Anxioselective anxiolytics: can less be more?

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

Food protein-derived anxiolytic peptides: their potential role in anxiety management

Could bioactive peptides from food proteins be used as prophylactic in the management of anxiety disorders?

Neuropharmacological potentials of β-carboline alkaloids for neuropsychiatric disorders

Neuropsychiatric disorders are diseases of the central nervous system (CNS) which are characterised by complex pathomechanisms that including homeostatic failure, malfunction, atrophy, pathology remodelling and reactivity anomaly of the neuronal system where treatment options remain challenging. β-Carboline (βC) alkaloids are scaffolds of structurally diverse tricyclic pyrido[3,4-b]indole alkaloid with vast occurrence in nature. Their unique structural features which favour interactions with enzymes and protein receptor targets account for their potent neuropharmacological properties. However, our current understanding of their biological mechanisms for these beneficial effects, especially for neuropsychiatric disorders is sparse. Therefore, we present a comprehensive review of the scientific progress in the last two decades on the prospective pharmacology and physiology of the βC alkaloids in the treatment of some neuropsychiatric conditions such as depression, anxiety, Alzheimer's disease, Parkinson's disease, brain tumour, essential tremor, epilepsy and seizure, licking behaviour, dystonia, agnosia, spasm, positive ingestive response as demonstrated in non-clinical models. The current evidence supports that βC alkaloids offer potential therapeutic agents against most of these disorders and amenable for further drug design.

Use of an Animal Model to Evaluate Anxiolytic Effects of Dietary Supplementation with Tilia tomentosa Moench Bud Extracts

Anxiety disorders are common and complex psychiatric syndromes affecting a broad spectrum of patients. On top of that, we know that aging produces an increase in anxiety vulnerability and sedative consumption. Moreover, stress disorders frequently show a clear gender susceptibility. Currently, the approved pharmacological strategies have severe side effects such as hallucinations, addiction, suicide, insomnia, and loss of motor coordination. Dietary integration with supplements represents an intriguing strategy for improving the efficacy and the safety of synthetic anxiolytics. Accordingly, a recent article demonstrated that glyceric bud extracts from Tilia tomentosa Moench (TTBEs) exert effects that are consistent with anxiolytic activity. However, the effects of these compounds in vivo are unknown. To examine this question, we conducted behavioral analysis in mice. A total of 21 days of oral supplements (vehicle and TTBEs) were assessed by Light Dark and Hole Board tests in male and female mice (young, 3 months; old, 24 months). Interestingly, the principal component analysis revealed gender and age-specific behavioral modulations. Moreover, the diet integration with the botanicals did not modify the body weight gain and the daily intake of water. Our results support the use of TTBEs as dietary supplements for anxiolytic purposes and unveil age and gender-dependent responses.

Effects of a hydroethanolic extract of Boophone disticha bulb on anxiety-related behaviour in naive BALB/c mice

ETHNOPHARMACOLOGICAL RELEVANCE

Boophone disticha is one of the most important medicinal bulbs of Southern Africa. Previous in vitro studies have shown that it's crude ethanolic extracts and some alkaloidal phytoconstituents possesses high affinity for the serotonin transporter protein (SERT) and serotonin receptor 1a (5HT_1a) which are both implicated in the pathogenesis and treatment of anxiety disorders. However, there are no in vivo studies that validates the anxiolytic actions of the plant.

AIM OF THE STUDY

This study was therefore set to determine the anxiolytic-like activity of an orally administered hydroethanolic extract of B. disticha bulbs in naive mice using the behavioural tests of anxiety.

MATERIALS AND METHODS

Naïve adult male BALB/c mice were randomly placed into five treatment groups (n=6-10): vehicle control (10ml/kg 0.9% NaCl), positive control (1mg/kg diazepam) and the hydroethanolic extract of B. disticha (10, 25 and 40mg/kg p.o.). Souk test, elevared plus maze and open field tests were used to evaluate the anxiolytic-like activity of the B. disticha extract.

RESULTS

Diazepam-treated mice exhibited higher number of sector visits and line crossings in the ST, rearings in the OF and head dips in the EPM than the control (p<0.05). B. disticha extract treated groups expressed higher sector visits at 10mg/kg, and, unprotected head dips at 25mg/kg in the ST, as well as, open arm time entries at 10mg/kg dose, and unprotected head dips at all doses in the EPM than the control group (p<0.05). The 25mg/kg B. disticha dose group exhibited highest anxiolytic-like activity in both the ST and OF, while the 10mg/kg was most active in the EPM.

CONCLUSION

The extract of B. disticha exerted good anxiolytic-like activity in both the ST and OF at medium dose (25mg/kg), while the low dose (10mg/kg) showed prominent anxiolytic-like activity in the EPM.

