Chronic benzodiazepine treatment of cells expressing recombinant GABA(A) receptors uncouples allosteric binding: studies on possible mechanisms

cAMP-PKA signaling pathway and anxiety: Where do we go next?

Cyclic adenosine monophosphate (cAMP) is an intracellular second messenger that is derived from the conversion of adenosine triphosphate catalysed by adenylyl cyclase (AC). Protein kinase A (PKA), the main effector of cAMP, is a dimeric protein kinase consisting of two catalytic subunits and two regulatory subunits. When cAMP binds to the regulatory subunits of PKA, it leads to the dissociation and activation of PKA, which allows the catalytic subunit of PKA to phosphorylate target proteins, thereby regulating various physiological functions and metabolic processes in cellular function. Recent researches also implicate the involvement of cAMP-PKA signaling in the pathologenesis of anxiety disorder. However, there are still debates on the prevention and treatment of anxiety disorders from this signaling pathway. To review the function of cAMP-PKA signaling in anxiety disorder, we searched the publications with the keywords including "cAMP", "PKA" and "Anxiety" from Pubmed, Embase, Web of Science and CNKI databases. The results showed that the number of publications on cAMP-PKA pathway in anxiety disorder tended to increase. Bioinformatics results displayed a close association between the cAMP-PKA pathway and the occurrence of anxiety. Mechanistically, cAMP-PKA signaling could influence brain-derived neurotrophic factor and neuropeptide Y and participate in the regulation of anxiety. cAMP-PKA signaling could also oppose the dysfunctions of gamma-aminobutyric acid (GABA), intestinal flora, hypothalamic-pituitary-adrenal axis, neuroinflammation, and signaling proteins (MAPK and AMPK) in anxiety. In addition, chemical agents with the ability to activate cAMP-PKA signaling demonstrated therapy potential against anxiety disorders. This review emphasizes the central roles of cAMP-PKA signaling in anxiety and the targets of the cAMP-PKA pathway would be potential candidates for treatment of anxiety. Nevertheless, more laboratory investigations to improve the therapeutic effect and reduce the adverse effect, and continuous clinical research will warrant the drug development.

Integrated workflow for the identification of new GABAA R positive allosteric modulators based on the in silico screening with further in vitro validation. Case study using Enamine's stock chemical space

Numerous studies reported an association between GABAA R subunit genes and epilepsy, eating disorders, autism spectrum disorders, neurodevelopmental disorders, and bipolar disorders. This study was aimed to find some potential positive allosteric modulators and was performed by combining the in silico approach with further in vitro evaluation of its real activity. We started from the GABAA R-diazepam complexes and assembled a lipid embedded protein ensemble to refine it via molecular dynamics (MD) simulation. Then we focused on the interaction of α1β2γ2 with some Z-drugs (non-benzodiazepine compounds) using an Induced Fit Docking (IFD) into the relaxed binding site to generate a pharmacophore model. The pharmacophore model was validated with a reference set and applied to decrease the pre-filtered Enamine database before the main docking procedure. Finally, we succeeded in identifying a set of compounds, which met all features of the docking model. The aqueous solubility and stability of these compounds in mouse plasma were assessed. Then they were tested for the biological activity using the rat Purkinje neurons and CHO cells with heterologously expressed human α1β2γ2 GABAA receptors. Whole-cell patch clamp recordings were used to reveal the GABA induced currents. Our study represents a convenient and tunable model for the discovery of novel positive allosteric modulators of GABAA receptors. A High-throughput virtual screening of the largest available database of chemical compounds resulted in the selection of 23 compounds. Further electrophysiological tests allowed us to determine a set of 3 the most outstanding active compounds. Considering the structural features of leader compounds, the study can develop into the MedChem project soon.

Diazepam-Induced Down-Regulation of The Gabaa Receptor α1 Subunit, as Mediated by the Activation of L-Type Voltage-Gated Calcium Channel/Ca2+/Protein Kinase A Signaling Cascade

Benzodiazepines are among the most prescribed drug class worldwide to treat disorders such as anxiety, insomnia, muscle spasticity, and convulsive disorders, and to induce presurgical sedation. Although benzodiazepines exhibit a high therapeutic index and low toxicity in short-term treatments, prolonged administration induces tolerance to most of their therapeutic actions. The mechanism of this tolerance remains unclear. The central actions of benzodiazepines are mediated by binding to GABA_A receptors, which mediate most fast inhibitory transmission in the brain. The majority of GABA_A receptors are composed of two α-(1-6), two β-(1-3) and one γ-subunits (1-3). In a previous report, we demonstrated that the prolonged exposure of cerebrocortical neurons to diazepam produces a transcriptional repression of the GABA_A receptor α1 subunit gene via a mechanism dependent on the activation of L-type voltage-gated calcium channels (L-VGCCs). The results reported here confirm that the diazepam-induced downregulation of the α1 subunit is contingent upon calcium influx from extracellular space. In addition, this regulatory mechanism involves the activation of protein kinase A (PKA) and is accompanied by the activation of two transcription factors, the cAMP-response element-binding protein (CREB) and the inducible cAMP early repressor (ICER). Together, our results suggest that diazepam's activation of an L-VGCC/Ca²⁺/PKA/CREB-ICER signaling pathway is responsible for the regulation of GABA_A receptors. This elucidation of the intracellular signaling cascade activated by a prolonged benzodiazepine exposure, itself potentially involved in the development of tolerance, may contribute to locating molecular targets for future therapeutic interventions.

Application of gabapentinoids and novel compounds for the treatment of benzodiazepine dependence: the glutamatergic model

BACKGROUND

Current approaches for managing benzodiazepine (BZD) withdrawal symptoms are daunting for clinicians and patients, warranting novel treatment and management strategies. This review discusses the pharmacodynamic properties of BZDs, gabapentinoids (GBPs), endozepines, and novel GABAergic compounds associated with potential clinical benefits for BZD-dependent patients. The objective of this study was to review the complex neuromolecular changes occurring within the GABAergic and glutamatergic systems during the BZD tolerance and withdrawal periods while also examining the mechanism by which GBPs and alternative pharmacological therapies may attenuate withdrawal symptoms.

METHODS AND RESULTS

An elaborative literature review was conducted using multiple platforms, including the National Center for Biotechnology (NCBI), AccessMedicine, ScienceDirect, pharmacology textbooks, clinical trial data, case reports, and PubChem. Our literature analysis revealed that many distinctive neuroadaptive mechanisms are involved in the GABAergic and glutamatergic systems during BZD tolerance and withdrawal. Based on this data, we hypothesize that GBPs may attenuate the overactive glutamatergic system during the withdrawal phase by an indirect presynaptic glutamatergic mechanism dependent on the α₂δ₁ subunit expression.

CONCLUSIONS

GBPs may benefit individuals undergoing BZD withdrawal, given that the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor current significantly increases during abrupt BZD withdrawal in animal studies. This may be a conceivable explanation for the effectiveness of GBPs in treating both alcohol withdrawal symptoms and BZD withdrawal symptoms in some recent studies. Finally, natural and synthetic GABAergic compounds with unique pharmacodynamic properties were found to exert potential clinical benefits as BZD substitutes in animal studies, though human studies are lacking.

Intravenous ganaxolone for the treatment of refractory status epilepticus: Results from an open-label, dose-finding, phase 2 trial

OBJECTIVE

Patients with refractory status epilepticus (RSE) have failed treatment with benzodiazepines and ≥1 second-line intravenous (IV) antiseizure medication (ASM). Guidelines recommend IV anesthesia when second-line ASMs have failed, but potential harms can outweigh the benefits. Novel treatments are needed to stop and durably control RSE without escalation to IV anesthetics. Ganaxolone is an investigational neuroactive steroid in development for RSE treatment. This study's objective was to determine the appropriate dosing for IV ganaxolone in RSE and obtain a preliminary assessment of efficacy and safety.

METHODS

This was an open-label, phase 2 trial conducted from February 19, 2018 to September 18, 2019, at three sites in the United States. Patients were aged ≥12 years, had convulsive or nonconvulsive SE, and failed to respond to ≥1 second-line IV ASM. Twenty-one patients were screened; 17 were enrolled. Patients received IV ganaxolone added to standard-of-care ASMs. Ganaxolone infusion was initiated as an IV bolus (over 3 min) with continuous infusion of decreasing infusion rates for 48-96 h followed by an 18-h taper. There were three ganaxolone dosing cohorts: low, 500 mg/day; medium, 650 mg/day; and high, 713 mg/day. The primary end point was the number of patients not requiring escalation to IV anesthetic treatment within 24 h of ganaxolone initiation.

RESULTS

Most of the 17 enrolled patients (65%) had nonconvulsive SE, and had failed a median of three prior ASMs, including first-line benzodiazepine and second-line IV ASM therapy. Median time to SE cessation following ganaxolone initiation was 5 min. No patient required escalation to third-line IV anesthetics during the 24-h period following ganaxolone initiation. Two treatment-related serious adverse events (sedation) were reported. Of the three deaths, none was considered related to ganaxolone; all occurred 9-22 days after completing ganaxolone.

SIGNIFICANCE

IV ganaxolone achieved rapid and durable seizure control in patients with RSE, and showed acceptable safety and tolerability.

