Mechanism of alpha-subunit selectivity of benzodiazepine pharmacology at gamma-aminobutyric acid type A receptors

Anxiety disorders: Treatments, models, and circuitry mechanisms

Anxiety disorders are one of the most prevalent mental health conditions worldwide, imposing a significant burden on individuals affected by them and society in general. Current research endeavors aim to enhance the effectiveness of existing anxiolytic drugs and reduce their side effects through optimization or the development of new treatments. Several anxiolytic novel drugs have been produced as a result of discovery-focused research. However, many drug candidates that show promise in preclinical rodent model studies fail to offer any substantive clinical benefits to patients. This review provides an overview of the diagnosis and classification of anxiety disorders together with a systematic review of anxiolytic drugs with a focus on their targets, therapeutic applications, and side effects. It also provides a concise overview of the constraints and disadvantages associated with frequently administered anxiolytic drugs. Additionally, the study comprehensively reviews animal models used in anxiety studies and their associated molecular mechanisms, while also summarizing the brain circuitry related to anxiety. In conclusion, this article provides a valuable foundation for future anxiolytic drug discovery efforts.

Modulators of GABAA receptor-mediated inhibition in the treatment of neuropsychiatric disorders: past, present, and future

The predominant inhibitory neurotransmitter in the brain, γ-aminobutyric acid (GABA), acts at ionotropic GABAA receptors to counterbalance excitation and regulate neuronal firing. GABAA receptors are heteropentameric channels comprised from subunits derived from 19 different genes. GABAA receptors have one of the richest and well-developed pharmacologies of any therapeutic drug target, including agonists, antagonists, and positive and negative allosteric modulators (PAMs, NAMs). Currently used PAMs include benzodiazepine sedatives and anxiolytics, barbiturates, endogenous and synthetic neurosteroids, and general anesthetics. In this article, I will review evidence that these drugs act at several distinct binding sites and how they can be used to alter the balance between excitation and inhibition. I will also summarize existing literature regarding (1) evidence that changes in GABAergic inhibition play a causative role in major depression, anxiety, postpartum depression, premenstrual dysphoric disorder, and schizophrenia and (2) whether and how GABAergic drugs exert beneficial effects in these conditions, focusing on human studies where possible. Where these classical therapeutics have failed to exert benefits, I will describe recent advances in clinical and preclinical drug development. I will also highlight opportunities to advance a generation of GABAergic therapeutics, such as development of subunit-selective PAMs and NAMs, that are engendering hope for novel tools to treat these devastating conditions.

The efficacy of γ-aminobutyric acid type A receptor (GABA AR) subtype-selective positive allosteric modulators in blocking tetramethylenedisulfotetramine (TETS)-induced seizure-like behavior in larval zebrafish with minimal sedation

The chemical threat agent tetramethylenedisulfotetramine (TETS) is a γ-aminobutyric acid type A receptor (GABA _AR) antagonist that causes life threatening seizures. Currently, there is no specific antidote for TETS intoxication. TETS-induced seizures are typically treated with benzodiazepines, which function as nonselective positive allosteric modulators (PAMs) of synaptic GABA_ARs. The major target of TETS was recently identified as the GABA_AR α2β3γ2 subtype in electrophysiological studies using recombinantly expressed receptor combinations. Here, we tested whether these in vitro findings translate in vivo by comparing the efficacy of GABA_AR subunit-selective PAMs in reducing TETS-induced seizure behavior in larval zebrafish. We tested PAMs targeting α1, α2, α2/3/5, α6, ß2/3, ß1/2/3, and δ subunits and compared their efficacy to the benzodiazepine midazolam (MDZ). The data demonstrate that α2- and α6-selective PAMs (SL-651,498 and SB-205384, respectively) were effective at mitigating TETS-induced seizure-like behavior. Combinations of SB-205384 and MDZ or SL-651,498 and 2–261 (ß2/3-selective) mitigated TETS-induced seizure-like behavior at concentrations that did not elicit sedating effects in a photomotor behavioral assay, whereas MDZ alone caused sedation at the concentration required to stop seizure behavior. Isobologram analyses suggested that SB-205384 and MDZ interacted in an antagonistic fashion, while the effects of SL-651,498 and 2–261 were additive. These results further elucidate the molecular mechanism by which TETS induces seizures and provide mechanistic insight regarding specific countermeasures against this chemical convulsant.

