Distinct structural changes in the GABAA receptor elicited by pentobarbital and GABA

An Update on Stiripentol Mechanisms of Action: A Narrative Review

Stiripentol (Diacomit®) (STP) is an orally active antiseizure medication (ASM) indicated as adjunctive therapy, for the treatment of seizures associated with Dravet syndrome (DS), a severe form of childhood epilepsy, in conjunction with clobazam and, in some regions valproic acid. Since the discovery of STP, several mechanisms of action (MoA) have been described that may explain its specific effect on seizures associated with DS. STP is mainly considered as a potentiator of gamma-aminobutyric acid (GABA) neurotransmission: (i) via uptake blockade, (ii) inhibition of degradation, but also (iii) as a positive allosteric modulator of GABAA receptors, especially those containing α3 and δ subunits. Blockade of voltage-gated sodium and T-type calcium channels, which is classically associated with anticonvulsant and neuroprotective properties, has also been demonstrated for STP. Finally, several studies indicate that STP could regulate glucose energy metabolism and inhibit lactate dehydrogenase. STP is also an inhibitor of several cytochrome P450 enzymes involved in the metabolism of other ASMs, contributing to boost their anticonvulsant efficacy as add-on therapy. These different MoAs involved in treatment of DS and recent data suggest a potential for STP to treat other neurological or non-neurological diseases.

Fundamental Neurochemistry Review: GABAA receptor neurotransmission and epilepsy: Principles, disease mechanisms and pharmacotherapy

Epilepsy is a common neurological disorder associated with alterations of excitation-inhibition balance within brain neuronal networks. GABA_A receptor neurotransmission is the most prevalent form of inhibitory neurotransmission and is strongly implicated in both the pathophysiology and treatment of epilepsy, serving as a primary target for antiseizure medications for over a century. It is now established that GABA exerts a multifaceted influence through an array of GABA_A receptor subtypes that extends far beyond simply negating excitatory activity. As the role of GABA_A neurotransmission within inhibitory circuits is elaborated, this will enable the development of precision therapies that correct the network dysfunction underlying epileptic pathology.

Aromas Influencing the GABAergic System

Aromas have a powerful influence in our everyday life and are known to exhibit an array of pharmacological properties, including anxiolytic, anti-stress, relaxing, and sedative effects. Numerous animal and human studies support the use of aromas and their constituents to reduce anxiety-related symptoms and/or behaviours. Although the exact mechanism of how these aromas exert their anxiolytic effects is not fully understood, the GABAergic system is thought to be primarily involved. The fragrance emitted from a number of plant essential oils has shown promise in recent studies in modulating GABAergic neurotransmission, with GABA_A receptors being the primary therapeutic target. This review will explore the anxiolytic and sedative properties of aromas found in common beverages, such as coffee, tea, and whisky as well aromas found in food, spices, volatile organic compounds, and popular botanicals and their constituents. In doing so, this review will focus on these aromas and their influence on the GABAergic system and provide greater insight into viable anxiety treatment options.

Effect of Hibiscus syriacus Linnaeus extract and its active constituent, saponarin, in animal models of stress-induced sleep disturbances and pentobarbital-induced sleep

Treatment of sleep disorders promotes the long-term use of commercially available sleep inducers that have several adverse effects, including addiction, systemic fatigue, weakness, loss of concentration, headache, and digestive problems. Therefore, we aimed to limit these adverse effects by investigating a natural product, the extract of the Hibiscus syriacus Linnaeus flower (HSF), as an alternative treatment. In the electric footshock model, we measured anxiety and assessed the degree of sleep improvement after administering HSF extract. In the restraint model, we studied the sleep rate using PiezoSleep, a noninvasive assessment system. In the pentobarbital model, we measured sleep improvement and changes in sleep-related factors. Our first model confirmed the desirable effects of HSF extract and its active constituent, saponarin, on anxiolysis and Wake times. HSF extract also increased REM sleep time. Furthermore, HSF extract and saponarin increased the expression of cortical GABAA receptor α1 (GABAAR α1) and c-Fos in the ventrolateral preoptic nucleus (VLPO). In the second model, HSF extract and saponarin restored the sleep rate and the sleep bout duration. In the third model, HSF extract and saponarin increased sleep maintenance time. Moreover, HSF extract and saponarin increased cortical cholecystokinin (CCK) mRNA levels and the expression of VLPO c-Fos. HSF extract also increased GABAAR α1 mRNA level. Our results suggest that HSF extract and saponarin are effective in maintaining sleep and may be used as a novel treatment for sleep disorder. Eventually, we hope to introduce HSF and saponarin as a clinical treatment for sleep disorders in humans.