Impaired Cognitive Function and Hippocampal Changes Following Chronic Diazepam Treatment in Middle-Aged Mice

Background: Gamma-aminobutyric acid (GABA) type A receptors are positively allosterically modulated by benzodiazepine binding, leading to a potentiated response to GABA. Diazepam (DZP, a benzodiazepine) is widely prescribed for anxiety, epileptic discharge, and insomnia, and is also used as a muscle relaxant and anti-convulsant. However, some adverse effects - such as tolerance, dependence, withdrawal effects, and impairments in cognition and learning - are elicited by the long-term use of DZP. Clinical studies have reported that chronic DZP treatment increases the risk of dementia in older adults. Furthermore, several studies have reported that chronic DZP administration may affect neuronal activity in the hippocampus, dendritic spine structure, and cognitive performance. However, the effects of chronic DZP administration on cognitive function in aged mice is not yet completely understood. Methods: A behavioral test, immunohistochemical analysis of neurogenic and apoptotic markers, dendritic spine density analysis, and long-term potentiation (LTP) assay of the hippocampal CA1 and CA3 were performed in both young (8 weeks old) and middle-aged (12 months old) mice to investigate the effects of chronic DZP administration on cognitive function. The chronic intraperitoneal administration of DZP was performed by implanting an osmotic minipump. To assess spatial learning and memory ability, the Morris water maze test was performed. Dendritic spines were visualized using Lucifer yellow injection into the soma of hippocampal neurons, and spine density was analyzed. Moreover, the effects of exercise on DZP-induced changes in spine density and LTP in the hippocampus were assessed. Results: Learning performance was impaired by chronic DZP administration in middle-aged mice but not in young mice. LTP was attenuated by DZP administration in the CA1 of young mice and the CA3 of middle-aged mice. The spine density of hippocampal neurons was decreased by chronic DZP administration in the CA1 of both young and middle-aged mice as well as in the CA3 of middle-aged mice. Neither neurogenesis nor apoptosis in the hippocampus was affected by chronic DZP administration. Conclusion: The results of this study suggest that the effects of chronic DZP are different between young and middle-aged mice. The chronic DZP-induced memory retrieval performance impairment in middle-aged mice can likely be attributed to decreased LTP and dendritic spine density in hippocampal neurons in the CA3. Notably, prophylactic exercise suppressed the adverse effects of chronic DZP on LTP and spine maintenance in middle-aged mice.

Regulation of GABAA Receptors Induced by the Activation of L-Type Voltage-Gated Calcium Channels

GABA_A receptors are pentameric ion channels that mediate most synaptic and tonic extrasynaptic inhibitory transmissions in the central nervous system. There are multiple GABA_A receptor subtypes constructed from 19 different subunits in mammals that exhibit different regional and subcellular distributions and distinct pharmacological properties. Dysfunctional alterations of GABA_A receptors are associated with various neuropsychiatric disorders. Short- and long-term plastic changes in GABA_A receptors can be induced by the activation of different intracellular signaling pathways that are triggered, under physiological and pathological conditions, by calcium entering through voltage-gated calcium channels. This review discusses several mechanisms of regulation of GABA_A receptor function that result from the activation of L-type voltage gated calcium channels. Calcium influx via these channels activates different signaling cascades that lead to changes in GABA_A receptor transcription, phosphorylation, trafficking, and synaptic clustering, thus regulating the inhibitory synaptic strength. These plastic mechanisms regulate the interplay of synaptic excitation and inhibition that is crucial for the normal function of neuronal circuits.

Regulation of GABAA Receptor Subunit Expression in Substance Use Disorders

The modulation of neuronal cell firing is mediated by the release of the neurotransmitter GABA (γ-aminobuytric acid), which binds to two major families of receptors. The ionotropic GABAA receptors (GABA_ARs) are composed of five distinct subunits that vary in expression by brain region and cell type. The action of GABA on GABA_ARs is modulated by a variety of clinically and pharmacologically important drugs such as benzodiazepines and alcohol. Exposure to and abuse of these substances disrupts homeostasis and induces plasticity in GABAergic neurotransmission, often via the regulation of receptor expression. Here, we review the regulation of GABA_AR subunit expression in adaptive and pathological plasticity, with a focus on substance use. We examine the factors influencing the expression of GABA_AR subunit genes including the regulation of the 5′ and 3′ untranslated regions, variations in DNA methylation, immediate early genes and transcription factors that regulate subunit expression, translational and post-translational modifications, and other forms of receptor regulation beyond expression. Advancing our understanding of the factors regulating GABA_AR subunit expression during adaptive plasticity, as well as during substance use and withdrawal will provide insight into the role of GABAergic signaling in substance use disorders, and contribute to the development of novel targeted therapies.

Preoperative anxiety-induced glucocorticoid signaling reduces GABAergic markers in spinal cord and promotes postoperative hyperalgesia by affecting neuronal PAS domain protein 4

Preoperative anxiety is common in patients undergoing elective surgery and is closely related to postoperative hyperalgesia. In this study, a single prolonged stress model was used to induce preoperative anxiety-like behavior in rats 24 h before the surgery. We found that single prolonged stress exacerbated the postoperative pain and elevated the level of serum corticosterone. Previous studies have shown that glucocorticoid is associated with synaptic plasticity, and decreased spinal GABAergic activity can cause hyperalgesia in rodents. Here, single prolonged stress rats’ lumbar spinal cord showed reduced glutamic acid decarboxylase-65, glutamic acid decarboxylase-67, GABA type A receptor alpha 1 subunit, and GABA type A receptor gamma 2 subunit, indicating an impairment of GABAergic system. Furthermore, neuronal PAS domain protein 4 was also reduced in rats after single prolonged stress stimulation, which has been reported to promote GABAergic synapse development. Then, intraperitoneal injection of RU486 (a glucocorticoid receptor antagonist) rather than spironolactone (a mineralocorticoid receptor antagonist) was found to relieve single prolonged stress-induced hyperalgesia and reverse neuronal PAS domain protein 4 reduction and the impairment of GABAergic system. Furthermore, overexpressing neuronal PAS domain protein 4 could also restore the damage of GABAergic system caused by single prolonged stress while interfering with neuronal PAS domain protein 4 caused an opposite effect. Finally, after stimulation of rat primary spinal cord neurons with exogenous corticosterone in vitro, neuronal PAS domain protein 4 and GABAergic markers were also downregulated, and RU486 reversed that. Together, our results demonstrated that preoperative anxiety led to GABAergic system impairment in spinal cord and thus caused hyperalgesia due to glucocorticoid-induced downregulation of neuronal PAS domain protein 4.

Diazepam Accelerates GABAAR Synaptic Exchange and Alters Intracellular Trafficking

Despite 50+ years of clinical use as anxiolytics, anti-convulsants, and sedative/hypnotic agents, the mechanisms underlying benzodiazepine (BZD) tolerance are poorly understood. BZDs potentiate the actions of gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, through positive allosteric modulation of γ2 subunit containing GABA type A receptors (GABAARs). Here we define key molecular events impacting γ2 GABAAR and the inhibitory synapse gephyrin scaffold following initial sustained BZD exposure in vitro and in vivo. Using immunofluorescence and biochemical experiments, we found that cultured cortical neurons treated with the classical BZD, diazepam (DZP), presented no substantial change in surface or synaptic levels of γ2-GABAARs. In contrast, both γ2 and the postsynaptic scaffolding protein gephyrin showed diminished total protein levels following a single DZP treatment in vitro and in mouse cortical tissue. We further identified DZP treatment enhanced phosphorylation of gephyrin Ser270 and increased generation of gephyrin cleavage products. Selective immunoprecipitation of γ2 from cultured neurons revealed enhanced ubiquitination of this subunit following DZP exposure. To assess novel trafficking responses induced by DZP, we employed a γ2 subunit containing an N terminal fluorogen-activating peptide (FAP) and pH-sensitive green fluorescent protein (γ2pHFAP). Live-imaging experiments using γ2pHFAP GABAAR expressing neurons identified enhanced lysosomal targeting of surface GABAARs and increased overall accumulation in vesicular compartments in response to DZP. Using fluorescence resonance energy transfer (FRET) measurements between α2 and γ2 subunits within a GABAAR in neurons, we identified reductions in synaptic clusters of this subpopulation of surface BZD sensitive receptor. Additional time-series experiments revealed the gephyrin regulating kinase ERK was inactivated by DZP at multiple time points. Moreover, we found DZP simultaneously enhanced synaptic exchange of both γ2-GABAARs and gephyrin using fluorescence recovery after photobleaching (FRAP) techniques. Finally we provide the first proteomic analysis of the BZD sensitive GABAAR interactome in DZP vs. vehicle treated mice. Collectively, our results indicate DZP exposure elicits down-regulation of gephyrin scaffolding and BZD sensitive GABAAR synaptic availability via multiple dynamic trafficking processes.

Anxiolytics targeting GABAA receptors: Insights on etifoxine

OBJECTIVES

Anxiety and adjustment disorders are among the most prevalent mental health conditions. This review focuses on γ-aminobutyric acid receptor type A (GABA_AR)-mediated anxiolysis, describing the action of both endogenous and exogenous modulators of GABA_AR. Future directions and innovative strategies to alleviate anxiety symptoms are discussed, with a particular emphasis on etifoxine.