Characterization of GABAA receptor ligands with automated patch-clamp using human neurons derived from pluripotent stem cells

INTRODUCTION

Automated patch clamp is a recent but widely used technology to assess pre-clinical drug safety. With the availability of human neurons derived from pluripotent stem cells, this technology can be extended to determine CNS effects of drug candidates, especially those acting on the GABA_A receptor.

METHODS

iCell Neurons (Cellular Dynamics International, A Fujifilm Company) were cultured for ten days and analyzed by patch clamp in the presence of agonist GABA or in combination with positive allosteric GABA_A receptor modulators. Both efficacy and affinity were determined. In addition, mRNA of GABA_A receptor subunits were quantified by qRT-PCR.

RESULTS

We have shown that iCell Neurons are compatible with the IonFlux microfluidic system of the automated patch clamp instrument. Resistance ranging from 15 to 25MΩ was achieved for each trap channel of patch clamped cells in a 96-well plate format. GABA induced a robust change of current with an EC₅₀ of 0.43μM. Positive GABA_A receptor modulators diazepam, HZ-166, and CW-04-020 exhibited EC₅₀ values of 0.42μM, 1.56μM, and 0.23μM, respectively. The α2/α3/α5 selective compound HZ-166-induced the highest potentiation (efficacy) of 810% of the current induced by 100nM GABA. Quantification of GABA_A receptor mRNA in iCell Neurons revealed high levels of α5 and β3 subunits and low levels of α1, which is similar to the configuration in human neonatal brain.

DISCUSSION

iCell Neurons represent a new cellular model to characterize GABAergic compounds using automated patch clamp. These cells have excellent representation of cellular GABA_A receptor distribution that enable determination of total small molecule efficacy and affinity as measured by cell membrane current change.

Access to 4-alkylaminopyridazine derivatives via nitrogen-assisted regioselective Pd-catalyzed reactions

3-Substituted, 6-substituted, and unsymmetrical 3,6-disubstituted 4-alkylaminopyridazines were prepared from a sequence of three chemo- and regioselective reactions combining amination and palladium-catalyzed cross-coupling reactions, such as reductive dehalogenation and Suzuki-Miyaura reactions. Extension of the methodology to Sonogashira reaction yielded a novel class of 3-substituted pyrrolopyridazines.

Synthesis, anticonvulsant, sedative and anxiolytic activities of novel annulated pyrrolo[1,4]benzodiazepines

Four new pentacyclic benzodiazepine derivatives (PBDTs 13–16) were synthesized by conventional thermal heating and microwave-assisted intramolecular cyclocondensation. Their anticonvulsant, sedative and anxiolytic activities were evaluated by drug-induced convulsion models, a pentobarbital-induced hypnotic model and an elevated plus maze in mice. PBDT 13, a triazolopyrrolo[2,1-c][1,4]benzodiazepin-8-one fused with a thiadiazolone ring, exhibited the best anticonvulsant, sedative and anxiolytic effects in our tests. There was no significant difference in potency between PBDT 13 and diazepam, and we proposed that the action mechanism of PBDT 13 could be similar to that of diazepam via benzodiazepine receptors.

SB-205384 is a positive allosteric modulator of recombinant GABAA receptors containing rat α3, α5, or α6 subunit subtypes coexpressed with β3 and γ2 subunits

Many drugs used to treat anxiety are positive modulators of GABAA receptors, which mediate fast inhibitory neurotransmission. The GABAA receptors can be assembled from a combination of at least 16 different subunits. The receptor's subunit composition determines its pharmacologic and functional properties, and subunit expression varies throughout the brain. A primary goal for new treatments targeting GABAA receptors is the production of subunit-selective modulators acting upon a discrete population of receptors. The anxiolytic 4-amino-7-hydroxy-2-methyl-5,6,7,8,-tetrahydrobenzo[b]thieno[2,3-b]pyridine-3-carboxylic acid, but-2-ynyl ester (SB-205384) is widely considered to be selective for α3-containing GABAA receptors. However, it has been tested only on α1-, α2-, and α3-containing receptors. We examined the activity of SB-205384 at recombinant receptors containing the six different α subunits and found that receptors containing the α3, α5, and α6 subunits were potentiated by SB-205384, with the α6 subunit conferring the greatest responsiveness. Properties associated with chimeric α1/α6 subunits suggested that multiple structural domains influence sensitivity to SB-205384. Point mutations of residues within the extracellular N-terminal domain identified a leucine residue located in loop E of the agonist binding site as an important determinant of high sensitivity to modulation. In the α6 subunit the identity of this residue is species-dependent, with the leucine found in rat subunits but not in human. Our results indicate that SB-205384 is not an α3-selective modulator, and instead acts at several GABAA receptor isoforms. These findings have implications for the side-effect profile of this anxiolytic as well as for its use in neuronal and animal studies as a marker for contribution from α3-containing receptors.

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.