New (arylalkyl)azole derivatives showing anticonvulsant effects could have VGSC and/or GABAAR affinity according to molecular modeling studies

(Arylalkyl)azoles (AAAs) emerged as a novel class of antiepileptic agents with the invention of nafimidone and denzimol. Several AAA derivatives with potent anticonvulsant activities have been reported so far, however neurotoxicity was usually an issue. We prepared a set of ester derivatives of 1-(2-naphthyl)-2-(1H-1,2,4-triazol-1-yl)ethanone oxime and evaluated their anticonvulsant and neurotoxic effects in mice. Most of our compounds were protective against maximal electroshock (MES)- and/or subcutaneous metrazol (s.c. MET)-induced seizures whereas none of them showed neurotoxicity. Nafimidone and denzimol have an activity profile similar to that of phenytoin or carbamazepine, both of which are known to inhibit voltage-gated sodium channels (VGSCs) as well as to enhance γ-aminobutiric acid (GABA)-mediated response. In order to get insights into the effects of our compounds on VGSCs and A-type GABA receptors (GABA_ARs) we performed docking studies using homology model of Na⁺ channel inner pore and GABA_AR as docking scaffolds. We found that our compounds bind VGSCs in similar ways as phenytoin, carbamazepine, and lamotrigine. They showed strong affinity to benzodiazepine (BZD) binding site and their binding interactions were mainly complied with the experimental data and the reported BZD binding model.

Targeting the modulation of neural circuitry for the treatment of anxiety disorders

Anxiety disorders are a major public health concern. Here, we examine the familiar area of anxiolysis in the context of a systems-level understanding that will hopefully lead to revealing an underlying pharmacological connectome. The introduction of benzodiazepines nearly half a century ago markedly improved the treatment of anxiety disorders. These agents reduce anxiety rapidly by allosterically enhancing the postsynaptic actions of GABA at inhibitory type A GABA receptors but side effects limit their use in chronic anxiety disorders. Selective serotonin reuptake inhibitors and serotonin/norepinephrine reuptake inhibitors have emerged as an effective first-line alternative treatment of such anxiety disorders. However, many individuals are not responsive and side effects can be limiting. Research into a relatively new class of agents known as neurosteroids has revealed novel modulatory sites and mechanisms of action that are providing insights into the pathophysiology of certain anxiety disorders, potentially bridging the gap between the GABAergic and serotonergic circuits underlying anxiety. However, translating the pharmacological activity of compounds targeted to specific receptor subtypes in rodent models of anxiety to effective therapeutics in human anxiety has not been entirely successful. Since modulating any one of several broad classes of receptor targets can produce anxiolysis, we posit that a systems-level discovery platform combined with an individualized medicine approach based on noninvasive brain imaging would substantially advance the development of more effective therapeutics.

Zolpidem and eszopiclone prime α1β2γ2 GABAA receptors for longer duration of activity

BACKGROUND AND PURPOSE

GABAA receptors mediate neuronal inhibition in the brain. They are the primary targets for benzodiazepines, which are widely used to treat neurological disorders including anxiety, epilepsy and insomnia. The mechanism by which benzodiazepines enhance GABAA receptor activity has been extensively studied, but there is little mechanistic information on how non-benzodiazepine drugs that bind to the same site exert their effects. Eszopiclone and zolpidem are two non-benzodiazepine drugs for which no mechanism of action has yet been proposed, despite their clinical importance as sleeping aids. Here we investigate how both drugs enhance the activity of α1β2γ2 GABAA receptors.