METHODS

We used available data from the recent literature to update the mode of action of anxiolytics. We focussed our search on anxiolytics acting at GABA_ARs, as well as on the pharmacological properties of formerly and currently prescribed anxiolytics.

RESULTS

Considering the adverse effects of current treatments aimed at increasing inhibitory controls, optimisation of existing pharmacotherapies is of crucial importance. Among the alternative compounds targeting the GABAergic system, translocator protein (TSPO) ligands, such as etifoxine (EFX), which promote endogenous neurosteroidogenesis, are emerging as promising candidates for anxiety relief. In several papers comparing the efficacy of benzodiazepines and EFX, EFX showed interesting properties with limited side effects. Indeed, neurosteroids are potent GABA_AR modulators with highly underrated anxiolytic properties.

CONCLUSIONS

Novel therapeutic strategies have been emerging following the recognition of neurosteroids as potent anxiolytics. Featured at the top of the list for well-tolerated anxiety relief, TSPO ligands such as etifoxine appear promising.

Valium without dependence? Individual GABAA receptor subtype contribution toward benzodiazepine addiction, tolerance, and therapeutic effects

Benzodiazepines are one of the most prescribed medications as first-line treatment of anxiety, insomnia, and epilepsy around the world. Over the past two decades, advances in the neuropharmacological understanding of gamma aminobutyric acid (GABA)_A receptors revealed distinct contributions from each subtype and produced effects. Recent findings have highlighted the importance of α₁ containing GABA_A receptors in the mechanisms of addiction and tolerance in benzodiazepine treatments. This has shown promise in the development of tranquilizers with minimal side effects such as cognitive impairment, dependence, and tolerance. A valium-like drug without its side effects, as repeatedly demonstrated in animals, is achievable.

Role of GABAA receptors in alcohol use disorders suggested by chronic intermittent ethanol (CIE) rodent model

GABAergic inhibitory transmission is involved in the acute and chronic effects of ethanol on the brain and behavior. One-dose ethanol exposure induces transient plastic changes in GABAA receptor subunit levels, composition, and regional and subcellular localization. Rapid down-regulation of early responder δ subunit-containing GABAA receptor subtypes mediating ethanol-sensitive tonic inhibitory currents in critical neuronal circuits corresponds to rapid tolerance to ethanol's behavioral responses. Slightly slower, α1 subunit-containing GABAA receptor subtypes mediating ethanol-insensitive synaptic inhibition are down-regulated, corresponding to tolerance to additional ethanol behaviors plus cross-tolerance to other GABAergic drugs including benzodiazepines, anesthetics, and neurosteroids, especially sedative-hypnotic effects. Compensatory up-regulation of synaptically localized α4 and α2 subunit-containing GABAA receptor subtypes, mediating ethanol-sensitive synaptic inhibitory currents follow, but exhibit altered physio-pharmacology, seizure susceptibility, hyperexcitability, anxiety, and tolerance to GABAergic positive allosteric modulators, corresponding to heightened alcohol withdrawal syndrome. All these changes (behavioral, physiological, and biochemical) induced by ethanol administration are transient and return to normal in a few days. After chronic intermittent ethanol (CIE) treatment the same changes are observed but they become persistent after 30 or more doses, lasting for at least 120 days in the rat, and probably for life. We conclude that the ethanol-induced changes in GABAA receptors represent aberrant plasticity contributing critically to ethanol dependence and increased voluntary consumption. We suggest that the craving, drug-seeking, and increased consumption in the rat model are tied to ethanol-induced plastic changes in GABAA receptors, importantly the development of ethanol-sensitive synaptic GABAA receptor-mediating inhibitory currents that participate in maintained positive reward actions of ethanol on critical neuronal circuits. These probably disinhibit nerve endings of inhibitory GABAergic neurons on dopamine reward circuit cells, and limbic system circuits mediating anxiolysis in hippocampus and amygdala. We further suggest that the GABAA receptors contributing to alcohol dependence in the rat and presumably in human alcohol use disorders (AUD) are the ethanol-induced up-regulated subtypes containing α4 and most importantly α2 subunits. These mediate critical aspects of the positive reinforcement of ethanol in the dependent chronic user while alleviating heightened withdrawal symptoms experienced whenever ethanol is absent. The speculative conclusions based on firm observations are readily testable.

To what extent is it possible to dissociate the anxiolytic and sedative/hypnotic properties of GABAA receptors modulators?

The relatively common view indicates a possible dissociation between the anxiolytic and sedative/hypnotic properties of benzodiazepines (BZs). Indeed, GABAA receptor (GABAAR) subtypes have specific cerebral distribution in distinct neural circuits. Thus, GABAAR subtype-selective drugs may be expected to perform distinct functions. However, standard behavioral test assays provide limited direction towards highlighting new action mechanisms of ligands targeting GABAARs. Automated behavioral tests, lack sensitivity as some behavioral characteristics or subtle behavioral changes of drug effects or that are not considered in the overall analysis (Ohl et al., 2001) and observation-based analyses are not always performed. In addition, despite the use of genetically engineered mice, any possible dissociation between the anxiolytic and sedative properties of BZs remains controversial. Moreover, the involvement the different subtypes of GABAAR subtypes in the anxious behavior and the mechanism of action of anxiolytic agents remains unclear since there has been little success in the pharmacological investigations so far. This raises the question of the involvement of the different subunits in anxiolytic-like and/or sedative effects; and the actual implication of these subunits, particularly, α-subunits in the modulation of sedation and/or anxiety-related disorders. This present review was prompted by several conflicting studies on the degree of involvement of these subunits in anxiolytic-like and/or sedative effects. To this end, we explored the GABAergic system, particularly, the role of different subunits containing synaptic GABAARs. We report herein the targeting gene encoding the different subunits and their contribution in anxiolytic-like and/or sedative actions, as well as, the mechanism underlying tolerance to BZs.

A Case Report of Clonazepam Dependence: Utilization of Therapeutic Drug Monitoring During Withdrawal Period

Clonazepam is long-acting benzodiazepine agonist used in short-acting benzodiazepine withdrawal; however, recent observations suggest the existence of its abuse. We demonstrate a 40-year-old man with a 20-year history of psychiatric care with recently benzodiazepine dependence (daily intake of ∼60 mg of clonazepam and 10 mg of alprazolam). High serum levels of both drugs were analyzed 3 weeks before admission to hospitalization (clonazepam 543.9 ng/mL, alprazolam 110 ng/mL) and at the time of admission (clonazepam 286.2 ng/mL, alprazolam 140 ng/mL) without any signs of benzodiazepine intoxication. Gradual withdrawal of clonazepam with monitoring of its serum levels and increase of gabapentin dose were used to minimize physical signs and symptoms of clonazepam withdrawal. Alprazolam was discontinued promptly. Clinical consequences of the treatment were controllable tension, intermittent headache, and rarely insomia. It is the first case report showing utilization of therapeutic drug monitoring during withdrawal period in the patient with extreme toleration to severe benzodiazepine dependence.

Activation-induced regulation of GABAA receptors: Is there a link with the molecular basis of benzodiazepine tolerance?

Benzodiazepines have been used clinically for more than 50 years to treat disorders such as insomnia, anxiety, and epilepsy, as well as to aid muscle relaxation and anesthesia. The therapeutic index for benzodiazepines if very high and the toxicity is low. However, their usefulness is limited by the development of either or both tolerance to most of their pharmacological actions and dependence. Tolerance develops at different rates depending on the pharmacological action, suggesting the existence of distinct mechanisms for each behavioral parameter. Alternatively, multiple mechanisms could coexist depending on the subtype of GABAA receptor expressed and the brain region involved. Because most of the pharmacological actions of benzodiazepines are mediated through GABAA receptor binding, adaptive alterations in the number, structure, and/or functions of these receptors may play an important role in the development of tolerance. This review is focused on the regulation of GABAA receptors induced by long-term benzodiazepine exposure and its relationship with the development of tolerance. Understanding the mechanisms behind benzodiazepine tolerance is critical for designing drugs that could maintain their efficacy during long-term treatments.

The effects of repeated zolpidem treatment on tolerance, withdrawal-like symptoms, and GABAA receptor mRNAs profile expression in mice: comparison with diazepam

RATIONALE

Zolpidem is a short-acting, non-benzodiazepine hypnotic that acts as a full agonist at α1-containing GABAA receptors. Overall, zolpidem purportedly has fewer instances of abuse and dependence than traditionally used benzodiazepines. However, several studies have shown that zolpidem may be more similar to benzodiazepines in terms of behavioral tolerance and withdrawal symptoms.

OBJECTIVES

In the current study, we examined whether subchronic zolpidem or diazepam administration produced deficits in zolpidem's locomotor-impairing effects, anxiety-like behaviors, and changes in GABAAR subunit messenger RNA (mRNA).

METHODS

Mice were given subchronic injections of either zolpidem (10 mg/kg), diazepam (20 mg/kg), or vehicle twice daily for 7 days. On day 8, mice were given a challenge dose of zolpidem (2 mg/kg) or vehicle before open field testing. Another set of mice underwent the same injection regimen but were sacrificed on day 8 for qRT-PCR analysis.