Anxiolytic activity of pyridoindole derivatives SMe1EC2 and SMe1M2: behavioral analysis using rat model

Anxiety and mood disorders have become very significant affections in the last decades. According to WHO at least one mental disease occurred per year in 27% of EU inhabitants (more than 82 mil. people). It is estimated that by 2020, depression will be the main cause of morbidity in the developed countries. These circumstances call for research for new prospective drugs with anxiolytic and antidepressive properties exhibiting no toxicity and withdrawal effect and possessing beneficial properties, like antioxidant and/or neuroprotective effects. The aim of this study was to obtain information about psychopharmacological properties of pyridoindole derivatives SMe1EC2 and SMe1M2, using non-invasive behavioral methods in rats.The battery of ethological tests (open field, elevated plus-maze, light/dark box exploration, forced swim test) was used to obtain information about anxiolytic and antidepressant activity of the pyridoindole derivatives. The substances were administered intraperitoneally 30 minutes before the tests at doses of 1, 10 and 25 mg/kg.In the behavioral tests, SMe1EC2 was found to exert anxiolytic activity in elevated plus maze with no affection of locomotor activity. The highest dose of SMe1M2 increased the time spent in the lit part of the Light/Dark box, however this result was influenced by inhibition of motor activity of the rats. Similar findings were observed also in elevated plus-maze, although these results were not statistically significant.In conclusion, from the results of our study it is evident that both pyridoindoles acted on the CNS. In the highest dose, SMe1M2 was found to possess rather sedative than anxiolytic or antidepressant activity.

Effects of oxazepam on affective perception, recognition, and event-related potentials

BACKGROUND

Little is known about how rapid electrocortical responses (event-related potentials; ERPs) to affective pictures are modulated by benzodiazepine agonists. The present study investigated effects of oxazepam (20 mg p.o.) on behavioral measures and ERPs associated with affective picture processing during perception and recognition memory retrieval.

METHODS

Forty-three healthy young adults were given oxazepam or placebo treatment under a double-blind experimental procedure. Affective pictures (negatively arousing or neutral) elicited ERP responses and participants rated pictures for emotionality (during incidental encoding) and recognition.

RESULTS

Oxazepam did not affect perceptual (P1, P2) or emotional (early posterior negativity and late parietal positivity) ERPs or ratings during perception. However, oxazepam impaired recognition performance and decreased positive mid-frontal ERP component at 420-450 ms for old vs. new pictures. The memory impairment was retained at the delayed memory test.

CONCLUSIONS

Oxazepam does not selectively influence electrocortical or perceptual indexes of emotional perception or emotional memory. Rather, it blocks memory consolidation independent of valence category. These findings indicate that ERPs can be of use in assessing effects of benzodiazepines on memory-related processes.

A multicenter, placebo-controlled, double-blind, randomized study of efficacy and safety of ocinaplon (DOV 273,547) in generalized anxiety disorder

Preclinical studies demonstrated that ocinaplon, a positive allosteric modulator of GABA(A) receptors, possesses anxiolytic-like actions at doses devoid of the side effects typically associated with benzodiazepines. The aim of this study was to evaluate the effects of ocinaplon in a multicenter, double-blind proof-of-concept trial of male and female outpatients who met DSM-IV criteria for GAD with no coexisting depression, and had a baseline score of > or =20 on the Hamilton Scale for Anxiety (HAM-A). Patients with <20% reduction in HAM-A to placebo in a single-blind 7-day run-in period were randomly assigned to treatment with ocinaplon 90 mg t.i.d. (n = 31) or placebo for 28 days (n = 29). Ocinaplon was more effective than placebo in reducing HAM-A scores (P= 0.009). Patients assigned to ocinaplon exhibited a mean improvement of 14.2 points (SE = 2.6) on the total score of the HAM-A scale at the conclusion of the trial, while patients assigned to placebo obtained a mean improvement of 6.3 points (SE = 2.0). A significant (P= 0.023) difference in improvement between ocinaplon and placebo was observed beginning at and continuing from 1-week after the initiation of dosing. The proportion of patients with treatment-emergent adverse events (TEAE) was not statistically significant between ocinaplon and placebo. One serious adverse event (SAE) occurred in the ocinaplon group that was considered possibly related to study medication (icterus following transaminase elevations). The patient had preexisting medical conditions that may have contributed to this SAE. A full recovery was observed with no residual effects. The overall safety profile revealed no patterns of TEAEs, including those effects typically associated with other anxiolytic and/or benzodiazepine compounds, such as sedation. Ocinaplon appears to be a well-tolerated and effective treatment for GAD. It produces a rapid onset of anxiolytic action absent the side effects (e.g., dizziness, sedation) typically reported following anxiolytic doses of benzodiazepines.