EXPERIMENTAL APPROACH

We used rapid ligand application onto macropatches and single-channel kinetic analysis to assess rates of current deactivation. We also studied synaptic currents in primary neuronal cultures and in heterosynapses, whereby native GABAergic nerve terminals form synapses with HEK293 cells expressing α1β2γ2 GABAA receptors. Drug binding and modulation was quantified with the aid of an activation mechanism.

KEY RESULTS

At the single-channel level, the drugs prolonged the duration of receptor activation, with similar KD values of ∼80 nM. Channel activation was prolonged primarily by increasing the equilibrium constant between two connected shut states that precede channel opening.

CONCLUSIONS AND IMPLICATIONS

As the derived mechanism successfully simulated the effects of eszopiclone and zolpidem on ensemble currents, we propose it as the definitive mechanism accounting for the effects of both drugs. Importantly, eszopiclone and zolpidem enhanced GABAA receptor currents via a mechanism that differs from that proposed for benzodiazepines.

Spatial Learning Deficits Induced by Muscimol and CL218,872: Lack of Effect of Prenatal Malnutrition

The sensitivity of prenatal protein malnourished rats to the amnestic properties of the direct GABAA receptor agonist muscimol and the selective benzodiazepine (BZ) receptor agonist, CL218,872, was studied in the male offspring of rats provided with a protein deficient diet (6% casein) for 5 weeks prior to mating and throughout pregnancy. At postnatal day 90, rats were tested during acquisition of the submerged platform version of the Morris water maze task using four systemic doses of muscimol (0.1, 0.3, 1.0 and 1.8 mg/kg i.p.) or three systemic doses of CL218,872 (1.0, 3.2, and 5.6 mg/kg i.p.). In a dose dependent manner both drugs impaired acquisition of the task and impaired accuracy of the search pattern on the probe trial (platform removed). However, neither drug dissociated the performance of the two nutritional groups. These data are important in light of previous findings of differential behavioral effects of the non-specific BZ agonist, chlordiazepoxide (CDP), on spatial learning and on drug discrimination in prenatally malnourished rats and in the context of previous findings of reduced sensitivity to the anxiolytic effects of non-specific BZ receptor agonists across a wide variety of models of malnutrition. The present findings also support the concept that prenatal malnutrition does not affect the global functioning of the GABAA receptor, but fundamentally alters the way in which a subset of GABAA receptors (i.e. those containing the alpha2, alpha3 and/or the alpha5 but not the alpha1 subunit) is modulated by BZs.

A unified model of the GABA(A) receptor comprising agonist and benzodiazepine binding sites

We present a full-length α(1)β(2)γ(2) GABA receptor model optimized for agonists and benzodiazepine (BZD) allosteric modulators. We propose binding hypotheses for the agonists GABA, muscimol and THIP and for the allosteric modulator diazepam (DZP). The receptor model is primarily based on the glutamate-gated chloride channel (GluCl) from C. elegans and includes additional structural information from the prokaryotic ligand-gated ion channel ELIC in a few regions. Available mutational data of the binding sites are well explained by the model and the proposed ligand binding poses. We suggest a GABA binding mode similar to the binding mode of glutamate in the GluCl X-ray structure. Key interactions are predicted with residues α(1)R66, β(2)T202, α(1)T129, β(2)E155, β(2)Y205 and the backbone of β(2)S156. Muscimol is predicted to bind similarly, however, with minor differences rationalized with quantum mechanical energy calculations. Muscimol key interactions are predicted to be α(1)R66, β(2)T202, α(1)T129, β(2)E155, β(2)Y205 and β(2)F200. Furthermore, we argue that a water molecule could mediate further interactions between muscimol and the backbone of β(2)S156 and β(2)Y157. DZP is predicted to bind with interactions comparable to those of the agonists in the orthosteric site. The carbonyl group of DZP is predicted to interact with two threonines α(1)T206 and γ(2)T142, similar to the acidic moiety of GABA. The chlorine atom of DZP is placed near the important α(1)H101 and the N-methyl group near α(1)Y159, α(1)T206, and α(1)Y209. We present a binding mode of DZP in which the pending phenyl moiety of DZP is buried in the binding pocket and thus shielded from solvent exposure. Our full length GABA(A) receptor is made available as Model S1.