RESULTS

We found that subchronic zolpidem and diazepam administration produced deficits in the acute locomotor-impairing effects of zolpidem and increased anxiety-like behaviors 1 day after drug termination. In addition, we found that subchronic treatment of zolpidem and diazepam induced distinct but overlapping GABAAR subunit mRNA changes in the cortex but few changes in the hippocampus, amygdala, or prefrontal cortex. Levels of mRNA measured in separate mice after a single injection of either zolpidem or diazepam revealed no mRNA changes.

CONCLUSIONS

In mice, subchronic treatment of zolpidem and diazepam can produce deficits in the locomotor-impairing effects of zolpidem, anxiety-like withdrawal symptoms, and subunit-specific mRNA changes.

Ethanol promotes clathrin adaptor-mediated endocytosis via the intracellular domain of δ-containing GABAA receptors

Pharmacological and genetic evidence reveals that GABA(A) receptor (GABA(A)-R) expression and localization are modulated in response to acute and chronic ethanol (EtOH) exposure. To determine molecular mechanisms of GABA(A)-R plasticity in response to in vivo acute EtOH, we measured early time changes in GABA(A)-R subunit localization. Single doses of EtOH (3 g/kg via i.p. injection in rats) produced decreases in surface levels of GABA(A)-R α4 and δ subunits at 5-15 min post-EtOH in hippocampus CA1 and dentate gyrus, verifying our earlier report (Liang et al., 2007). Here we also examined the β3 subunit and its phosphorylation state during internalization. β3 also was internalized during 5-15 min after EtOH exposure, while phosphorylation of β3 was increased, then decreased at later times, ruling out β3 dephosphorylation-dependent endocytosis. As early as 5 min post-EtOH, there is an initial increase in association between the δ subunits with clathrin adaptor proteins AP2-μ2 revealed by coimmunoprecipitation, followed by a decrease in association 15 min post-EtOH. In vitro studies using glutathione S-transferase fused to the δ subunit intracellular domain (ICD) show that two regions, one containing a classical YxxΦ motif and the other an atypical R/K-rich motif, directly and differentially bind to AP2-μ2, with the former YRSV exhibiting higher affinity. Mutating both regions in the δ-ICD abolishes μ2 binding, providing a possible mechanism that can explain the rapid downregulation of extrasynaptic α4βδ-GABA(A)-R following in vivo EtOH administration, in which the δ-ICD increases in affinity for clathrin AP2-μ2 leading to endocytosis.

Enhanced GABAergic transmission in the central nucleus of the amygdala of genetically selected Marchigian Sardinian rats: alcohol and CRF effects

The GABAergic system in the central amygdala (CeA) plays a major role in ethanol dependence and the anxiogenic-like response to ethanol withdrawal. Alcohol dependence is associated with increased corticotropin releasing factor (CRF) influence on CeA GABA release and CRF type 1 receptor (CRF(1)) antagonists prevent the excessive alcohol consumption associated with dependence. Genetically selected Marchigian Sardinian (msP) rats have an overactive extrahypothalamic CRF(1) system, are highly sensitive to stress, and display an innate preference for alcohol. The present study examined differences in CeA GABAergic transmission and the effects of ethanol, CRF and a CRF(1) antagonist in msP, Sprague Dawley, and Wistar rats using an electrophysiological approach. We found no significant differences in membrane properties or mean amplitude of evoked GABA(A)-inhibitory postsynaptic potentials (IPSPs). However, paired-pulse facilitation (PPF) ratios of evoked IPSPs were significantly lower and spontaneous miniature inhibitory postsynaptic current (mIPSC) frequencies were higher in msP rats, suggesting increased CeA GABA release in msP as compared to Sprague Dawley and Wistar rats. The sensitivity of spontaneous GABAergic transmission to ethanol (44 mM), CRF (200 nM) and CRF(1) antagonist (R121919, 1 μM) was comparable in msP, Sprague Dawley, and Wistar rats. However, a history of ethanol drinking significantly increased the baseline mIPSC frequency and decreased the effects of a CRF(1) antagonist in msP rats, suggesting increased GABA release and decreased CRF(1) sensitivity. These results provide electrophysiological evidence that msP rats display distinct CeA GABAergic activity as compared to Sprague Dawley and Wistar rats. The elevated GABAergic transmission observed in naïve msP rats is consistent with the neuroadaptations reported in Sprague Dawley rats after the development of ethanol dependence.

Differential effects of short- and long-term zolpidem treatment on recombinant α1β2γ2s subtype of GABA(A) receptors in vitro

AIM

Zolpidem is a non-benzodiazepine agonist at benzodiazepine binding site in GABA(A) receptors, which is increasingly prescribed. Recent studies suggest that prolonged zolpidem treatment induces tolerance. The aim of this study was to explore the adaptive changes in GABA(A) receptors following short and long-term exposure to zolpidem in vitro.

METHODS

Human embryonic kidney (HEK) 293 cells stably expressing recombinant α1β2γ2s GABA(A) receptors were exposed to zolpidem (1 and 10 μmol/L) for short-term (2 h daily for 1, 2, or 3 consecutive days) or long-term (continuously for 48 h). Radioligand binding studies were used to determine the parameters of [(3)H]flunitrazepam binding sites.

RESULTS

A single (2 h) or repeated (2 h daily for 2 or 3 d) short-term exposure to zolpidem affected neither the maximum number of [(3)H]flunitrazepam binding sites nor the affinity. In both control and short-term zolpidem treated groups, addition of GABA (1 nmol/L-1 mmol/L) enhanced [(3)H]flunitrazepam binding in a concentration-dependent manner. The maximum enhancement of [(3)H]flunitrazepam binding in short-term zolpidem treated group was not significantly different from that in the control group. In contrast, long-term exposure to zolpidem resulted in significantly increase in the maximum number of [(3)H]flunitrazepam binding sites without changing the affinity. Furthermore, long-term exposure to zolpidem significantly decreased the ability of GABA to stimulate [(3)H]flunitrazepam binding.

CONCLUSION

The results suggest that continuous, but not intermittent and short-term, zolpidem-exposure is able to induce adaptive changes in GABA(A) receptors that could be related to the development of tolerance and dependence.

βCCT, an antagonist selective for α(1)GABA(A) receptors, reverses diazepam withdrawal-induced anxiety in rats

The abrupt discontinuation of prolonged benzodiazepine treatment elicits a withdrawal syndrome with increased anxiety as a major symptom. The neural mechanisms underlying benzodiazepine physical dependence are still insufficiently understood. Flumazenil, the non-selective antagonist of the benzodiazepine binding site of GABA(A) receptors was capable of preventing and reversing the increased anxiety during benzodiazepine withdrawal in animals and humans in some, but not all studies. On the other hand, a number of data suggest that GABA(A) receptors containing α(1) subunits are critically involved in processes developing during prolonged use of benzodiazepines, such are tolerance to sedative effects, liability to physical dependence and addiction. Hence, we investigated in the elevated plus maze the level of anxiety 24 h following 21 days of diazepam treatment and the influence of flumazenil or a preferential α(1)-subunit selective antagonist βCCt on diazepam withdrawal syndrome in rats. Abrupt cessation of protracted once-daily intraperitoneal administration of 2 mg/kg diazepam induced a withdrawal syndrome, measured by increased anxiety-like behavior in the elevated plus maze 24 h after treatment cessation. Acute challenge with either flumazenil (10mg/kg) or βCCt (1.25, 5 and 20 mg/kg) alleviated the diazepam withdrawal-induced anxiety. Moreover, both antagonists induced an anxiolytic-like response close, though not identical, to that seen with acute administration of diazepam. These findings imply that the mechanism by which antagonism at GABA(A) receptors may reverse the withdrawal-induced anxiety involves the α(1) subunit and prompt further studies aimed at linking the changes in behavior with possible adaptive changes in subunit expression and function of GABA(A) receptors.

In vivo knockdown of GAD67 in the amygdala disrupts fear extinction and the anxiolytic-like effect of diazepam in mice

In mammals, γ-aminobutyric acid (GABA) transmission in the amygdala is particularly important for controlling levels of fear and anxiety. Most GABA synthesis in the brain is catalyzed in inhibitory neurons from L-glutamic acid by the enzyme glutamic acid decarboxylase 67 (GAD67). In the current study, we sought to examine the acquisition and extinction of conditioned fear in mice with knocked down expression of the GABA synthesizing enzyme GAD67 in the amygdala using a lentiviral-based (LV) RNA interference strategy to locally induce loss-of-function. In vitro experiments revealed that our LV-siRNA-GAD67 construct diminished the expression of GAD67 as determined with western blot and fluorescent immunocytochemical analyses. In vivo experiments, in which male C57BL/6J mice received bilateral amygdala microinjections, revealed that LV-siRNA-GAD67 injections produce significant inhibition of endogenous GAD67 when compared with control injections. In contrast, no significant changes in GAD65 expression were detected in the amygdala, validating the specificity of LV knockdown. Behavioral experiments showed that LV knockdown of GAD67 results in a deficit in the extinction, but not the acquisition or retention, of fear as measured by conditioned freezing. GAD67 knockdown did not affect baseline locomotion or basal measures of anxiety as measured in open field apparatus. However, diminished GAD67 in the amygdala blunted the anxiolytic-like effect of diazepam (1.5 mg kg(-1)) as measured in the elevated plus maze. Together, these studies suggest that of GABAergic transmission in amygdala mediates the inhibition of conditioned fear and the anxiolytic-like effect of diazepam in adult mice.