Orthosteric and Allosteric Activation of Human 5-HT3A Receptors

The serotonin type 3 receptor (5-HT₃) is a ligand-gated ion channel that converts the binding of the neurotransmitter serotonin (5-HT) into a transient cation current that mediates fast excitatory responses in peripheral and central nervous systems. Information regarding the activation and modulation of the human 5-HT₃ type A receptor has been based only on macroscopic current measurements because of its low ion conductance. By constructing a high-conductance human 5-HT₃A receptor, we here revealed mechanistic information regarding the orthosteric activation by 5-HT and by the partial agonist tryptamine, and the allosteric activation by the terpenoids, carvacrol, and thymol. Terpenoids potentiated macroscopic currents elicited by the orthosteric agonist and directly elicited currents with slow-rising phases and submaximal amplitudes. At the single-channel level, activation by orthosteric and allosteric agonists appeared as openings in quick succession (bursts) that showed no ligand concentration dependence. Bursts were grouped into long-duration clusters in the presence of 5-HT and even longer in the presence of terpenoids, whereas they remained isolated in the presence of tryptamine. Kinetic analysis revealed that allosteric and orthosteric activation mechanisms can be described by the same scheme that includes transitions of the agonist-bound receptor to closed intermediate states before opening (priming). Reduced priming explained the partial agonism of tryptamine; however, equilibrium constants for gating and priming were similar for 5-HT and terpenoid activation. Thus, our kinetic analysis revealed that terpenoids are efficacious agonists for 5-HT₃A receptors. These findings not only extend our knowledge about the human 5-HT₃A molecular function but also provide novel insights into the mechanisms of action of allosteric ligands, which are of increasing interest as therapeutic drugs in all the superfamily.

Conformational Changes in the 5-HT3A Receptor Extracellular Domain Measured by Voltage-Clamp Fluorometry

The 5-hydroxytryptamine (5-HT) type 3 receptor is a member of the cysteine (Cys)-loop receptor super family of ligand-gated ion channels in the nervous system and is a clinical target in a range of diseases. The 5-HT₃ receptor mediates fast serotonergic neurotransmission by undergoing a series of conformational changes initiated by ligand binding that lead to the rapid opening of an intrinsic cation-selective channel. However, despite the availability of high-resolution structures of a mouse 5-HT₃ receptor, many important aspects of the mechanistic basis of 5-HT₃ receptor function and modulation by drugs remain poorly understood. In particular, there is little direct evidence for the specific conformational changes predicted to occur during ligand-gated channel activation and desensitization. In the present study, we used voltage-clamp fluorometry (VCF) to measure conformational changes in regions surrounding the orthosteric binding site of the human 5-HT_3A (h5-HT_3A) receptor during binding of 5-HT and different classes of 5-HT₃ receptor ligands. VCF utilizes parallel measurements of receptor currents with photon emission from fluorescent reporter groups covalently attached to specific positions in the receptor structure. Reporter groups that are highly sensitive to the local molecular environment can, in real time, report conformational changes as changes in fluorescence that can be correlated with changes in receptor currents reporting the functional states of the channel. Within the loop C, D, and E regions that surround the orthosteric binding site in the h5-HT_3A receptor, we identify positions that are amenable to tagging with an environmentally sensitive reporter group that reports robust fluorescence changes upon 5-HT binding and receptor activation. We use these reporter positions to characterize the effect of ligand binding on the local structure of the orthosteric binding site by agonists, competitive antagonists, and allosterically acting channel activators. We observed that loop C appears to show distinct fluorescence changes for ligands of the same class, while loop D reports similar fluorescence changes for all ligands binding at the orthosteric site. In contrast, the loop E reporter position shows distinct changes for agonists, antagonists, and allosteric compounds, suggesting the conformational changes in this region are specific to ligand function. Interpretation of these results within the framework of current models of 5-HT₃ and Cys-loop mechanisms are used to expand the understanding of how ligand binding in Cys-loop receptors relates to channel gating. SIGNIFICANCE STATEMENT: The 5-HT₃ receptor is an important ligand-gated ion channel and drug target in the central and peripheral nervous system. Determining how ligand binding induced conformational changes in the receptor is central for understanding the structural mechanisms underlying 5-HT₃ receptor function. Here, we employ voltage-gated fluorometry to characterize conformational changes in the extracellular domain of the human 5-HT₃ receptor to identify intrareceptor motions during binding of a range of 5-HT₃ receptor agonists and antagonists.