Effects of eszopiclone and zolpidem on sleep-wake behavior, anxiety-like behavior and contextual memory in rats

At present, eszopiclone and zolpidem are the most commonly prescribed drugs for treating insomnia. Despite the established relationship between sleep disturbance and anxiety, it remains unknown whether targeted treatment for insomnia may affect acute anxiety. Therefore, the objective of this study was to examine the effects of three different doses (1, 3, and 10mg/kg) of eszopiclone and zolpidem on the states of sleep and wakefulness, levels of anxiety-like behavior, and long-term contextual memory in footshock-induced anxious rats. The results of this study demonstrated that the administration of eszopiclone and zolpidem both were equally effective in attenuating footshock stressor-induced suppression of slow-wave sleep (SWS). The administration of eszopiclone at 1mg/kg or zolpidem at 1 and 3mg/kg doses showed a tendency for attenuating stressor-induced suppression of REM sleep. However, the REM sleep attenuating effects of these drugs disappeared when they were administered at higher doses. The administration of eszopiclone at 3 and 10mg/kg doses and zolpidem at all three doses reduced the power of electroencephalographic theta band frequencies during wakefulness. In addition, the administration of eszopiclone at 1 and 3mg/kg doses suppressed stressor-induced anxiety-like behavior. The administration of zolpidem at 1, 3, or 10mg/kg doses was not effective in attenuating stressor-induced anxiety-like behavior. Contextual memory after administration of eszopiclone at 1mg/kg dose had no effects, but was reduced significantly with increased dosage. Contextual memory after administration of zolpidem, at all three doses, was severely disrupted. The results of this study suggest that eszopiclone at a low dose could be used effectively to control anxiety and anxiety-induced insomnia.

The modulation of synaptic GABA(A) receptors in the thalamus by eszopiclone and zolpidem

Eszopiclone (Lunesta; Sepracor, Marlborough, MA) and zolpidem [N,N,6-trimethyl-2-(4-methylphenyl)-imidazo(1,2-a)pyridine-3-acetamide] are among the most commonly prescribed hypnotics in use in the United States. The thalamus plays a pivotal role in sleep regulation and rhythmicity. Two distinct subtypes of synaptic GABA(A) receptors (GABA(A)-Rs), alpha(1)beta(2)gamma(2) and alpha(3)beta(3)gamma(2), are expressed in thalamocortical relay neurons and in interneurons of the RTN (reticular thalamic nucleus), respectively. Thalamocortical neurons also express extrasynaptic GABA(A)-Rs composed of alpha(4)beta(2)delta subunits. In this study, we compared the effects of eszopiclone and zolpidem on miniature inhibitory postsynaptic currents (IPSCs), spontaneous IPSCs, and tonic inhibition in the mouse thalamus. Eszopiclone (0.1-1 microM) slowed the decay phase of IPSCs recorded from RTN neurons, whereas zolpidem was less effective and increased the decay time constant only at > or = 0.3 microM. IPSCs of RTN neurons were more sensitive to eszopiclone than zolpidem at all concentrations tested. On the other hand, IPSCs of relay neurons in the ventrobasal nucleus (VB) were more sensitive to zolpidem than eszopiclone. Zolpidem (0.1-1 microM) prolonged the decay of IPSCs from VB neurons, whereas eszopiclone increased the decay time constant only at > or = 0.3 microM. Neither of these two hypnotics affected tonic inhibition in relay neurons. Our results demonstrate that eszopiclone has greater efficacy at synaptic GABA(A)-Rs of RTN neurons than in relay neurons, whereas zolpidem exerts bigger effects on relay neurons than RTN neurons. This distinct pattern of activity on thalamic neurons may contribute to some of the observed differences in the clinical effects of these two hypnotics.

