Application of the Co-Agonist Concerted Transition Model to Analysis of GABAA Receptor Properties

Regulating the activity of GABAergic neurons in the ventral pallidum alters the general anesthesia effect of propofol

The full mechanism of action of propofol, a commonly administered intravenous anesthetic drug in clinical practice, remains elusive. The focus of this study was the role of GABAergic neurons which are the main neuron group in the ventral pallidum (VP) closely associated with anesthetic effects in propofol anesthesia. The activity of VP GABAergic neurons following propofol anesthesia in Vgat-Cre mice was observed via detecting c-Fos immunoreactivity by immunofluorescence and western blotting. Subsequently, chemogenetic techniques were employed in Vgat-Cre mice to regulate the activity of VP GABAergic neurons. The role of VP GABAergic neurons in generating the effects of general anesthesia induced by intravenous propofol was further explored through behavioral tests of the righting reflex. The results revealed that c-Fos expression in VP GABAergic neurons in Vgat-Cre mice dramatically decreased after propofol injection. Further studies demonstrated that chemogenetic activation of VP GABAergic neurons during propofol anesthesia shortened the duration of anesthesia and promoted wakefulness. Conversely, the inhibition of VP GABAergic neurons extended the duration of anesthesia and facilitated the effects of anesthesia. The results obtained in this study suggested that regulating the activity of GABAergic neurons in the ventral pallidum altered the effect of propofol on general anesthesia.

Potentiation of the GABAAR reveals variable energetic contributions by etiocholanolone and propofol

The properties of a potentiator are typically evaluated by measuring its ability to enhance the magnitude of the control response. Analysis of the ability of drugs to potentiate responses from receptor channels takes place in the context of particular models to extract parameters for functional effects. In the often-used coagonist model, the agonist generating control activity and the potentiator enhancing the control activity make additive energetic contributions to stabilize the active state of the receptor. The energetic contributions are fixed and, once known, enable calculation of predicted receptor behavior at any concentration combination of agonist and potentiator. Here, we have examined the applicability of the coagonist model by measuring the relationship between the magnitude of receptor potentiation and the level of background activity. Ternary αβγ GABAA receptors were activated by GABA or the allosteric agonist propofol, or by a gain-of-function mutation, and etiocholanolone- or propofol-mediated potentiation of peak responses was measured. We show that the free energy change contributed by the modulators etiocholanolone or propofol is reduced at higher levels of control activity, thereby being in disagreement with basic principles of the coagonist model. Possible mechanisms underlying this discrepancy are discussed.

Direct measurements of neurosteroid binding to specific sites on GABAA receptors

BACKGROUND AND PURPOSE

Neurosteroids are allosteric modulators of GABAA currents, acting through several functional binding sites although their affinity and specificity for each site are unknown. The goal of this study was to measure steady-state binding affinities of various neurosteroids for specific sites on the GABAA receptor.

EXPERIMENTAL APPROACH

Two methods were developed to measure neurosteroid binding affinity: (1) quenching of specific tryptophan residues in neurosteroid binding sites by the neurosteroid 17-methylketone group, and (2) FRET between MQ290 (an intrinsically fluorescent neurosteroid) and tryptophan residues in the binding sites. The assays were developed using ELIC-α1GABAAR, a chimeric receptor containing transmembrane domains of the α1-GABAA receptor. Tryptophan mutagenesis was used to identify specific interactions.

KEY RESULTS

Allopregnanolone (3α-OH neurosteroid) was shown to bind at intersubunit and intrasubunit sites with equal affinity, whereas epi-allopregnanolone (3β-OH neurosteroid) binds at the intrasubunit site. MQ290 formed a strong FRET pair with W246, acting as a site-specific probe for the intersubunit site. The affinity and site-specificity of several neurosteroid agonists and inverse agonists was measured using the MQ290 binding assay. The FRET assay distinguishes between competitive and allosteric inhibition of MQ290 binding and demonstrated an allosteric interaction between the two neurosteroid binding sites.

CONCLUSIONS AND IMPLICATIONS

The affinity and specificity of neurosteroid binding to two sites in the ELIC-α1GABAAR were directly measured and an allosteric interaction between the sites was revealed. Adaptation of the MQ290 FRET assay to a plate-reader format will enable screening for high affinity agonists and antagonists for neurosteroid binding sites.

Mutational Analysis of Anesthetic Binding Sites and Their Effects on GABAA Receptor Activation and Modulation by Positive Allosteric Modulators of the α7 Nicotinic Receptor

The positive allosteric modulators (PAMs) of the α7 nicotinic receptor N-(5-Cl-2-hydroxyphenyl)-N'-[2-Cl-5-(trifluoromethyl)phenyl]-urea (NS-1738) and (E)-3-(furan-2-yl)-N-(p-tolyl)-acrylamide (PAM-2) potentiate the α1β2γ2L GABAA receptor through interactions with the classic anesthetic binding sites located at intersubunit interfaces in the transmembrane domain of the receptor. In the present study, we employed mutational analysis to investigate in detail the involvement and contributions made by the individual intersubunit interfaces to receptor modulation by NS-1738 and PAM-2. We show that mutations to each of the anesthetic-binding intersubunit interfaces (β+/α-, α+/β-, and γ+/β-), as well as the orphan α+/γ- interface, modify receptor potentiation by NS-1738 and PAM-2. Furthermore, mutations to any single interface can fully abolish potentiation by the α7-PAMs. The findings are discussed in the context of energetic additivity and interactions between the individual binding sites.