Benzodiazepine treatment induces subtype-specific changes in GABA(A) receptor trafficking and decreases synaptic inhibition

Benzodiazepines potentiate γ-aminobutyric acid type A receptor (GABA(A)R) activity and are widely prescribed to treat anxiety, insomnia, and seizure disorders. Unfortunately, clinical use of benzodiazepines (BZs) is severely limited by tolerance. The mechanisms leading to BZ tolerance are unknown. BZs bind at the interface between an α and γ subunit of GABA(A)Rs, preferentially enhancing synaptic receptors largely composed of α(1-3, 5), β3, and γ2 subunits. Using confocal imaging and patch-clamp approaches, we show that treatment with the BZ flurazepam decreases GABA(A)R surface levels and the efficacy of neuronal inhibition in hippocampal neurons. A dramatic decrease in surface and total levels of α2 subunit-containing GABA(A)Rs occurred within 24 h of flurazepam treatment, whereas GABA(A)Rs incorporating α1 subunits showed little alteration. The GABA(A)R surface depletion could be reversed by treatment with the BZ antagonist Ro 15-1788. Coincident with decreased GABA(A)R surface levels, flurazepam treatment reduced miniature inhibitory postsynaptic current amplitude, which returned to control levels with acute Ro 15-1788 treatment. GABA(A)R endocytosis and insertion rates were unchanged by flurazepam treatment. Treatment with leupeptin restored flurazepam lowered receptor surface levels, strongly suggesting that flurazepam increases lysosomal degradation of GABA(A)Rs. Together, these data suggest that flurazepam exposure enhances degradation of α2 subunit-containing GABA(A)Rs after their removal from the plasma membrane, leading to a reduction in inhibitory synapse size and number along with a decrease in the efficacy of synaptic inhibition. These reported subtype-specific changes in GABA(A)R trafficking provide significant mechanistic insight into the initial neuroadaptive responses occurring with BZ treatment.

A novel GABA(A) receptor pharmacology: drugs interacting with the α(+) β(-) interface

GABA(A) receptors are ligand-gated chloride channels composed of five subunits that can belong to different subunit classes. The existence of 19 different subunits gives rise to a multiplicity of GABA(A) receptor subtypes with distinct subunit composition; regional, cellular and subcellular distribution; and pharmacology. Most of these receptors are composed of two α, two β and one γ2 subunits. GABA(A) receptors are the site of action of a variety of pharmacologically and clinically important drugs, such as benzodiazepines, barbiturates, neuroactive steroids, anaesthetics and convulsants. Whereas GABA acts at the two extracellular β(+) α(-) interfaces of GABA(A) receptors, the allosteric modulatory benzodiazepines interact with the extracellular α(+) γ2(-) interface. In contrast, barbiturates, neuroactive steroids and anaesthetics seem to interact with solvent accessible pockets in the transmembrane domain. Several benzodiazepine site ligands have been identified that selectively interact with GABA(A) receptor subtypes containing α2βγ2, α3βγ2 or α5βγ2 subunits. This indicates that the different α subunit types present in these receptors convey sufficient structural differences to the benzodiazepine binding site to allow specific interaction with certain benzodiazepine site ligands. Recently, a novel drug binding site was identified at the α(+) β(-) interface. This binding site is homologous to the benzodiazepine binding site at the α(+) γ2(-) interface and is thus also strongly influenced by the type of α subunit present in the receptor. Drugs interacting with this binding site cannot directly activate but only allosterically modulate GABA(A) receptors. The possible importance of such drugs addressing a spectrum of receptor subtypes completely different from that of benzodiazepines is discussed.

High dose benzodiazepine dependence: description of 29 patients treated with flumazenil infusion and stabilised with clonazepam

The withdrawal syndrome from benzodiazepine (BZD) can be severe and in some cases may impede cessation of the use of the drug. We present here a case series of benzodiazepine detoxification by flumazenil infusion, stabilised with clonazepam. Patients were treated with flumazenil 1.35 mg/day for a median of 7 days. Self-reported physical withdrawal symptoms were recorded daily. In addition to flumazenil, antidepressants were given before treatment commenced and clonazepam was administered nightly with both being continued after discharge. Twenty-nine patients were treated. No patients dropped out from the treatment programme. Nine patients (31%) required a temporary reduction/cessation of the infusion. The linear trend in the reduction of the daily withdrawal scores in the overall study population was significant. The linear trends were also significant in the group of patients for whom a temporary reduction/suspension of the flumazenil was required. Six months after treatment, 15 patients (53%) were abstinent from clonazepam and other BZDs. For five (21%) the BZD dependence were reinstated. More than two-thirds of the subjects tolerated the procedure well and about half had a good long term response. Slow flumazenil infusion appears to merit consideration as a possible future treatment. Suggestions for future research are examined.

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 effects of zolpidem treatment on GABA(A) receptors in cultured cerebellar granule cells: changes in functional coupling

AIMS

Hypnotic zolpidem is a positive allosteric modulator of γ-aminobutyric acid (GABA) action, with preferential although not exclusive binding for α1 subunit-containing GABA(A) receptors. The pharmacological profile of this drug is different from that of classical benzodiazepines, although it acts through benzodiazepine binding sites at GABA(A) receptors. The aim of this study was to further explore the molecular mechanisms of GABA(A) receptor induction by zolpidem.

MAIN METHODS

In the present study, we explored the effects of two-day zolpidem (10 μM) treatment on GABA(A) receptors on the membranes of rat cerebellar granule cells (CGCs) using [(3)H]flunitrazepam binding and semi-quantitative PCR analysis.

KEY FINDINGS

Two-day zolpidem treatment of CGCs did not significantly affect the maximum number (B(max)) of [(3)H]flunitrazepam binding sites or the expression of α1 subunit mRNA. However, as shown by decreased GABA [(3)H]flunitrazepam binding, two-day exposure of CGCs to zolpidem caused functional uncoupling of GABA and benzodiazepine binding sites at GABA(A) receptor complexes.

SIGNIFICANCE

If functional uncoupling of GABA and benzodiazepine binding sites at GABA(A) receptors is the mechanism responsible for the development of tolerance following long-term administration of classical benzodiazepines, chronic zolpidem treatment may induce tolerance.

Mechanisms Underlying Tolerance after Long-Term Benzodiazepine Use: A Future for Subtype-Selective GABA(A) Receptor Modulators?

Despite decades of basic and clinical research, our understanding of how benzodiazepines tend to lose their efficacy over time (tolerance) is at least incomplete. In appears that tolerance develops relatively quickly for the sedative and anticonvulsant actions of benzodiazepines, whereas tolerance to anxiolytic and amnesic effects probably does not develop at all. In light of this evidence, we review the current evidence for the neuroadaptive mechanisms underlying benzodiazepine tolerance, including changes of (i) the GABA(A) receptor (subunit expression and receptor coupling), (ii) intracellular changes stemming from transcriptional and neurotrophic factors, (iii) ionotropic glutamate receptors, (iv) other neurotransmitters (serotonin, dopamine, and acetylcholine systems), and (v) the neurosteroid system. From the large variance in the studies, it appears that either different (simultaneous) tolerance mechanisms occur depending on the benzodiazepine effect, or that the tolerance-inducing mechanism depends on the activated GABA(A) receptor subtypes. Importantly, there is no convincing evidence that tolerance occurs with α subunit subtype-selective compounds acting at the benzodiazepine site.

Subunit Compensation and Plasticity of Synaptic GABA(A) Receptors Induced by Ethanol in α4 Subunit Knockout Mice

There is considerable evidence that ethanol (EtOH) potentiates γ-aminobutyric acid type A receptor (GABA_AR) action, but only GABA_ARs containing δ subunits appear sensitive to low millimolar EtOH. The α4 and δ subunits co-assemble into GABA_ARs which are relatively highly expressed at extrasynaptic locations in the dentate gyrus where they mediate tonic inhibition. We previously demonstrated reversible- and time-dependent changes in GABA_AR function and subunit composition in rats after single-dose EtOH intoxication. We concluded that early tolerance to EtOH occurs by over-activation and subsequent internalization of EtOH-sensitive extrasynaptic α4βδ-GABA_ARs. Based on this hypothesis, any highly EtOH-sensitive GABA_ARs should be subject to internalization following exposure to suitably high EtOH doses. To test this, we studied the GABA_ARs in mice with a global deletion of the α4 subunit (KO). The dentate granule cells of these mice exhibited greatly reduced tonic currents and greatly reduced potentiation by acutely applied EtOH, whereas synaptic currents showed heightened sensitivity to low EtOH concentrations. The hippocampus of naive KO mice showed reduced δ subunit protein levels, but increased α2, and γ2 levels compared to wild-type (WT) controls, suggesting at least partial compensation by these subunits in synaptic, highly EtOH-sensitive GABA_ARs of KO mice. In WT mice, cross-linking and Western blot analysis at 1 h after an EtOH challenge (3.5 g/kg, i.p.) revealed increased intracellular fraction of the α1, α4, and δ, but not α2, α5, or γ2 subunits. By contrast, we observed significant internalization of α1, α2, δ, and γ2 subunits after a similar EtOH challenge in KO mice. Synaptic currents from naïve KO mice were more sensitive to potentiation by zolpidem (0.3 μM, requiring α1/α2, inactive at α4/5 GABA_ARs) than those from naïve WT mice. At 1 h after EtOH, synaptic currents of WT mice were unchanged, whereas those of KO mice were significantly less sensitive to zolpidem, suggesting decreases in functional α1/2βγ GABA_ARs. These data further support our hypothesis that EtOH intoxication induces GABA_AR plasticity via internalization of highly EtOH-sensitive GABA_ARs.