Modeling of αk/γ2 (k=1, 2, 3 and 5) interface of GABAA receptor and docking studies with zolpidem: Implications for selectivity

The three-dimensional models of the alphak/gamma2 (k=1, 2, 3 and 5) interface of GABA(A) receptors, which included the agonist-binding site, were constructed and validated by molecular modeling technology. To investigate the mechanism of alpha subunit selectivity of zolpidem, docking calculations were used to illustrate the potential binding modes of zolpidem with different alpha subtypes. The results revealed that there were three reasons resulting in the distinct binding affinity of zolpidem to different alpha subtype. Firstly, the number of hydrogen bonds of agonist-receptor complex would determine the magnitude of binding affinity. Secondly, the His residue in loop A of alpha subunit was indicated as a key role of benzodiazepine binding. Thirdly, the side chain of Glu in loop C reduced the affinity of zolpidem to those receptors containing alpha2, alpha3 or alpha5 subunits.

Prenatal malnutrition alters diazepam-mediated suppression of ultrasonic vocalizations in an age dependent manner

The sensitivity of prenatally malnourished rats to the ultrasonic vocalization (USV) suppressant effect of diazepam (a non-specific benzodiazepine (BZ) receptor agonist) was investigated. Male offspring of dams provided with a protein deficient diet (6% casein) for 5 weeks prior to mating and throughout pregnancy were compared to the offspring of mothers provided with a diet of adequate protein content (25% casein). At postnatal day 7 or 11, pups were injected with vehicle or one of five doses of DZ (0.03, 0.1, 0.3, 1 or 3mg/kg) 30 min after removal from their dam. Thirty minutes later they were subjected to 2 min of cooling on a 20 degrees C surface and their USVs were quantified. DZ dose-dependently suppressed USV at both ages. At P7, the USV suppressant effect of DZ was the same for both groups. However, by P11 the prenatally malnourished rats showed significantly greater suppression of USV by 0.03 and 0.1mg/kg DZ than well-nourished controls. These differences were not related to degree of temperature loss or body weight. Thus, differential sensitivity to BZ receptor agonists develops in the second postnatal week in prenatally malnourished rats. This reflects either an altered program of development of the GABAergic system, or adaptive, compensatory changes in the GABAergic system in response to more extensive functional disturbances in the developing brain.

Structural determinants for high-affinity zolpidem binding to GABA-A receptors

The imidazopyridine zolpidem (Ambien) is one of the most commonly prescribed sleep aids in the United States (Rush, 1998). Similar to classic benzodiazepines (BZDs), zolpidem binds at the extracellular N-terminal alpha/gamma subunit interface of the GABA-A receptor (GABAR). However, zolpidem differs significantly from classic BZDs in chemical structure and neuropharmacological properties. Thus, classic BZDs and zolpidem are likely to have different requirements for high-affinity binding to GABARs. To date, three residues--gamma2Met57, gamma2Phe77, and gamma2Met130--have been identified as necessary for high-affinity zolpidem binding (Proc Natl Acad Sci USA 94:8824-8829, 1997; Mol Pharmacol 52:874-881, 1997). In this study, we used radioligand binding techniques, gamma2/alpha1 chimeric subunits (chi), site-directed mutagenesis, and molecular modeling to identify additional gamma2 subunit residues important for high-affinity zolpidem binding. Whereas alpha1beta2chi receptors containing only the first 161 amino-terminal residues of the gamma2 subunit bind the classic BZD flunitrazepam with wild-type affinity, zolpidem affinity is decreased approximately 8-fold. By incrementally restoring gamma2 subunit sequence, we identified a seven-amino acid stretch in the gamma2 subunit loop F region (amino acids 186-192) that is required to confer high-affinity zolpidem binding to GABARs. When mapped to a homology model, these seven amino acids make up part of loop F located at the alpha/gamma interface. Based on in silico zolpidem docking, three residues within loop F, gamma2Glu189, gamma2Thr193, and gamma2Arg194, emerge as being important for stabilizing zolpidem in the BZD binding pocket and probably interact with other loop F residues to maintain the structural integrity of the BZD binding site.