Cortical neurons and networks are dormant but fully responsive during isoelectric brain state

A continuous isoelectric electroencephalogram reflects an interruption of endogenously-generated activity in cortical networks and systematically results in a complete dissolution of conscious processes. This electro-cerebral inactivity occurs during various brain disorders, including hypothermia, drug intoxication, long-lasting anoxia and brain trauma. It can also be induced in a therapeutic context, following the administration of high doses of barbiturate-derived compounds, to interrupt a hyper-refractory status epilepticus. Although altered sensory responses can be occasionally observed on an isoelectric electroencephalogram, the electrical membrane properties and synaptic responses of individual neurons during this cerebral state remain largely unknown. The aim of the present study was to characterize the intracellular correlates of a barbiturate-induced isoelectric electroencephalogram and to analyse the sensory-evoked synaptic responses that can emerge from a brain deprived of spontaneous electrical activity. We first examined the sensory responsiveness from patients suffering from intractable status epilepticus and treated by administration of thiopental. Multimodal sensory responses could be evoked on the flat electroencephalogram, including visually-evoked potentials that were significantly amplified and delayed, with a high trial-to-trial reproducibility compared to awake healthy subjects. Using an analogous pharmacological procedure to induce prolonged electro-cerebral inactivity in the rat, we could describe its cortical and subcortical intracellular counterparts. Neocortical, hippocampal and thalamo-cortical neurons were all silent during the isoelectric state and displayed a flat membrane potential significantly hyperpolarized compared with spontaneously active control states. Nonetheless, all recorded neurons could fire action potentials in response to intracellularly injected depolarizing current pulses and their specific intrinsic electrophysiological features were preserved. Manipulations of the membrane potential and intracellular injection of chloride in neocortical neurons failed to reveal an augmented synaptic inhibition during the isoelectric condition. Consistent with the sensory responses recorded from comatose patients, large and highly reproducible somatosensory-evoked potentials could be generated on the inactive electrocorticogram in rats. Intracellular recordings revealed that the underlying neocortical pyramidal cells responded to sensory stimuli by complex synaptic potentials able to trigger action potentials. As in patients, sensory responses in the isoelectric state were delayed compared to control responses and exhibited an elevated reliability during repeated stimuli. Our findings demonstrate that during prolonged isoelectric brain state neurons and synaptic networks are dormant rather than excessively inhibited, conserving their intrinsic properties and their ability to integrate and propagate environmental stimuli.

Comparison of kinetic and pharmacological profiles of recombinant α1γ2L and α1β2γ2L GABAA receptors - A clue to the role of intersubunit interactions

The fastest inhibitory mechanism in the CNS is mediated by ionotropic GABAA receptors and it is known that subunit composition critically determines their properties. While a typical GABAA receptor consists of two α, two β and one γ/δ subunit, there are some exceptions, e.g. αβ receptors. Functional α1γ2 GABAA receptors can be expressed in recombinant model (Verdoorn et al., 1990) and although their role remains unknown, it seems appealing to extend their characterization to further explore the structure-function relationship of GABAA receptors. Intriguingly, this receptor is lacking canonical GABA binding sites but it can be activated by GABA and dose-response relationships for α1β2γ2L and α1γ2L receptors overlap. Deactivation kinetics was similar for both receptors but the percentage of the fast component was smaller in the case of α1γ2L receptors and, consequently, the mean deactivation time constant was slower. The rate and extent of macroscopic desensitization were smaller in the case of α1γ2L receptors but they showed slower recovery. Both receptor types had a similar proton sensitivity showing only subtle but significant differences in pH effects on deactivation. Flurazepam exerted a similar effect on both receptors but the rapid deactivation components were differently affected and an opposite effect was observed on desensitization extent. Rebound currents evoked by pentobarbital were undistinguishable for both receptor types. Taking altogether, although some significant differences were found, α1β2γ2L and α1γ2L receptors showed unforeseen similarity. We propose that functioning of GABAA receptors might rely on subunit-subunit cooperative interactions to a larger extent than believed so far.