Absence of tolerance to the anticonvulsant and neuroprotective effects of imidazenil against DFP-induced seizure and neuronal damage

The clinical use of diazepam or midazolam to control organophosphate (OP) nerve agent-induced seizure activity is limited by their unwanted effects including sedation, amnesia, withdrawal, and anticonvulsant tolerance. Imidazenil is an imidazo-benzodiazepine derivative with high intrinsic efficacy and selectivity for α2-, α3-, and α5- but low intrinsic efficacy for α1-containing GABA(A) receptors. We have previously shown that imidazenil is more efficacious than diazepam at protecting rats and mice from diisopropyl fluorophosphate (DFP)-induced seizures and neuronal damage without producing sedation. In the present study, we compared the tolerance liability of imidazenil and diazepam to attenuate the seizure activity and neurotoxic effects of DFP. Rats received protracted (14 days) oral treatment with increasing doses of imidazenil (1-4 mg/kg), diazepam (5-20 mg/kg), or vehicle. Eighteen hours after the last dose of the protracted treatment schedule, rats were tested for anticonvulsant tolerance after a 30 min pretreatment with a single test dose of imidazenil (0.5 mg/kg) or diazepam (5 mg/kg) prior to a DFP challenge (1.5 mg/kg). The anticonvulsant (modified Racine score scale) and neuroprotective (fluoro-jade B staining) effects of diazepam were significantly reduced in protracted diazepam-treated animals whereas the effects of imidazenil were not altered in protracted imidazenil-treated animals. The present findings indicate that protracted imidazenil treatment does not produce tolerance to its protective action against the neurotoxic effects of OP exposure.

The effects of zolpidem treatment and withdrawal on the in vitro expression of recombinant alpha1beta2gamma2s GABA(A) receptors expressed in HEK 293 cells

Zolpidem, a widely used hypnotic drug which acts through benzodiazepine binding sites, is a positive allosteric modulator of gamma-aminobutyric acid (GABA) action with preferential affinity for GABA(A) receptors containing alpha1 subunit. The pharmacological profile of zolpidem is different from that of classical benzodiazepines. The aim of this study was to find out whether zolpidem treatment triggers adaptive changes in the recombinant alpha1 subunit-containing GABA(A) receptors other than those observed following treatment with classical benzodiazepine-diazepam. Radioligand binding studies showed that 2-day exposure of human embryonic kidney (HEK) 293 cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors to zolpidem (10 muM) up-regulated the maximum number (B (max)) of [(3)H]flunitrazepam, [(3)H]muscimol, and [(3)H]t-butylbicycloorthobenzoate ([(3)H]TBOB) binding sites without changing their affinity (K (d)), suggesting an increase in total GABA(A) receptor number. Semi-quantitative RT-PCR analysis demonstrated increased levels of alpha1 subunit mRNA, while Western blot demonstrated up-regulated gamma2 subunit proteins, suggesting that zolpidem induced de novo synthesis of receptors proteins, at both the transcriptional and translational levels. GABA-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from zolpidem-treated cells was markedly reduced, indicating allosteric uncoupling between GABA and benzodiazepine binding sites. The number of benzodiazepine and convulsant binding sites as well as the functional coupling between GABA and benzodiazepine binding sites normalized in 24 h following discontinuation of zolpidem treatment. The results of our in vitro studies suggest that a 2-day exposure of recombinant alpha1 subunit-containing GABA(A) receptors stably transfected in HEK 293 cells to zolpidem induces adaptive changes in this selective GABA(A) receptor subtype, which are not substantially different from those obtained after prolonged exposure of cells to high concentrations of diazepam.

The role of flumazenil in the treatment of benzodiazepine dependence: physiological and psychological profiles

Two-related studies are presented here, detailing our early experience with benzodiazepine-dependent patients treated with a four-day flumazenil infusion using a novel delivery technique. Patients with long-term benzodiazepine dependence who attended the Australian Medical Procedures Research Foundation (AMPRF, Perth, Australia) for treatment were recruited for these studies. Self-reported psychological and physical symptoms, as well as objective vital signs data were collected at intervals before, during and 2 weeks postinfusion. Study A is a case series with cardiovascular measures; study B is an open trial that tracks the psychological profiles of 13 subjects. Withdrawal symptoms were tracked, however, the nature and severity of these symptoms differed between patients. No major complications or discomfort prompting study dropout was observed. Significant benzodiazepine abstinence occurred with this flumazenil infusion method despite high levels of initial dependence, comorbid substance use and comorbid psychiatric illness. Low-dose flumazenil infusion appears to be a safe and effective treatment resulting in withdrawal symptoms of lesser severity than any other cessation method currently available. Recommendations for future research are discussed.

Acute cross tolerance to midazolam, and not pentobarbital and pregnanolone, after a single dose of chlordiazepoxide in monkeys discriminating midazolam

Although cross tolerance can develop among positive gamma-aminobutyric acidA (GABAA) modulators acting at the same modulatory site, cross tolerance does not always develop to drugs acting at sites that are different from the site of action of the drug administered chronically. To examine the relationship between cross tolerance and site of action, four rhesus monkeys discriminated midazolam and, on separate occasions, received 32 mg/kg of chlordiazepoxide 24 h before dose-effect determinations for drugs acting at different sites. Midazolam, pentobarbital, and pregnanolone produced >80% midazolam-lever responding. Although monkeys responded on the midazolam lever 2-4 h after 32 mg/kg of chlordiazepoxide, they responded on the saline lever 24 h later. Twenty-four hours after an acute injection of 32 mg/kg of chlordiazepoxide, midazolam dose-effect curves were shifted 4.6-fold to the right, whereas pregnanolone dose-effect curves were shifted three-fold to the left. Sensitivity to pentobarbital increased in one monkey and decreased in others 24 h after chlordiazepoxide administration. Decreased sensitivity to midazolam shows that acute cross tolerance develops after chlordiazepoxide administration, although it does not develop to drugs acting at other sites on GABAA receptors. These differences among positive GABAA modulators suggest that even short-term benzodiazepine administration changes GABAA receptors, and those changes impact modulatory sites differently.

Differential effects of diazepam treatment and withdrawal on recombinant GABAA receptor expression and functional coupling

Prolonged exposure to benzodiazepines, drugs known to produce tolerance and dependence and also to be abused, leads to adaptive changes in GABA(A) receptors. To further explore the mechanisms responsible for these phenomena, we studied the effects of prolonged diazepam treatment on the recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors, stably expressed in human embryonic kidney (HEK) 293 cells. The results demonstrating that long-term (48 and 72 h) exposure of cells to a high concentration of diazepam (50 microM) enhanced the maximum number (B(max)) of [(3)H]flunitrazepam, [(3)H]muscimol and [(3)H]t-butylbicycloorthobenzoate ([(3)H]TBOB) binding sites, without changing their affinity (K(d)), suggested the up-regulation of GABA(A) receptors. As demonstrated by cell counting and WST-1 proliferation assay, the observed increase in receptor expression was not a consequence of stimulated growth of cells exposed to diazepam. Semi-quantitative RT-PCR and Western blot analysis, showing elevated levels of alpha(1) subunit mRNA as well as beta(2) and gamma(2) subunit proteins, respectively, suggested that prolonged high dose diazepam treatment induced de novo receptor synthesis by acting at both transcriptional and translational levels. The finding that the number of GABA(A) receptor binding sites returned to control value 24 h following diazepam withdrawal, makes this process less likely to account for the development of benzodiazepine tolerance and dependence. On the other hand, the results demonstrating that observed functional uncoupling between GABA and benzodiazepine binding sites persisted after the termination of diazepam treatment supported the hypothesis of its possible role in these phenomena.