Allopregnanolone produces hyperphagia by reducing neophobia without altering food palatability

The neurosteroid allopregnanolone may increase feeding by altering food palatability; however, it may also increase feeding by reducing anxiety (neophobia). Moreover, it is unclear whether this induced hyperphagia is selective to safe, palatable foods only. Male rats were injected with allopregnanolone 20 min prior to behavioral testing. The taste reactivity test was used to examine possible shifts in the palatability of a 0.3 M sucrose solution. A lickometer was used to monitor intake and licking of either a sucrose or sucrose-quinine solution. Sucrose palatability was not enhanced; however, allopregnanolone significantly increased sucrose intake and licking on Test Day 1 when the solution was novel, but not on Test Day 2 when the solution was familiar. Sucrose-quinine intake was not enhanced. Allopregnanolone-induced hyperphagia is not a result of altered sucrose palatability, but rather reflects a reduction in the neophobia elicited by a novel solution; an effect that further seems to be selective to safe, palatable foods.

Graded response to GABA by native extrasynaptic GABA receptors

GABA is the main inhibitory neurotransmitter in the mammalian CNS. GABA in the brain is commonly associated with a fast, point-to-point form of signalling called synaptic transmission (phasic inhibition), but there is growing evidence that GABA participates in another, slower and more diffuse form of signalling often referred to as tonic inhibition. Unresolved questions regarding tonic neuronal inhibition concern activation and functional properties of extrasynaptic GABAA receptors (GABARex) present on neurones. Extrasynaptic receptors are exposed to submicromolar GABA concentrations and may modulate the overall excitability of neurones and neuronal networks. Here, we examined GABA-activated single-channel currents in dentate gyrus granule neurones in rat hippocampal slices. We activated three types (I, II, III) of GABARex channels by nanomolar GABA concentrations (EC50 I: 27 +/- 12; II: 4 +/- 3; III: 43 +/- 19 nm). The channels opened after a delay and the single-channel conductance was graded (gammamax I: 61 +/- 3; II: 85 +/- 8, III: 40 +/- 3 pS). The channels were differentially modulated by 1 microm diazepam, 200 nm zolpidem, 1 microm flumazenil and 50 nm THDOC (3alpha, 21-dihydroxy-5alpha-pregnan-20-one), consistent with the following minimal subunit composition of GABARex I alpha1betagamma2, GABARex II alpha4betagamma2 and GABARex III alphabetadelta channels.