Limiting activity at beta1-subunit-containing GABAA receptor subtypes reduces ataxia

GABA(A) receptor (R) positive allosteric modulators that selectively modulate GABA(A)Rs containing beta(2)- and/or beta(3)- over beta(1)-subunits have been reported across diverse chemotypes. Examples include loreclezole, mefenamic acid, tracazolate, and etifoxine. In general,"beta(2/3)-selective" GABA(A)R positive allosteric modulators are nonbenzodiazepines (nonBZs), do not show alpha-subunit isoform selectivity, yet have anxiolytic efficacy with reduced ataxic/sedative effects in animal models and humans. Here, we report on an enantiomeric pair of nonBZ GABA(A)R positive allosteric modulators that demonstrate differential beta-subunit isoform selectivity. We have tested this enantiomeric pair along with a series of other beta(2/3)-subunit selective, alpha-subunit isoform-selective, BZ and nonBZ GABA(A) positive allosteric modulators using electrophysiological, pharmacokinetic, and behavioral assays to test the hypothesis that ataxia may be correlated with the extent of modulation at beta(1)-subunit-containing GABA(A)Rs. Our findings provide an alternative strategy for designing anxioselective allosteric modulators of the GABA(A)R with BZ-like anxiolytic efficacy by reducing or eliminating activity at beta(1)-subunit-containing GABA(A)Rs.

GABA A receptors: subtypes provide diversity of function and pharmacology

This mini-review attempts to update experimental evidence on the existence of GABA(A) receptor pharmacological subtypes and to produce a list of those native receptors that exist. GABA(A) receptors are chloride channels that mediate inhibitory neurotransmission. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. They are assembled from a family of 19 homologous subunit gene products and form numerous receptor subtypes with properties that depend upon subunit composition, mostly hetero-oligomeric. These vary in their regulation and developmental expression, and importantly, in brain regional, cellular, and subcellular localization, and thus their role in brain circuits and behaviors. We propose several criteria for including a receptor hetero-oligomeric subtype candidate on a list of native subtypes, and a working GABA(A) receptor list. These criteria can be applied to all the members of the LGIC superfamily. The list is divided into three categories of native receptor subtypes: "Identified", "Existence with High Probability", and "Tentative", and currently includes 26 members, but will undoubtedly grow, with future information. This list was first presented by Olsen & Sieghart (in press).

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Evaluation of anxiolytic-like effects of some short-acting benzodiazepine hypnotics in mice

Anxiolytic-like effects of some short-acting benzodiazepine hypnotics were examined with experimental paradigms of anxiety using an elevated plus-maze in male ICR mice. Diazepam was used as a positive control. The drug at a dose of 1 mg/kg significantly increased the percentage of time spent in the open arms and percentage of the number of open arm entries in the elevated plus-maze. Triazolam, brotizolam, rilmazafone, and lormetazepam also showed an anxiolytic-like effect as indicated by the significant increase in the percentage of time spent in the open arms and percentage of the number of open arm entries. Effects of short-acting benzodiazepine hypnotics used in the study were more potent than those of diazepam. In addition, the doses affecting the elevated plus-maze by benzodiazepine hypnotics were much smaller than those that showed muscle-relaxant activity measured by the rotarod test, indicating that anxiolytic-like effects of benzodiazepine hypnotics had high specificity and selectivity.

Strain- and model-dependent effects of chlordiazepoxide, L-838,417 and zolpidem on anxiety-like behaviours in laboratory mice

The promise of subtype-selective GABA(A) receptor drugs with anxiolytic properties but with a much reduced side-effect burden (compared to benzodiazepines) is an attainable goal. However, its achievement necessitates the availability of in vivo preclinical assays capable of demonstrating differences as well as similarities between subtype-selective agents and non-selective benzodiazepines. In this study, we have compared three mouse strains (NMRI, C57BL/6J and DBA/2) in four models of anxiety-like behaviour (plus-maze, zero-maze, light-dark, and Vogel conflict). Furthermore, in each model, we have contrasted in detail the behavioural responses of each strain to the non-selective benzodiazepine chlordiazepoxide (CDP; 5-20 mg/kg), and the subtype-selective agents L-838,417 (GABA(A)-alpha(2/3/5); 3-30 mg/kg) and zolpidem (GABA(A)-alpha1; 0.3-3.0 mg/kg). The data show a complex mouse strainxmodelxpharmacological agent interaction. Most importantly, not all mouse strainxmodel test systems showed a positive response to CDP or predicted the response to L-838,417. This dissociation between CDP and L-838,417 opens up opportunities for preclinical test systems that differentiate subtype-selective and non-selective GABA(A) receptor agents, an attribute that might well be important in providing the necessary confidence for further drug development. Present findings suggest the need for a much greater focus on defining test systems appropriate for screening novel chemical entities, rather than self-selection of models or genotypes based on responses to known pharmacological agents. For example, if current data with L-838,417 are confirmed with compounds showing similar selectivity profiles, such agents may in future be best identified and characterised using test systems comprising NMRI mice in the zero-maze and/or C57 mice in the Vogel conflict and/or light-dark tests.