Competitive Interactions Between Propofol and Diazepam: Studies in GABAA Receptors and Zebrafish

Although propofol is among the most commonly administered general anesthetics, its mechanism of action is not fully understood. It has been hypothesized that propofol acts via a similar mechanism as (R)-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate (etomidate) by binding within the GABAA receptor transmembrane receptor domain at the two β +/α - subunit interfaces with resultant positive allosteric modulation. To test this hypothesis, we leveraged the ability of diazepam to bind to those sites and act as a competitive antagonist. We used oocyte-expressed α 1 β 3 γ 2L GABAA receptors to define the actions of diazepam (± flumazenil) on currents activated or potentiated by propofol and a zebrafish activity assay to define the impact of diazepam and flumazenil on propofol-induced anesthesia. We found that diazepam increased the amplitudes of GABAA receptor-mediated currents at nanomolar concentrations but reduced them at micromolar concentrations. The current amplitude changes produced by nanomolar diazepam concentrations were inhibited by flumazenil whereas those produced by micromolar diazepam concentrations were not. Studies of agonist potentiation showed that the micromolar inhibitory action of diazepam was surmountable by high concentrations of propofol and produced a rightward shift in the propofol concentration-response curve characterized by a Schild slope not statistically significantly different from 1, consistent with competition between diazepam and propofol. Although micromolar concentrations of diazepam (plus flumazenil) similarly reduced GABAA receptor currents modulated by propofol and etomidate, it only reduced the anesthetic actions of etomidate. We conclude that while both propofol and etomidate can modulate GABAA receptors by binding to the β +/α - subunit interfacial sites, propofol-induced anesthesia likely involves additional target sites. SIGNIFICANCE STATEMENT: Although the drug combination of diazepam and flumazenil reverses the GABAA receptor positive modulatory actions of both propofol and (R)-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate (etomidate), it only reverses the in vivo anesthetic actions of etomidate. These results strongly suggest that distinct mechanisms of action account for the anesthetic actions of these two commonly administered anesthetic agents.

Modulation of the mammalian GABAA receptor by type I and type II positive allosteric modulators of the α7 nicotinic acetylcholine receptor

BACKGROUND AND PURPOSE

Positive allosteric modulators of the α7 nicotinic acetylcholine (nACh) receptor (α7-PAMs) possess promnesic and procognitive properties and have potential in the treatment of cognitive and psychiatric disorders including Alzheimer's disease and schizophrenia. Behavioural studies in rodents have indicated that α7-PAMs can also produce antinociceptive and anxiolytic effects that may be associated with positive modulation of the GABAA receptor. The overall goal of this study was to investigate the modulatory actions of selected α7-PAMs on the GABAA receptor.

EXPERIMENTAL APPROACH

We employed a combination of cell fluorescence imaging, electrophysiology, functional competition and site-directed mutagenesis to investigate the functional and structural mechanisms of modulation of the GABAA receptor by three representative α7-PAMs.

KEY RESULTS

We show that the α7-PAMs at micromolar concentrations enhance the apparent affinity of the GABAA receptor for the transmitter and potentiate current responses from the receptor. The compounds were equi-effective at binary αβ and ternary αβγ GABAA receptors. Functional competition and site-directed mutagenesis indicate that the α7-PAMs bind to the classic anaesthetic binding sites in the transmembrane region in the intersubunit interfaces, which results in stabilization of the active state of the receptor.

CONCLUSION AND IMPLICATIONS

We conclude that the tested α7-PAMs are micromolar-affinity, intermediate- to low-efficacy allosteric potentiators of the mammalian αβγ GABAA receptor. Given the similarities in the in vitro sensitivities of the α7 nACh and α1β2γ2L GABAA receptors to α7-PAMs, we propose that doses used to produce nACh receptor-mediated behavioural effects in vivo are likely to modulate GABAA receptor function.

Activation of the α1β2γ2L GABAA Receptor by Physiological Agonists

The Cl⁻ permeable GABA_A receptor is a major contributor to cellular inhibition in the brain. The receptor is normally activated by synaptically-released or ambient GABA but is sensitive to a number of physiological compounds such as β-alanine, taurine, and neurosteroids that, to various degrees, activate the receptor and modulate responses either to the transmitter or to each other. Here, we describe α1β2γ2L GABA_A receptor activation and modulation by combinations of orthosteric and allosteric activators. The overall goal was to gain insight into how changes in the levels of endogenous agonists modulate receptor activity and influence cellular inhibition. Experimental observations and simulations are described in the framework of a cyclic concerted transition model. We also provide general analytical solutions for the analysis of electrophysiological data collected in the presence of combinations of active compounds.

A critical residue in the α1M2-M3 linker regulating mammalian GABAA receptor pore gating by diazepam

Benzodiazepines (BZDs) are a class of widely prescribed psychotropic drugs that modulate activity of GABA_A receptors (GABA_ARs), neurotransmitter-gated ion channels critical for synaptic transmission. However, the physical basis of this modulation is poorly understood. We explore the role of an important gating domain, the α₁M2–M3 linker, in linkage between the BZD site and pore gate. To probe energetics of this coupling without complication from bound agonist, we use a gain of function mutant (α₁L9'Tβ₂γ_2L) directly activated by BZDs. We identify a specific residue whose mutation (α₁V279A) more than doubles the energetic contribution of the BZD positive modulator diazepam (DZ) to pore opening and also enhances DZ potentiation of GABA-evoked currents in a wild-type background. In contrast, other linker mutations have little effect on DZ efficiency, but generally impair unliganded pore opening. Our observations reveal an important residue regulating BZD-pore linkage, thereby shedding new light on the molecular mechanism of these drugs.