Pharmacological properties of GABAA receptors containing gamma1 subunits

GABA(A) receptors composed of alpha(1), beta(2), gamma(1) subunits are expressed in only a few areas of the brain and thus represent interesting drug targets. The pharmacological properties of this receptor subtype, however, are largely unknown. In the present study, we expressed alpha(1)beta(2)gamma(1)-GABA(A) receptors in Xenopus laevis oocytes and analyzed their modulation by 21 ligands from 12 structural classes making use of the two-microelectrode voltage-clamp method and a fast perfusion system. Modulation of GABA-induced chloride currents (I(GABA)) was studied at GABA concentrations eliciting 5 to 10% of the maximal response. Triazolam, clotiazepam, midazolam, 2-(4-methoxyphenyl)-2,3,5,6,7,8,9,10-octahydro-cyclohepta-(b)pyrazolo[4,3-d]pyridin-3-one (CGS 20625), 2-(4-chlorophenyl)-pyrazolo[4,3-c]quinolin-3-one (CGS 9896), diazepam, zolpidem, and bretazenil at 1 microM concentrations were able to significantly (>20%) enhance I(GABA) in alpha(1)beta(2)gamma(1) receptors. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, 3-methyl-6-[3-trifluoromethyl-phenyl]-1,2,4-triazolo[4,3-b]pyridazine (Cl 218,872), clobazam, flumazenil, 5-(6-ethyl-7-methoxy-5-methylimidazo[1,2-a]pyrimidin-2-yl)-3-methyl-[1,2,4]-oxadiazole (Ru 33203), 2-phenyl-4-(3-ethyl-piperidinyl)-quinoline (PK 9084), flurazepam, ethyl-7-methoxy-11,12,13,13a-tetrahydro-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c] [1,4]benzodiazepine-1-carboxylate (l-655,708), 2-(6-ethyl-7-methoxy-5-methylimidazo[1,2-a]pyrimidin-2-yl)-4-methyl-thiazole (Ru 33356), and 6-ethyl-7-methoxy-5-methylimidazo[1,2-a]pyrimidin-2-yl)phenylmethanone (Ru 32698) (1 microM each) had no significant effect, and flunitrazepam and 2-phenyl-4-(4-ethyl-piperidinyl)-quinoline (PK 8165) inhibited I(GABA). The most potent compounds triazolam, clotiazepam, midazolam, and CGS 20625 were investigated in more detail on alpha(1)beta(2)gamma(1) and alpha(1)beta(2)gamma(2S) receptors. The potency and efficiency of these compounds for modulating I(GABA) was smaller for alpha(1)beta(2)gamma(1) than for alpha(1)beta(2)gamma(2S) receptors, and their effects on alpha(1)beta(2)gamma(1) could not be blocked by flumazenil. CGS 20625 displayed the highest efficiency by enhancing at 100 microM I(GABA) (alpha(1)beta(2)gamma(2)) by 775 +/- 17% versus 526 +/- 14% I(GABA) (alpha(1)beta(2)gamma(1)) and 157 +/- 17% I(GABA) (alpha(1)beta(2)) (p < 0.05). These data provide new insight into the pharmacological properties of GABA(A) receptors containing gamma(1) subunits and may aid in the design of specific ligands for this receptor subtype.

Palatability-dependent appetite and benzodiazepines: new directions from the pharmacology of GABA(A) receptor subtypes

This paper updates an early review on benzodiazepine-enhanced food intake, published in the first issue of Appetite, and describes the considerable advances since then in the pharmacology of benzodiazepines, their sites and mechanisms of action, and in understanding the psychological processes leading to the increase in food consumption. A great diversity of benzodiazepine receptor ligands have been developed, many of which affect food intake. Agonists can be divided into full agonists (which produce the full spectrum of benzodiazepine effects) and partial agonists (which are more selective in their effects). In addition, inverse agonists have been identified, with high affinity for benzodiazepine receptors but having negative efficacy: these drugs exhibit anorectic properties. Benzodiazepine receptors are part of GABA(A) receptor complexes, and ligands thereby modulate inhibitory neurotransmission in the brain. Molecular approaches have identified a palette of receptor subunits from which GABA(A) receptors are assembled. In all likelihood, benzodiazepine-induced hyperphagia is mediated by the alpha2/alpha3 subtype not the alpha1 subtype. Novel alpha2/alpha3 selective compounds will test this hypothesis. A probable site of action in the caudal brainstem for benzodiazepines is the parabrachial nucleus. Behavioural evidence strongly indicates that a primary action of benzodiazepines is to enhance the positive hedonic evaluation (palatability) of tastes and foodstuffs. This generates the increased food intake and instrumental responding for food rewards. Therapeutic applications may derive from the actions of benzodiazepine agonists and inverse agonists on food procurement and ingestion.