The role of GABA(A) receptors in the control of transient lower oesophageal sphincter relaxations in the dog

BACKGROUND AND PURPOSE

Transient lower oesophageal sphincter relaxations (TLESRs) are triggered by activation of mechanosensitive gastric vagal afferents and are the major cause of gastroesophageal reflux and therefore an important target for therapeutic intervention in gastroesophageal reflux disease (GERD). Activation of the metabotropic GABA(B) receptor has shown to inhibit TLESRs. The aim of the present study was to assess the role of the ionotropic GABA(A) receptor in the regulation of TLESRs.

EXPERIMENTAL APPROACH

TLESRs were quantified using Dentsleeve manometry in dogs, and GABA(A) agonists were given i.v. prior to gastric distension. Immunohistochemistry and RT-PCR were used to localize GABA(A) receptors in the dog nodose ganglion, the source of vagal afferents which initiate TLESRs.

KEY RESULTS

The prototypical GABA(A) agonist muscimol produced a dose-dependent inhibition of TLESRs ranging from 19 to 56%. The two other GABA(A) agonists evaluated, isoguvacine and 4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol (THIP), as well as the GABA(A) positive allosteric modulator diazepam, had no major effects on TLESRs. Evaluation of higher doses was limited by emesis (THIP and isoguvacine) or restlessness/sedation (diazepam). Of the predominant GABA(A) receptor subunits (alpha, beta and gamma components), alpha and beta but not gamma were detected in the dog nodose ganglion by RT-PCR, while immunohistochemistry in addition demonstrated nerve fibres expressing the gamma subunit.

CONCLUSIONS AND IMPLICATIONS

The present observations demonstrate that GABA(A) receptors exert an inhibitory control of TLESRs. These results warrant further studies using GABA(A) isoform-selective agonists to define the identity of receptors involved.

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

Classical benzodiazepines (BZs) exert anxiolytic, sedative, hypnotic, muscle relaxant, anticonvulsive, and amnesic effects through potentiation of neurotransmission at GABA(A) receptors containing alpha(1), alpha(2), alpha(3) or alpha(5) subunits. Genetic studies suggest that modulation at the alpha(1) subunit contributes to much of the adverse effects of BZs, most notably sedation, ataxia, and amnesia. Hence, BZ site ligands functionally inactive at GABA(A) receptors containing the alpha(1) subunit are considered to be promising leads for novel, anxioselective anxiolytics devoid of sedative properties. In pursuing this approach, we used two-electrode voltage clamp experiments in Xenopus oocytes expressing recombinant GABA(A) receptor subtypes to investigate functional selectivity of three newly synthesized BZ site ligands and also compared their in vivo behavioral profiles. The compounds were functionally selective for alpha(2)-, alpha(3)-, and alpha(5)-containing subtypes of GABA(A) receptors (SH-053-S-CH3 and SH-053-S-CH3-2'F) or essentially selective for alpha(5) subtypes (SH-053-R-CH3). Possible influences on behavioral measures were tested in the elevated plus maze, spontaneous locomotor activity, and rotarod test, which are considered primarily predictive of the anxiolytic, sedative, and ataxic influence of BZs, respectively. The results confirmed the substantially diminished ataxic potential of BZ site agonists devoid of alpha(1) subunit-mediated effects, with preserved anti-anxiety effects at 30 mg/kg of SH-053-S-CH3 and SH-053-S-CH3-2'F. However, all three ligands, dosed at 30 mg/kg, decreased spontaneous locomotor activity, suggesting that sedation may be partly dependent on activity mediated by alpha(5)-containing GABA(A) receptors. Hence, it could be of importance to avoid substantial agonist activity at alpha(5) receptors by candidate anxioselective anxiolytics, if clinical sedation is to be avoided.

Are benzodiazepines really anxiolytic? Evidence from a 3D maze spatial navigation task

The effects of diazepam and chlordiazepoxide were assessed in a 3D maze which is a modification of an 8-arm radial maze. Each arm of the maze is attached to a bridge radiating from a central platform. Animals exposed for the first time to the maze do not venture beyond the line that separate a bridge from an arm. The prime criteria set for an anxiolytic effect is whether mice would increase the frequency of entries onto arms and increase arm/bridge entries ratio. C57 mice readily cross the line on first exposure and make more than 8 arm visits onto arms on second exposure, while other strains (CD-1 and Balb/c) hold back and rarely cross the line on first exposure and require more sessions to make more than 8 arm entries. An anxiolytic drug is expected to encourage intermediate (CD-1) and high (Balb/c) anxiety mice to adventure onto the arms of the maze and make more visits to the arms to comparable levels seen with low anxiety c57 mice. In the present report, administration of different doses of diazepam (0.625, 1.25, 2.5 and 5 mg kg(-1) i.p.) and chlordiazepoxide (5, 10 and 15 mg kg(-1) i.p.) did not reduce anxiety in animals, with the lowest dose of diazepam increasing motor activity in Balb/c and increasing anxiety in c57 mice while the highest doses of both diazepam (2.5 and 5 mg kg(-1) i.p.) and chlordiazepoxide (15 mg kg(-1) i.p.) induced mild sedation. Our results raise some concerns about the methodological foundations in the current assessment of anxiety and anxiolytic compounds both in animal and human studies.

