The roles of glutamate receptors on USVs remain to be investigated.

MATERIALS AND METHODS

We examined the roles of glutamate receptor subtypes on mouse pup USVs using N-methyl-D: -aspartate (NMDA) receptor antagonists with different affinities [dizocilpine (MK-801), memantine, and neramexane] and group II metabotropic glutamate receptor agonist (LY-379268) and antagonist (LY-341495). These effects were compared with classic BZs: flunitrazepam, bromazepam, and chlordiazepoxide. To assess the role of GABA(A) receptor subunits on USVs, drugs that have preferential actions at different GABA(A)-alpha subunits (L-838417 and QH-ii-066) were tested. Seven-day-old CFW mouse pups were separated from their dam and littermates and placed individually on a 19 degrees C test platform for 4 min. Grid crossings and body rolls were measured in addition to USVs.

RESULTS

Dizocilpine dose-dependently reduced USVs, whereas memantine and neramexane showed biphasic effects and enhanced USVs at low to moderate doses. The NMDA receptor antagonists increased locomotion. LY-379268 reduced USVs but also suppressed locomotion. All BZs reduced USVs and increased motor incoordination. Neither L-838417 nor QH-ii-066 changed USVs, but both induced motor incoordination.

CONCLUSION

Low-affinity NMDA receptor antagonists, but not the high-affinity antagonist, enhanced mouse pup distress calls, which may be reflective of an anxiety-like state. BZs reduced USVs but also induced motor incoordination, possibly mediated by the alpha5 subunit containing GABA(A) receptors.

Contribution of alpha1 subunit-containing gamma-aminobutyric acidA (GABAA) receptors to motor-impairing effects of benzodiazepines in squirrel monkeys

RATIONALE

Benzodiazepines (BZs) are effective anxiolytics and hypnotics, but their use is limited by unwanted side effects, such as motor impairment.

OBJECTIVES

To assess the contribution of alpha1 subunit-containing gamma-aminobutyric acid(A) (GABA(A)) receptor subtypes to the motor-impairing effects of BZs, the present study evaluated two observable measures of motor coordination (balance on a pole, resistance to hind-limb flexion) engendered by nonselective and selective BZ-site agonists in squirrel monkeys.

MATERIALS AND METHODS

Multiple doses of nonselective BZs (triazolam, alprazolam, diazepam, and chlordiazepoxide) and alpha1 subunit-preferring agonists (zolpidem and zaleplon) were administered to adult male squirrel monkeys (N = 4-6), and experimenters rated the monkey's ability to balance on a horizontal pole ("ataxic-like effects"), as well as the degree of resistance to hind-limb flexion ("myorelaxant-like effects").

RESULTS

Administration of all BZ-type drugs resulted in ataxic-like and myorelaxant-like effects. Pretreatment with the alpha1 subunit-preferring antagonist beta-carboline-3-carboxylate-t-butyl ester (betaCCT) attenuated the ataxic-like effects engendered by both types of drugs. However, betaCCT was largely ineffective at blocking the ability of both BZs and non-BZs to induce myorelaxant-like effects.

CONCLUSIONS

These experiments demonstrate dose-dependent motor impairment in squirrel monkeys using quantitative behavioral observation techniques. Altogether, these findings suggest a lack of a prominent role for alpha1 subunit-containing receptors in the alteration of hind-limb flexion, a putative measure of myorelaxation, induced by BZ-type drugs in monkeys.

In vivo characterization and dynamic receptor occupancy imaging of TPA023B, an alpha 2/alpha 3/alpha 5 subtype selective gamma-aminobutyric acid-a partial agonist

BACKGROUND

A novel, high-affinity (.7-2.0 nmol) compound that selectively activates the alpha2, alpha 3, and alpha 5 (but not alpha1) gamma-aminobutyric acid-A (GABA(A)) receptor subtypes, TPA023B (2',6-difluoro-5'-[3-(1-hydroxy-1-methylethyl) imidazo[1,2-b][1,2,4]triazin-7-yl][1,1'-biphenyl]-2-carbonitrile) was pharmacologically characterized and studied by means of positron emission tomography (PET) to determine dynamic occupancies of the benzodiazepine binding site of human brain GABA(A) receptors after a single oral dose.

METHODS

Four healthy male volunteers were studied in a double-blind, randomized placebo-controlled study of which three were given a single dose of 1.5 mg TPA023B and the fourth received placebo. The time course of GABA(A) receptor occupancy was determined with multiple dynamic [(11)C]flumazenil PET studies at pre-dose baseline and 5 and 24 hours after dose. Arterial sampling and full kinetic modeling with a two-compartment model was used to calculate parametric maps of receptor availability (distribution volume V(T)) and of occupancy.

RESULTS

The GABA(A) receptor occupancy as determined from [(11)C]flumazenil V(T) values in all brain regions was reduced homogeneously, on average by 52.5 +/- 1.2% after 5 hours and 46.4 +/- 6.0% after 24 hours. No serious adverse events were encountered in humans.

CONCLUSIONS

Single oral doses of 1.5 mg of TPA023B correspond to average receptor occupancies in neocortical regions of 52% and 46% after 5 and 24 hours, respectively. Provided suitable ligands and quantification methods are available for the appropriate target, quantitative PET offers a unique tool for dynamic in vivo measurement of relevant on-site receptor occupancy.

GABA(A) Receptor Dynamics and Constructing GABAergic Synapses

GABA_A receptors are located on the majority of neurons in the central and peripheral nervous system, where they mediate important actions of the neurotransmitter gamma-aminobutyric acid. Early in development the trophic properties of GABA allow a healthy development of the nervous system. Most neurons have a high intracellular Cl-concentration early in life due to the late functional expression of the Cl-pump KCC2, therefore GABA has excitatory effects at this stage. Upon higher expression and activation of KCC2 GABA takes on its inhibitory effects while glutamate functions as the major excitatory neurotransmitter. Like all multisubunit membrane proteins the GABA_A receptor is assembled in the ER and travels through the Golgi and remaining secretory pathway to the cell surface, where it mediates GABA actions either directly at the synapses or at extrasynaptic sites responding to ambient GABA to provide a basal tonic inhibitory state. In order to adapt to changing needs and information states, the GABAergic system is highly dynamic. That includes subtype specific trafficking to different locations in the cell, regulation of mobility by interaction with scaffold molecules, posttranslational modifications, that either directly affect channel function or the interaction with other proteins and finally the dynamic exchange between surface and intracellular receptor pools, that either prepare receptors for recycling to the surface or degradation. Here we give an overview of the current understanding of GABA_A receptor functional and molecular dynamics that play a major part in maintaining the balance between excitation and inhibition and in changes in network activity.