GABA and glutamate systems as therapeutic targets in depression and mood disorders

Advances made in diverse areas of neuroscience suggest that neurotransmitter systems, additional to the monoaminergic, contribute to the pathophysiology of mood disorders. This ever accruing body of preclinical and clinical research is providing increased recognition of the contribution made by amino acid neurotransmitters to the neurobiology of mood disorders. This review examines evidence supporting the role of GABA and glutamate in these processes and explores the potential to target these systems in the development of novel compounds; the viability of these agents for treatment-related co-morbidities will also be considered.

Discovery of Functionally Selective 7,8,9,10-Tetrahydro-7,10-ethano-1,2,4-triazolo[3,4-a]phthalazines as GABAA Receptor Agonists at the α3 Subunit

We have previously identified the 7,8,9,10-tetrahydro-7,10-ethano-1,2,4-triazolo[3,4-a]phthalazine (1) as a potent partial agonist for the alpha(3) receptor subtype with 5-fold selectivity in binding affinity over alpha(1). This paper describes a detailed investigation of the substituents on this core structure at both the 3- and 6-positions. Despite evaluating a wide range of groups, the maximum selectivity that could be achieved in terms of affinity for the alpha(3) subtype over the alpha(1) subtype was 12-fold (for 57). Although most analogues showed no selectivity in terms of efficacy, some did show partial agonism at alpha(1) and antagonism at alpha(3) (e.g., 25 and 75). However, two analogues tested (93 and 96), both with triazole substituents in the 6-position, showed significantly higher efficacy for the alpha(3) subtype over the alpha(1) subtype. This was the first indication that selectivity in efficacy in the required direction could be achieved in this series.

[The life cycle of the GABA(A) receptor and its regulating molecules]

Gamma-aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the central nervous system. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their function or cell surface stability will have major consequences for neuronal excitation. This review highlights recent findings on the regulation of GABA(A)-receptor expression and function, focusing on the mechanisms of sorting, targeting, synaptic clustering, and endocytic events of GABA(A) receptors, all which are regulated by their associated proteins. Now these topics are an area of active interest in studies on inhibitory neurotransmission.

Identification of the bovine gamma-aminobutyric acid type A receptor alpha subunit residues photolabeled by the imidazobenzodiazepine [3H]Ro15-4513

Ligands binding to the benzodiazepine-binding site in gamma-aminobutyric acid type A (GABA(A)) receptors may allosterically modulate function. Depending upon the ligand, the coupling can either be positive (flunitrazepam), negative (Ro15-4513), or neutral (flumazenil). Specific amino acid determinants of benzodiazepine binding affinity and/or allosteric coupling have been identified within GABA(A) receptor alpha and gamma subunits that localize the binding site at the subunit interface. Previous photolabeling studies with [(3)H]flunitrazepam identified a primary site of incorporation at alpha(1)His-102, whereas studies with [(3)H]Ro15-4513 suggested incorporation into the alpha(1) subunit at unidentified amino acids C-terminal to alpha(1)His-102. To determine the site(s) of photoincorporation by Ro15-4513, we affinity-purified ( approximately 200-fold) GABA(A) receptor from detergent extracts of bovine cortex, photolabeled it with [(3)H]Ro15-4513, and identified (3)H-labeled amino acids by N-terminal sequence analysis of subunit fragments generated by sequential digestions with a panel of proteases. The patterns of (3)H release seen after each digestion of the labeled fragments determined the number of amino acids between the cleavage site and labeled residue, and the use of sequential proteolytic fragmentation identified patterns of cleavage sites unique to the different alpha subunits. Based upon this radiochemical sequence analysis, [(3)H]Ro15-4513 was found to selectively label the homologous tyrosines alpha(1)Tyr-210, alpha(2)Tyr-209, and alpha(3)Tyr-234, in GABA(A) receptors containing those subunits. These results are discussed in terms of a homology model of the benzodiazepine-binding site based on the molluscan acetylcholine-binding protein structure.