Comparative effects of nonselective and subtype-selective gamma-aminobutyric acidA receptor positive modulators in the rat-conditioned emotional response test

Benzodiazepine receptor anxiolytics show no selectivity between gamma-aminobutyric acid-A receptors containing alpha1, alpha2, alpha3 or alpha5 subunits. Pharmacological studies and data emerging from transgenic mouse models, however, predict that compounds with selective affinity and/or efficacy for gamma-aminobutyric acid-A receptor subtypes would have novel pharmacological profiles. Thus, the gamma-aminobutyric acid-A-alpha1 'affinity selective' drug zolpidem has a sedative-hypnotic profile, whereas L838,417, which has 'selective efficacy' for gamma-aminobutyric acid-A-alpha2, alpha3 and alpha5 receptors, has an anxiolytic-like profile. Here, we compare the nonselective benzodiazepine-site-positive modulators diazepam, lorazepam, midazolam, alprazolam and zopiclone with (i) gamma-aminobutyric acid-AA-alpha1 affinity selective compounds zolpidem and CL218,872 and (ii) L838,417, in the rat-conditioned emotional response test after systemic administration. Given the role of the basolateral amygdala in anxiety and the expression of alpha1, alpha2 and alpha3 subunits in this region, we also assessed the effects of bilateral infusion of L838,417 and midazolam directly into basolateral amygdala in the conditioned emotional response test. Nonselective modulators at low-moderate doses produced anxiolytic effects and sedation at higher doses. Zolpidem was inactive as an anxiolytic and engendered severe sedation, whereas CL218,872 produced an anxiolytic-like profile with minimal sedation. L838,417 produced an anxiolytic-like profile with no sedation, albeit producing behavioural disturbance at high doses. Infusion of midazolam and L838,417 into basolateral amygdala engendered anxiolytic-like effects, although both compounds were more effective after systemic injections, implicating additional brain sites in their anxiolytic-like actions after systemic administration. In conclusion, the diversity of effects of the compounds studied implicates both intrinsic efficacy and/or subtype selectivity as important determinants of anxiolytic-like effects in the rat-conditioned emotional response test.

Pharmacodynamic and pharmacokinetic effects of TPA023, a GABA(A) alpha(2,3) subtype-selective agonist, compared to lorazepam and placebo in healthy volunteers

TPA023, a GABA(A) alpha2,3 alphasubtype-selective partial agonist, is expected to have comparable anxiolytic efficacy as benzodiazepines with reduced sedating effects. The compound lacks efficacy at the alpha1 subtype, which is believed to mediate these effects. This study investigated the effects of 0.5 and 1.5 mg TPA023 and compared them with placebo and lorazepam 2 mg (therapeutic anxiolytic dose). Twelve healthy male volunteers participated in this placebo-controlled, double-blind, double-dummy, four-way, cross-over study. Saccadic eye movements and visual analogue scales (VAS) were used to assess the sedative properties of TPA023. The effects on posturaL stability and cognition were assessed using body sway and a standardized battery of neurophysiological memory tests. Lorazepam caused a significant reduction in saccadic peak velocity, the VAS alertness score and impairment of memory and body sway. TPA023 had significant dose dependent effects on saccadic peak velocity (85 deg/sec maximum reduction at the higher dose) that approximated the effects of lorazepam. In contrast to lorazepam, TPA023 had no detectabLe effects on saccadic latency or inaccuracy. Also unlike lorazepam, TPA023 did not affect VAS alertness, memory or body sway. These results show that the effect profile of TPA023 differs markedly from that of lorazepam, at doses that were equipotent with regard to effects on saccadic peak veLocity. Contrary to lorazepam, TPA023 caused no detectable memory impairment or postural imbalance. These differences reflect the selectivity of TPA023 for different GABA(A) receptor subtypes.