Imidazenil: a low efficacy agonist at alpha1- but high efficacy at alpha5-GABAA receptors fail to show anticonvulsant cross tolerance to diazepam or zolpidem

Whereas advances in the molecular biology of GABA(A) receptor complex using knock-out and knock-in mice have been valuable in unveiling the structure, composition, receptor assembly, and several functions of different GABA(A) receptor subtypes, the mechanism(s) underlying benzodiazepine (BZ) tolerance and withdrawal remain poorly understood. Studies using specific GABA(A) receptor subunit knock-in mice suggest that tolerance to sedative action of diazepam requires long-term activation of alpha1 and alpha5 GABA(A) receptor subunits. We investigated the role of long-term activation of these GABA(A) receptor subunits during anticonvulsant tolerance using high affinity and high intrinsic efficacy ligands for GABA(A) receptors expressing the alpha5 subunit (imidazenil) or alpha1 subunit (zolpidem), and a non-selective BZ recognition site ligand (diazepam). We report here that long-term activation of GABA(A) receptors by zolpidem and diazepam but not by imidazenil elicits anticonvulsant tolerance. Although anticonvulsant cross-tolerance occurs between diazepam and zolpidem, there is no cross-tolerance between imidazenil and diazepam or zolpidem. Furthermore, diazepam or zolpidem long-term treatment decreased the expression of mRNA encoding the alpha1 GABA(A) receptor subunit in prefrontal cortex by 43% and 20% respectively. In addition, diazepam but not zolpidem long-term treatment produced a 30% increase in the expression of the alpha5 GABA(A) receptor subunit mRNA in prefrontal cortex. In contrast, imidazenil which is devoid of anticonvulsant tolerance does not elicit significant changes in the expression of alpha1 or alpha5 GABA(A) receptor subunit. These findings suggest that long-term activation of GABA(A) receptors containing the alpha1 or other subunits but not the alpha5 receptor subunit is essential for the induction of anticonvulsant tolerance.

The role of transcriptional and translational mechanisms in flumazenil-induced up-regulation of recombinant GABA(A) receptors

The aim of this study was to further elucidate the mechanisms involved in adaptive changes of GABA(A) receptors following prolonged exposure to flumazenil, the antagonist of benzodiazepine binding sites on GABA(A) receptors. The effects of prolonged flumazenil treatment were studied on recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors stably expressed in human embryonic kidney (HEK 293) cells. Using radioligand binding experiments we found enhancement in the maximum number of [(3)H]muscimol labeled binding sites in different preparations of HEK 293 cells. The parallel increase of [(3)H]flunitrazepam binding sites in the membranes was reduced in the presence of actinomycin D and cycloheximide, inhibitors of RNA and protein synthesis, respectively. Chronic flumazenil also raised the steady-state level of mRNA encoding alpha(1) receptor subunit. The results suggest that the up-regulation of GABA(A) receptors, observed after prolonged flumazenil treatment is at least partly due to increased de novo synthesis of receptor proteins at both transcriptional and translational level.

Training-induced changes in the expression of GABAA-associated genes in the amygdala after the acquisition and extinction of Pavlovian fear

Previous work suggests the gamma-aminobutyric acid (GABA)ergic system may be dynamically regulated during emotional learning. In the current study we examined training-induced changes in the expression of GABA(A)-related genes and the binding of GABA receptor radioligands in the amygdala after the acquisition and extinction of Pavlovian fear. Using in situ hybridization, we examined the expression pattern changes of mRNAs for GABAergic markers in the lateral, basolateral and central subdivisions of the amygdala in C57Bl/6J mice. These markers included GABA-synthesizing enzymes (GAD67 and GAD65), major GABA(A) receptor subunits (alpha1, alpha2, alpha3, alpha5, beta2 and gamma2) and the expression of mRNAs that are involved in a variety of GABA-related intracellular processes, including GABA transporter-1 (GAT1), GABA(A) receptor-associated protein and the GABA(A) clustering protein, gephyrin. With fear conditioning, we found decreased mRNA levels of alpha1, alpha5 and GAD67, as well as deceased benzodiazepine binding in the amygdala. Fear extinction induced an increase in mRNA levels of alpha2, beta2, GAD67 and gephyrin, as well as a decrease in GAT1. Together, these findings indicate that the acquisition of fear induced a downregulation of mRNA markers related to a decrease in amygdala GABAergic function, whereas the acquisition of fear extinction produced an upregulation of GABAergic markers related to enhanced GABAergic transmission.

Mechanisms of reversible GABAA receptor plasticity after ethanol intoxication

The time-dependent effects of ethanol (EtOH) intoxication on GABA(A) receptor (GABA(A)R) composition and function were studied in rats. A cross-linking assay and Western blot analysis of microdissected CA1 area of hippocampal slices obtained 1 h after EtOH intoxication (5 g/kg, gavage), revealed decreases in the cell-surface fraction of alpha4 and delta, but not alpha1, alpha5, or gamma2 GABA(A)R subunits, without changes in their total content. This was accompanied (in CA1 neuron recordings) by decreased magnitude of the picrotoxin-sensitive tonic current (I(tonic)), but not miniature IPSCs (mIPSCs), and by reduced enhancement of I(tonic) by EtOH, but not by diazepam. By 48 h after EtOH dosing, cell-surface alpha4 (80%) and gamma2 (82%) subunit content increased, and cell-surface alpha1 (-50%) and delta (-79%) and overall content were decreased. This was paralleled by faster decay of mIPSCs, decreased diazepam enhancement of both mIPSCs and I(tonic), and paradoxically increased mIPSC responsiveness to EtOH (10-100 mm). Sensitivity to isoflurane- or diazepam-induced loss of righting reflex was decreased at 12 and 24 h after EtOH intoxication, respectively, suggesting functional GABA(A)R tolerance. The plastic GABA(A)R changes were gradually and fully reversible by 2 weeks after single EtOH dosing, but unexplainably persisted long after withdrawal from chronic intermittent ethanol treatment, which leads to signs of alcohol dependence. Our data suggest that early tolerance to EtOH may result from excessive activation and subsequent internalization of alpha4betadelta extrasynaptic GABA(A)Rs. This leads to transcriptionally regulated increases in alpha4 and gamma2 and decreases in alpha1 subunits, with preferential insertion of the newly formed alpha4betagamma2 GABA(A)Rs at synapses.

Allosteric uncoupling and up-regulation of benzodiazepine and GABA recognition sites following chronic diazepam treatment of HEK 293 cells stably transfected with alpha1beta2gamma2S subunits of GABA (A) receptors

Benzodiazepines are drugs known to produce tolerance and dependence and also to be abused and co-abused. The aim of this study was to further explore the mechanisms that underlie adaptive changes in GABA(A) receptors following prolonged exposure to these drugs. Human embryonic kidney (HEK 293) cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors were exposed for 72 h to a high concentration of diazepam (50 microM) in the absence or presence of other drugs. Radioligand binding studies were used to determine the parameters of [(3)H]flunitrazepam and [(3)H]muscimol binding sites and allosteric interactions between these sites. Prolonged treatment with diazepam increased the maximum number (B (max)) of [(3)H]flunitrazepam and [(3)H]muscimol binding sites in the membranes, and of [(3)H]muscimol binding sites on the surface of HEK 293 cells. There was no change in the affinity (K (d)) of binding sites. The diazepam-induced increase in the B (max) value of [(3)H]flunitrazepam binding sites was reduced by two GABA(A) receptor antagonists, gabazine (1 and 10 microM) and picrotoxin (100 microM). In addition, it was reduced by cycloheximide (5 microg/ml), a protein synthesis inhibitor, and actinomycin D (7.5 microg/ml), an RNA synthesis inhibitor. Flumazenil (5 microM), the antagonist of benzodiazepine binding sites, also up-regulated [(3)H]flunitrazepam recognition sites. Simultaneous treatment with diazepam and flumazenil failed to produce an additive up-regulation. GABA (1 nM - 1 mM)-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from diazepam (50 microM)-pretreated cells was markedly reduced, suggesting functional uncoupling between GABA and benzodiazepine binding sites. The results suggest that diazepam up-regulated benzodiazepine binding sites on stably expressed GABA(A) receptors by stimulating their synthesis at both the transcriptional and translational levels. A comparable increase of [(3)H]muscimol binding sites expressed on the surface of intact HEK 293 cells suggests that internalisation of surface receptors presumably can not explain the uncoupling.

Trafficking of GABA(A) receptors, loss of inhibition, and a mechanism for pharmacoresistance in status epilepticus

During status epilepticus (SE), GABAergic mechanisms fail and seizures become self-sustaining and pharmacoresistant. During lithiumpilocarpine-induced SE, our studies of postsynaptic GABA(A) receptors in dentate gyrus granule cells show a reduction in the amplitude of miniature IPSCs (mIPSCs). Anatomical studies show a reduction in the colocalization of the beta2/beta3 and gamma2 subunits of GABA(A) receptors with the presynaptic marker synaptophysin and an increase in the proportion of those subunits in the interior of dentate granule cells and other hippocampal neurons with SE. Unlike synaptic mIPSCs, the amplitude of extrasynaptic GABA(A) tonic currents is augmented during SE. Mathematical modeling suggests that the change of mIPSCs with SE reflects a decrease in the number of functional postsynaptic GABA(A) receptors. It also suggests that increases in extracellular [GABA] during SE can account for the tonic current changes and can affect postsynaptic receptor kinetics with a loss of paired-pulse inhibition. GABA exposure mimics the effects of SE on mIPSC and tonic GABA(A) current amplitudes in granule cells, consistent with the model predictions. These results provide a potential mechanism for the inhibitory loss that characterizes initiation of SE and for the pharmacoresistance to benzodiazepines, as a reduction of available functional GABA(A) postsynaptic receptors. Novel therapies for SE might be directed toward prevention or reversal of these losses.

