Keith A. Wafford

Sedative but not anxiolytic properties of benzodiazepines are mediated by the GABA(A) receptor alpha1 subtype.

Inhibitory neurotransmission in the brain is largely mediated by GABAA receptors. Potentiation of GABA receptor activation through an allosteric benzodiazepine (BZ) site produces the sedative, anxiolytic, muscle relaxant, anticonvulsant and cognition-impairing effects of clinically used BZs such as diazepam. We created genetically modified mice (α1 H101R) with a diazepam-insensitive α1 subtype and a selective BZ site ligand, L-838,417, to explore GABAA receptor subtypes mediating specific physiological effects. These two complimentary approaches revealed that the α1 subtype mediated the sedative, but not the anxiolytic effects of benzodiazepines. This finding suggests ways to improve anxiolytics and to develop drugs for other neurological disorders based on their specificity for GABAA receptor subtypes in distinct neuronal circuits.

Pharmacological characterization of a novel cell line expressing human α4β3δ GABAA receptors

The pharmacology of the stable cell line expressing human α4β3δ GABAA receptor was investigated using whole‐cell patch‐clamp techniques. α4β3δ receptors exhibited increased sensitivity to GABA when compared to α4β3γ2 receptors, with EC50s of 0.50 (0.46, 0.53) μM and 2.6 (2.5, 2.6) μM respectively. Additionally, the GABA partial agonists piperidine‐4‐sulphonate (P4S) and 4,5,6,7‐tetrahydroisothiazolo‐[5,4‐c]pyridin‐3‐ol (THIP) displayed markedly higher efficacy at α4β3δ receptors, indeed THIP demonstrated greater efficacy than GABA at these receptors. The δ subunit conferred slow desensitization to GABA, with rate constants of 4.8±0.5 s for α4β3δ and 2.5±0.2 s for α4β3γ2. However, both P4S and THIP demonstrated similar levels of desensitization on both receptor subtypes suggesting this effect is agonist specific. α4β3δ and α4β3γ2 demonstrated equal sensitivity to inhibition by the cation zinc (2–3 μM IC50). However, α4β3δ receptors demonstrated greater sensitivity to inhibition by lanthanum. The IC50 for GABA antagonists SR‐95531 and picrotoxin, was similar for α4β3δ and α4β3γ2. Likewise, inhibition was observed on both subtypes at high and low pH. α4β3δ receptors were insensitive to modulation by benzodiazepine ligands. In contrast Ro15‐4513 and bretazenil potentiated GABA responses on α4β3γ2 cells, and the inverse agonist DMCM showed allosteric inhibition of α4β3γ2 receptors. The efficacy of neurosteroids at α4β3δ receptors was greatly enhanced over that observed at α4β3γ2 receptors. The greatest effect was observed using THDOC with 524±71.6% potentiation at α4β3δ and 297.9±49.7% at α4β3γ2 receptors. Inhibition by the steroid pregnenolone sulphate however, showed no subtype selectivity. The efficacy of both pentobarbitone and propofol was slightly augmented and etomidate greatly enhanced at α4β3δ receptors versus α4β3γ2 receptors. We show that the α4β3δ receptor has a distinct pharmacology and kinetic profile. With its restricted distribution within the brain and unique pharmacology this receptor may play an important role in the action of neurosteroids and anaesthetics.

Growth factors regulate the survival and fate of cells derived from human neurospheres

Cells isolated from the embryonic, neonatal, and adult rodent central nervous system divide in response to epidermal growth factor (EGF) and fibroblast growth factor 2 (FGF-2), while retaining the ability to differentiate into neurons and glia. These cultures can be grown in aggregates termed neurospheres, which contain a heterogeneous mix of both multipotent stem cells and more restricted progenitor populations. Neurospheres can also be generated from the embryonic human brain and in some cases have been expanded for extended periods of time in culture. However, the mechanisms controlling the number of neurons generated from human neurospheres are poorly understood. Here we show that maintaining cell–cell contact during the differentiation stage, in combination with growth factor administration, can increase the number of neurons generated under serum-free conditions from 8% to >60%. Neurotrophic factors 3 and 4 (NT3, NT4) and platelet-derived growth factor (PDGF) were the most potent, and acted by increasing neuronal survival rather than inducing neuronal phenotype. Following differentiation, the neurons could survive dissociation and either replating or transplantation into the adult rat brain. This experimental system provides a practically limitless supply of enriched, non-genetically transformed neurons. These should be useful for both neuroactive drug screening in vitro and possibly cell therapy for neurodegenerative diseases.