The anxioselective agent 7-(2-chloropyridin-4-yl)pyrazolo-[1,5-a]-pyrimidin-3-yl](pyridin-2-yl)methanone (DOV 51892) is more efficacious than diazepam at enhancing GABA-gated currents at alpha1 subunit-containing GABAA receptors

Studies using mice with point mutations of GABA(A) receptor alpha subunits suggest that the sedative and anxiolytic properties of 1,4-benzodiazepines are mediated, respectively, by GABA(A) receptors bearing the alpha(1) and alpha(2) subunits. This hypothesis predicts that a compound with high efficacy at GABA(A) receptors containing the alpha(1) subunit would produce sedation, whereas an agonist acting at alpha(2) subunit-containing receptors (with low or null efficacy at alpha(1)-containing receptors) would be anxioselective. Electrophysiological studies using recombinant GABA(A) receptors expressed in Xenopus oocytes indicate that maximal potentiation of GABA-stimulated currents by the pyrazolo-[1,5-a]-pyrimidine, DOV 51892, at alpha(1)beta(2)gamma(2S) constructs of the GABA(A) receptor was significantly higher (148%) than diazepam. In contrast, DOV 51892 was considerably less efficacious and/or potent than diazepam in enhancing GABA-stimulated currents mediated by constructs containing alpha(2), alpha(3), or alpha(5) subunits. In vivo, DOV 51892 increased punished responding in the Vogel conflict test, an effect blocked by flumazenil, and increased the percentage of time spent in the open arms of the elevated plus-maze. However, DOV 51892 had no consistent effects on motor function or muscle relaxation at doses more than 1 order of magnitude greater than the minimal effective anxiolytic dose. Although the mutant mouse data predict that the high-efficacy potentiation of GABA(A1a) receptor-mediated currents by DOV 51892 would be sedating, behavioral studies demonstrate that DOV 51892 is anxioselective, indicating that GABA potentiation mediated by alpha(1) subunit-containing GABA(A) receptors may be neither the sole mechanism nor highly predictive of the sedative properties of benzodiazepine recognition site modulators.

Oxidative metabolism of limbic structures after acute administration of diazepam, alprazolam and zolpidem

The effects of acute administration of two benzodiazepines and a non-benzodiazepine hypnotic on behavior and brain metabolism were evaluated in rats. After testing the behavioral action of the benzodiazepines on the open field and the elevated plus-maze, the effects of the three drugs on neuronal metabolism of particular limbic regions were measured using cytochrome c oxidase (CO) histochemistry. Diazepam (5 mg/kg i.p.) and alprazolam (0.5 mg/kg i.p.) induced clear anxiolytic effects and a decrease in locomotion, whereas zolpidem (2 mg/kg i.p.) caused an intense hypnotic effect. The anxiolytic effects of alprazolam were distinguishable from diazepam due to the pharmacological and clinical profile of this triazolobenzodiazepine. CO activity decreased significantly in almost all the limbic regions evaluated after zolpidem administration. However, significant prominent decreases in CO activity were found after diazepam treatment in the medial mammillary nucleus, anteroventral thalamus, cingulate cortex, dentate gyrus and basolateral amygdala. Alprazolam caused similar decreases in CO activity, with the exception of the prelimbic and cingulate cortices, where significant increases were detected. In agreement with previous studies using other functional mapping techniques, our results indicate that particular benzodiazepines and non-benzodiazepine hypnotics induce selective changes in brain oxidative metabolism.

Use of a novel mouse genotype to model acute benzodiazepine withdrawal

Withdrawal from benzodiazepines in physically dependent rodents often requires that the drug be dislodged from its receptor with a competitive antagonist. Withdrawal Seizure-Prone (WSP) mice were selectively bred for their susceptibility to handling-induced withdrawal convulsions following chronic treatment with ethanol. Reflecting pleiotropic genetic influences, they also experience more severe withdrawal from other sedative-hypnotics including the benzodiazepine, diazepam. We used this susceptible genotype to test whether other benzodiazepine receptor (BZR) agonists also produce physical dependence following acute administration, comparing studies of spontaneous withdrawal with those where convulsions were precipitated by a BZR antagonist (flumazenil). Separate groups of mice were tested following a single injection of one of eight BZR agonists. Several doses of each drug were tested for spontaneous withdrawal, and a single dose of each drug was tested for precipitated withdrawal. Withdrawal convulsions were seen after all of the drugs by at least one method, suggesting that BZR agonists as a class elicit acute physical dependence in this susceptible genotype.

Synthesis, Pharmacology, and Structure−Activity Relationships of Novel Imidazolones and Pyrrolones as Modulators of GABAA Receptors

New series of imidazolones and pyrrolones were synthesized. The compounds were tested regarding their anxiolytic properties due to modulation of the GABAA receptor response. Several derivatives exhibit considerable pharmacological activity while lacking the typical side effects of benzodiazepine receptor agonists. 1-(4-chlorophenyl)-4-morpholin-1-yl-1,5-dihydro-imidazol-2-one (2) and 1-(4-chlorophenyl)-4-piperidin-1-yl-1,5-dihydro-imidazol-2-one (3) were protective in the pentylenetetrazole test in rats with oral ED50 of 27.4 and 12.8 mg/kg and TD50 (rotarod) of >500 and 265 mg/kg, respectively. The minimum effective dose in the Vogel conflict test was 3 mg/kg for both compounds. Common structure-activity relationship and comparative molecular field analysis models of the various series of derivatives could be established which are in accordance with a GABAA mediated pharmacological action. The findings fit well into an established pharmacophore model. This model is refined by an additional steric restriction feature.