Chronic treatment with flumazenil enhances binding sites for convulsants at recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors

GABA(A) receptors mediate most of the fast inhibitory neurotransmission in the brain. Prolonged occupancy of these receptors by ligands leads to regulatory changes often resulting in reduction of receptor function. The mechanism of these changes is still unknown. In this study, stably transfected human embryonic kidney (HEK) 293 cells were used as a model to study the effects of prolonged flumazenil (antagonist of benzodiazepine binding sites at GABA(A) receptors) exposure on the recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors, the most common type of GABA(A) receptors found in the brain. Exposure (48 h) of HEK 293 cells stably expressing recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors to flumazenil (1 or 5 microM) in the presence of GABA (1 microM), enhanced the maximum number (B(max)) without affecting the affinity (K(d)) of [(3)H]TBOB labeled binding sites for convulsants. Diazepam (1 nM-1 mM) in the presence of GABA (1 microM) modulated [(3)H]TBOB binding to control and flumazenil pretreated cells according to a two-site model. No significant differences between the groups were observed in either the potency or efficacy of diazepam to modulate [(3)H]TBOB binding, as evidenced by a lack of significant changes between their IC(50) and I(max) values. The results suggest that chronic exposure of HEK 293 cells stably expressing recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors to flumazenil up-regulates the binding sites for convulsants, but it does not appear to affect the functional coupling between these sites and benzodiazepine binding sites. Along with our recent data, these results suggest that chronic treatment with flumazenil enhances the number of GABA(A) receptors.

Internalized GABA-receptor subunits are transferred to an intracellular pool associated with the postsynaptic density

Endocytosis represents an important mechanism regulating cell-surface expression of neurotransmitter receptors, including GABAA receptors, in neurons. Little is known, however, about trafficking of internalized receptors. Here, we used antibody tagging in living rat hippocampal neurons in culture to monitor GABAA receptor internalization. We show that cell-surface receptors have a homogeneous distribution reflecting their mobility in the membrane. Unexpectedly, internalized GABAA receptors were detected mainly in a subsynaptic pool associated with gephyrin at postsynaptic sites, whereas AMPA-type glutamate receptors were accumulated in the soma. This process was time-dependent and could be prevented by blocking clathrin-coated vesicle endocytosis. In control experiments, the existence of an intracellular pool of GABAA receptors associated with gephyrin was confirmed independently of internalization of surface receptors, and constitutive endocytosis, unrelated to antibody-tagging, could be demonstrated for both AMPA and GABAA receptors using a biotinylation assay. These results suggest that cycling of GABAA receptors between the cell surface and the subsynaptic pool provides a mechanism for the short-term regulation of GABAergic neurotransmission. Furthermore, the close association of gephyrin with internalized GABAA receptors suggests a role in intracellular receptor trafficking.

GABA induces activity dependent delayed-onset uncoupling of GABA/benzodiazepine site interactions in neocortical neurons

Changes in the function of type A gamma-aminobutyric acid receptors (GABA(A)Rs) are associated with neuronal development and tolerance to the sedative-hypnotic effects of GABA(A)R positive modulators. Persistent activation of GABA(A)Rs by millimolar concentrations of GABA occurs under physiological conditions as GABAergic fast-spiking neurons in neocortex and cerebellum exhibit basal firing rates of 5 to 50 Hz and intermittent rates up to 250 Hz, leaving a substantial fraction of synaptic receptors occupied persistently by GABA. Persistent exposure of neurons to GABA has been shown to cause a down-regulation of receptor number and an uncoupling of GABA/benzodiazepine (BZD) site interactions with a half-life of approximately 24 h. Here, we report that a single brief exposure of neocortical neurons in primary culture to GABA for 5-10 min (t(1/2) = 3.2 +/- 0.2 min) initiates a process that results in uncoupling hours later (t(1/2) = 12.1 +/- 2.2 h). Initiation of delayed-onset uncoupling is blocked by co-incubation with picrotoxin or alpha-amanitin but is insensitive to nifedipine, indicating that uncoupling is contingent upon receptor activation and transcription but is not dependent on voltage-gated Ca2+ influx. Delayed-onset uncoupling occurs without a change in receptor number or a change in the proportion of alpha1 subunit pharmacology, as zolpidem binding affinity is unaltered. Such activity dependent latent modulation of GABA(A)R function that manifests as delayed-onset uncoupling may be relevant to physiological, pathophysiological, and pharmacological conditions where synaptic receptors are transiently exposed to GABA agonists for several minutes.

Requirement of alpha5-GABAA receptors for the development of tolerance to the sedative action of diazepam in mice

Despite its pharmacological relevance, the mechanism of the development of tolerance to the action of benzodiazepines is essentially unknown. The acute sedative action of diazepam is mediated via alpha1-GABA(A) receptors. Therefore, we tested whether chronic activation of these receptors by diazepam is sufficient to induce tolerance to its sedative action. Knock-in mice, in which thealpha1-,alpha2-,alpha3-, oralpha(5)-GABA(A) receptors had been rendered insensitive to diazepam by histidine-arginine point mutation, were chronically treated with diazepam (8 d; 15 mg x kg(-1) x d(-1)) and tested for motor activity. Wild-type, alpha2(H101R), and alpha3(H126R) mice showed a robust diminution of the motor-depressant drug action. In contrast, alpha5(H105R) mice failed to display any sedative tolerance. alpha1(H101R) mice showed no alteration of motor activity with chronic diazepam treatment. Autoradiography with [3H]flumazenil revealed no change in benzodiazepine binding sites. However, a decrease in alpha5-subunit radioligand binding was detected selectively in the dentate gyrus with specific ligands. This alteration was observed only in diazepam-tolerant animals, indicating that the manifestation of tolerance to the sedative action of diazepam is associated with a downregulation of alpha5-GABA(A) receptors in the dentate gyrus. Thus, the chronic activation of alpha(5)-GABA(A) receptors is crucial for the normal development of sedative tolerance to diazepam, which manifests itself in conjunction with alpha1-GABA(A) receptors.

Enhancement of benzodiazepine binding sites following chronic treatment with flumazenil

The aim of this study was to improve our knowledge of the mechanisms leading to adaptive changes in gamma-aminobutyric acid(A) (GABA(A)) receptors following chronic drug treatment. Exposure (48 h) of human embryonic kidney (HEK 293) cells stably expressing recombinant alpha1beta2gamma2S GABA(A) receptors to the antagonist of benzodiazepine binding sites, flumazenil (5 microM), enhanced the maximum number (B(max)) and the equilibrium dissociation constant (K(d)) of [3H]flunitrazepam binding sites. The flumazenil-induced enhancement in B(max) was potentiated by GABA (50 microM) and reduced by the GABA(A) receptor antagonist, bicuculline (100 microM). Flumazenil-induced enhancement in K(d) was affected by neither of these treatments. GABA (50 microM) enhanced the density of [3H]flunitrazepam binding sites, and this enhancement was greater in the presence of diazepam (1 microM). The results suggest that chronic flumazenil treatment up-regulates in a bicuculline-sensitive manner benzodiazepine binding sites at stably expressed GABA(A) receptors.

Chronic exposure of cells expressing recombinant GABAA receptors to benzodiazepine antagonist flumazenil enhances the maximum number of benzodiazepine binding sites

The aim of this study was to better understand the mechanisms that underlie adaptive changes in GABAA receptors following their prolonged exposure to drugs. Exposure (48 h) of human embryonic kidney (HEK) 293 cells stably expressing recombinant alpha1beta2gamma2S GABAA receptors to flumazenil (1 or 5 microM) in the presence of GABA (1 microM) enhanced the maximum number (Bmax) of [3H]flunitrazepam binding sites without affecting their affinity (Kd). The flumazenil-induced enhancement in Bmax was not counteracted by diazepam (1 microM). GABA (1 nM-1 mM) enhanced [3H]flunitrazepam binding to membranes obtained from control and flumazenil-pretreated cells in a concentration-dependent manner. No significant differences were observed in either the potency (EC50) or efficacy (Emax) of GABA to potentiate [3H]flunitrazepam binding. However, in flumazenil pretreated cells the basal [3H]flunitrazepam and [3H]TBOB binding were markedly enhanced. GABA produced almost complete inhibition of [3H]TBOB binding to membranes obtained from control and flumazenil treated cells. The potencies of GABA to inhibit this binding, as shown by a lack of significant changes in the IC50 values, were not different between vehicle and drug treated cells. The results suggest that chronic exposure of HEK 293 cells stably expressing recombinant alpha1beta2gamma2S GABAA receptors to flumazenil (in the presence of GABA) up-regulates benzodiazepine and convulsant binding sites, but it does not affect the allosteric interactions between these sites and the GABA binding site. Further studies are needed to elucidate these phenomena.

Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses

Neural inhibition in the brain is mainly mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors. Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at perisynaptic and extrasynaptic locations where they prolong phasic inhibition and mediate tonic inhibition, respectively. Of special interest are mechanisms that modulate the stability and function of postsynaptic GABA(A) receptor subtypes and that are implicated in functional plasticity of inhibitory transmission in the brain. We will summarize recent progress on the classification of synaptic versus extrasynaptic receptors, the molecular composition of the postsynaptic cytoskeleton, the function of receptor-associated proteins in trafficking of GABA(A) receptors to and from synapses, and their role in post-translational signaling mechanisms that modulate the stability, density, and function of GABA(A) receptors in the postsynaptic membrane.