Molecular and Functional Diversity of the Expanding GABA-A Receptor Gene Family

ABSTRACT: Fast inhibitory neurotransmission in the mammalian CNS is mediated primarily by the neurotransmitter γ‐aminobutyric acid (GABA), which, upon binding to its receptor, leads to opening of the intrinsic ion channel, allowing chloride to enter the cell. Over the past 10 years it has become clear that a family of GABA‐A receptor subtypes exists, generated through the coassembly of polypeptides selected from α1‐α6, β1‐β3, γ1‐γ3, δ, ɛ, and π to form what is most likely a pentomeric macromolecule. The gene transcripts, and indeed the polypeptides, show distinct patterns of temporal and spatial expression, such that the GABA‐A receptor subtypes have a defined localization that presumably reflects their physiological role. A picture is beginning to emerge of the properties conferred to receptor subtypes by the different subunits; these include different functional properties, differential modulation by protein kinases, and the targeting to different membrane compartments. These properties presumably underlie the different physiological roles of the various receptor subtypes. Recently we have identified a further member of the GABA‐A receptor gene family, which we have termed θ, which appears to be most closely related to the β subunits. The structure, function, and distribution of θ‐containing receptors, and receptors containing the recently reported ɛ subunit, are described.

Thymol, a constituent of thyme essential oil, is a positive allosteric modulator of human GABAA receptors and a homo‐oligomeric GABA receptor from Drosophila melanogaster

The GABA‐modulating and GABA‐mimetic activities of the monoterpenoid thymol were explored on human GABAA and Drosophila melanogaster homomeric RDLac GABA receptors expressed in Xenopus laevis oocytes, voltage‐clamped at −60 mV. The site of action of thymol was also investigated. Thymol, 1–100 μM, resulted in a dose‐dependent potentiation of the EC20 GABA response in oocytes injected with either α1β3γ2s GABAA subunit cDNAs or the RDLac subunit RNA. At 100 μM thymol, current amplitudes in response to GABA were 416±72 and 715±85% of controls, respectively. On both receptors, thymol, 100 μM, elicited small currents in the absence of GABA. The EC50 for GABA at α1β3γ2s GABAA receptors was reduced by 50 μM thymol from 15±3 to 4±1 μM, and the Hill slope changed from 1.35±0.14 to 1.04±0.16; there was little effect on the maximum GABA response. Thymol (1–100 μM) potentiation of responses to EC20 GABA for α1β1γ2s, α6β3γ2s and α1β3γ2s human GABAA receptors was almost identical, arguing against actions at benzodiazepine or loreclezole sites. Neither flumazenil, 3‐hydroxymethyl‐β‐carboline (3‐HMC), nor 5α‐pregnane‐3α, 20α‐diol (5α‐pregnanediol) affected thymol potentiation of the GABA response at α1β3γ2s receptors, providing evidence against actions at the benzodiazepine/β‐carboline or steroid sites. Thymol stimulated the agonist actions of pentobarbital and propofol on α1β3γ2s receptors, consistent with a mode of action distinct from that of either compound. These data suggest that thymol potentiates GABAA receptors through a previously unidentified binding site.

Extrasynaptic GABAA Receptors of Thalamocortical Neurons: A Molecular Target for Hypnotics

Among hypnotic agents that enhance GABAA receptor function, etomidate is unusual because it is selective for β2/β3 compared with β1 subunit-containing GABAA receptors. Mice incorporating an etomidate-insensitive β2 subunit (β2N265S) revealed that β2 subunit-containing receptors mediate the enhancement of slow-wave activity (SWA) by etomidate, are required for the sedative, and contribute to the hypnotic actions of this anesthetic. Although the anatomical location of the β2-containing receptors that mediate these actions is unknown, the thalamus is implicated. We have characterized GABAA receptor-mediated neurotransmission in thalamic nucleus reticularis (nRT) and ventrobasalis complex (VB) neurons of wild-type, β–/–2, and β2N265S mice. VB but not nRT neurons exhibit a large GABA-mediated tonic conductance that contributes ∼80% of the total GABAA receptor-mediated transmission. Consequently, although etomidate enhances inhibition in both neuronal types, the effect of this anesthetic on the tonic conductance of VB neurons is dominant. The GABA-enhancing actions of etomidate in VB but not nRT neurons are greatly suppressed by the β2N265S mutation. The hypnotic THIP (Gaboxadol) induces SWA and at low, clinically relevant concentrations (30 nm to 3 μm) increases the tonic conductance of VB neurons, with no effect on VB or nRT miniature IPSCs (mIPSCs) or on the holding current of nRT neurons. Zolpidem, which has no effect on SWA, prolongs VB mIPSCs but is ineffective on the phasic and tonic conductance of nRT and VB neurons, respectively. Collectively, these findings suggest that enhancement of extrasynaptic inhibition in the thalamus may contribute to the distinct sleep EEG profiles of etomidate and THIP compared with zolpidem.

Structure and Pharmacology of Vertebrate GABAA Receptor Subtypes

Publisher Summary This chapter reviews GABAA receptors and describes how the techniques of molecular neurobiology enabled a revolution in the understanding of the GABAA receptor. In common with the other members of the ligand-gated ion channel family, the function of the GABAA receptor is modulated by phosphorylation. Indeed, purified native GBAA receptor protein can be phosphorylated in vitro by cyclic AMP-dependent protein kinase A (PKA) and calcium/phospholipiddependent protein kinase C (PKC). Analysis of the deduced amino acid sequences of GABAA receptor subunits indicates that the large cytoplasmic loop domains contain consensus sites for phosphorylation by PKA (human α3,α 4, α6, β1, β2, β3,γl , γ2, and γ3), PKC (all human subunits), and tyrosine kinase (γl, γ2, γ3) . It is this domain of the nAChR that undergoes phosphorylation. The pentameric structure of the GABAA receptor is based on analogy with the nicotinic receptor, the physicochemical properties of the solubilized receptor, and electron microscopic studies of purified receptor preparations. A diverse range of both naturally occurring and synthetic compounds can allosterically regulate GABAA receptors. By using recombinant receptors, it is possible to study the roles played by individual subunits in the actions of many of these compounds and indeed, in some cases, individual amino acids located at the binding sites have been identified.