Comparative cue generalization profiles of L-838, 417, SL651498, zolpidem, CL218,872, ocinaplon, bretazenil, zopiclone, and various benzodiazepines in chlordiazepoxide and zolpidem drug discrimination

The zolpidem discriminative cue is mediated by GABA(A)-alpha1 receptors, whereas the chlordiazepoxide cue may be mediated via non-alpha1 GABA(A) receptors because compounds with selective affinity for GABA(A)-alpha1 receptors fully generalize to the former cue. We predicted that L-838,417 [7-tert-butyl-3-(2,5-difluorophenyl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-1,2,4-triazolo[4,3-b]pyridazine], a partial agonist at non-alpha1 GABA(A) receptors and an antagonist at GABA(A)-alpha1 receptors, would generalize to the chlordiazepoxide but not the zolpidem-discriminative cue. SL651498 [6-fluoro-9-methyl-2-phenyl-4-(pyrrolidin-1-yl-carbonyl)-2,9-dihydro-1H-pyridol[3,4-b]indol-1-one] is a full agonist at GABA(A)-alpha2 receptors, with lower efficacy at GABA(A)-alpha3 receptors and least efficacy at GABA(A)-alpha1 and GABA(A)-alpha5 receptors. Because SL651498 has efficacy at GABA(A)-alpha1 receptors, we anticipated that it would generalize to both discriminative cues. Rats were trained to discriminate either zolpidem (3 mg/kg) or chlordiazepoxide (5 mg/kg) from vehicle using a two-lever operant procedure. The generalization profiles of L-838,417 and SL651498 were compared with nonselective full agonists, GABA(A)-alpha1-selective ligands zolpidem and CL218,872 [3-methyl-6-[3-(trifluoromethyl)phenyl]-1,2,4-triazolo[4,3-b]pyridazine], the nonselective partial agonist bretazenil, and the novel anxioselective drug ocinaplon. A nonselective partial agonist was included because L-838,417 and SL651498 are partial agonists at some GABA(A) receptors, and this property may influence their generalization profiles. All nonselective full agonists and ocinaplon fully generalized to both cues. CL218,872 and zolpidem generalized to zolpidem only, whereas L-838,417 fully generalized to chlordiazepoxide only. SL651498 fully generalized to chlordiazepoxide and occasioned significant zolpidem-appropriate responding. Bretazenil was similar to SL651498. In conclusion, at this training dose, the chlordiazepoxide-discriminative stimulus is mediated primarily via non-alpha1 GABA(A) receptors and the generalization profiles of the ligands tested seem to correspond with their in vitro profiles at GABA(A) receptor subtypes.

A Novel Selective GABAA α1 Receptor Agonist Displaying Sedative and Anxiolytic-like Properties in Rodents

In our pursuit to identify selective ligands for Bz/GABA(A) receptor subtypes, a novel pyrazolo[1,5-a]pyrimidine derivative (4), the azaisostere of zolpidem, was synthesized and evaluated in vitro on bovine brain homogenate and on recombinant benzodiazepine receptors (alphaxbeta2/3gamma2, x = 1-3, 5) expressed in HEK293 cells. Compound 4 displayed affinity only for alpha1beta2gamma2 subtype (K(i) = 31 nM), and in an in-depth, in vivo study it revealed sedative and anxiolytic-like properties without any amnesic and myorelaxant effects in rodents.

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.

The diagnosis and management of benzodiazepine dependence

PURPOSE OF REVIEW

Despite repeated recommendations to limit benzodiazepines to short-term use (2-4 weeks), doctors worldwide are still prescribing them for months or years. This over-prescribing has resulted in large populations of long-term users who have become dependent on benzodiazepines and has also led to leakage of benzodiazepines into the illicit drug market. This review outlines the risks of long-term benzodiazepine use, gives guidelines on the management of benzodiazepine withdrawal and suggests ways in which dependence can be prevented.

RECENT FINDINGS

Recent literature shows that benzodiazepines have all the characteristics of drugs of dependence and that they are inappropriately prescribed for many patients, including those with physical and psychiatric problems, elderly residents of care homes and those with comorbid alcohol and substance abuse. Many trials have investigated methods of benzodiazepine withdrawal, of which the keystones are gradual dosage tapering and psychological support when necessary. Several studies have shown that mental and physical health and cognitive performance improve after withdrawal, especially in elderly patients taking benzodiazepine hypnotics, who comprise a large proportion of the dependent population.

SUMMARY

Benzodiazepine dependence could be prevented by adherence to recommendations for short-term prescribing (2-4 weeks only when possible). Withdrawal of benzodiazepines from dependent patients is feasible and need not be traumatic if judiciously, and often individually, managed.