Evidence for a Significant Role of α3-Containing GABAA Receptors in Mediating the Anxiolytic Effects of Benzodiazepines

The GABAA receptor subtypes responsible for the anxiolytic effects of nonselective benzodiazepines (BZs) such as chlordiazepoxide (CDP) and diazepam remain controversial. Hence, molecular genetic data suggest that α2-rather than α3-containing GABAA receptors are responsible for the anxiolytic effects of diazepam, whereas the anxiogenic effects of an α3-selective inverse agonist suggest that an agonist selective for this subtype should be anxiolytic. We have extended this latter pharmacological approach to identify a compound, 4,2′-difluoro-5′-[8-fluoro-7-(1-hydroxy-1-methylethyl)imidazo[1,2-á]pyridin-3-yl]biphenyl-2-carbonitrile (TP003), that is an α3 subtype selective agonist that produced a robust anxiolytic-like effect in both rodent and non-human primate behavioral models of anxiety. Moreover, in mice containing a point mutation that renders α2-containing receptors BZ insensitive (α2H101R mice), TP003 as well as the nonselective agonist CDP retained efficacy in a stress-induced hyperthermia model. Together, these data show that potentiation of α3-containing GABAA receptors is sufficient to produce the anxiolytic effects of BZs and that α2 potentiation may not be necessary.

An inverse agonist selective for alpha5 subunit-containing GABAA receptors enhances cognition.

α5IA is a compound that binds with equivalent subnanomolar affinity to the benzodiazepine (BZ) site of GABAA receptors containing an α1, α2, α3, or α5 subunit but has inverse agonist efficacy selective for the α5 subtype. As a consequence, the in vitro and in vivo effects of this compound are mediated primarily via GABAA receptors containing an α5 subunit. In a mouse hippocampal slice model, α5IA significantly enhanced the θ burst-induced long-term potentiation of the excitatory postsynaptic potential in the CA1 region but did not cause an increase in the paroxysmal burst discharges that are characteristic of convulsant and proconvulsant drugs. These in vitro data suggesting that α5IA may enhance cognition without being proconvulsant were confirmed in in vivo rodent models. Hence, α5IA significantly enhanced performance in a rat hippocampal-dependent test of learning and memory, the delayed-matching-to-position version of the Morris water maze, with a minimum effective oral dose of 0.3 mg/kg, which corresponded to a BZ site occupancy of 25%. However, in mice α5IA was not convulsant in its own right nor did it potentiate the effects of pentylenetetrazole acutely or produce kindling upon chronic dosing even at doses producing greater than 90% occupancy. Finally, α5IA was not anxiogenic-like in the rat elevated plus maze nor did it impair performance in the mouse rotarod assay. Together, these data suggest that the GABAA α5-subtype provides a novel target for the development of selective inverse agonists with utility in the treatment of disorders associated with a cognitive deficit.

α5GABAA Receptors Mediate the Amnestic But Not Sedative-Hypnotic Effects of the General Anesthetic Etomidate

A fundamental objective of anesthesia research is to identify the receptors and brain regions that mediate the various behavioral components of the anesthetic state, including amnesia, immobility, and unconsciousness. Using complementary in vivo and in vitro approaches, we found that GABAA receptors that contain the α5 subunit (α5GABAARs) play a critical role in amnesia caused by the prototypic intravenous anesthetic etomidate. Whole-cell recordings from hippocampal pyramidal neurons showed that etomidate markedly increased a tonic inhibitory conductance generated by α5GABAARs, whereas synaptic transmission was only slightly enhanced. Long-term potentiation (LTP) of field EPSPs recorded in CA1 stratum radiatum was reduced by etomidate in wild-type (WT) but not α5 null mutant (α5−/−) mice. In addition, etomidate impaired memory performance of WT but not α5−/− mice for spatial and nonspatial hippocampal-dependent learning tasks. The brain concentration of etomidate associated with memory impairment in vivo was comparable with that which increased the tonic inhibitory conductance and blocked LTP in vitro. The α5−/− mice did not exhibit a generalized resistance to etomidate, in that the sedative-hypnotic effects measured with the rotarod, loss of righting reflex, and spontaneous motor activity were similar in WT and α5−/− mice. Deletion of the α5 subunit of the GABAARs reduced the amnestic but not the sedative-hypnotic properties of etomidate. Thus, the amnestic and sedative-hypnotic properties of etomidate can be dissociated on the basis of GABAAR subtype pharmacology.