Functional Site-Directed Fluorometry

Initially developed in the mid-1990s to examine the conformational changes of the canonical Shaker voltage-gated potassium channel, functional site-directed fluorometry has since been expanded to numerous other voltage-gated and ligand-gated ion channels as well as transporters, pumps, and other integral membrane proteins. The power of functional site-directed fluorometry, also known as voltage-clamp fluorometry, lies in its ability to provide information on the conformational changes in a protein in response to changes in its environment with high temporal resolution while simultaneously monitoring the function of that protein. Over time, applications of site-directed fluorometry have expanded to examine the interactions of ion channels with modulators ranging from membrane potential to ligands to accessory protein subunits to lipids. In the future, the range of questions answerable by functional site-directed fluorometry and its interpretive power should continue to improve, making it an even more powerful technique for dissecting the conformational dynamics of ion channels and other membrane proteins.

Rescue of Methyl-CpG Binding Protein 2 Dysfunction-induced Defects in Newborn Neurons by Pentobarbital

Rett syndrome is a neurodevelopmental disorder that usually arises from mutations or deletions in methyl-CpG binding protein 2 (MeCP2), a transcriptional regulator that affects neuronal development and maturation without causing cell loss. Here, we show that silencing of MeCP2 decreased neurite arborization and synaptogenesis in cultured hippocampal neurons from rat fetal brains. These structural defects were associated with alterations in synaptic transmission and neural network activity. Similar retardation of dendritic growth was also observed in MeCP2-deficient newborn granule cells in the dentate gyrus of adult mouse brains in vivo, demonstrating direct and cell-autonomous effects on individual neurons. These defects, caused by MeCP2 deficiency, were reversed by treatment with the US Food and Drug Administration-approved drug, pentobarbital, in vitro and in vivo, possibly caused by modulation of γ-aminobutyric acid signaling. The results indicate that drugs modulating γ-aminobutyric acid signaling are potential therapeutics for Rett syndrome.

Investigating ion channel conformational changes using voltage clamp fluorometry

Ion channels are membrane proteins whose functions are governed by conformational changes. The widespread distribution of ion channels, coupled with their involvement in most physiological and pathological processes and their importance as therapeutic targets, renders the elucidation of these conformational mechanisms highly compelling from a drug discovery perspective. Thanks to recent advances in structural biology techniques, we now have high-resolution static molecular structures for members of the major ion channel families. However, major questions remain to be resolved about the conformational states that ion channels adopt during activation, drug modulation and desensitization. Patch-clamp electrophysiology has long been used to define ion channel conformational states based on functional criteria. It achieves this by monitoring conformational changes at the channel gate and cannot detect conformational changes occurring in regions distant from the gate. Voltage clamp fluorometry involves labelling cysteines introduced into domains of interest with environmentally sensitive fluorophores and inferring structural rearrangements from voltage or ligand-induced fluorescence changes. Ion channel currents are monitored simultaneously to verify the conformational status. By defining real time conformational changes in domains distant from the gate, this technique provides unexpected new insights into ion channel structure and function. This review aims to summarise the methodology and highlight recent innovative applications of this powerful technique. This article is part of the Special Issue entitled 'Fluorescent Tools in Neuropharmacology'.

Excitability and responsiveness of rat barrel cortex neurons in the presence and absence of spontaneous synaptic activity in vivo

The amplitude and temporal dynamics of spontaneous synaptic activity in the cerebral cortex vary as a function of brain states. To directly assess the impact of different ongoing synaptic activities on neocortical function, we performed in vivo intracellular recordings from barrel cortex neurons in rats under two pharmacological conditions generating either oscillatory or tonic synaptic drive. Cortical neurons membrane excitability and firing responses were compared, in the same neurons, before and after complete suppression of background synaptic drive following systemic injection of a high dose of anaesthetic. Compared to the oscillatory state, the tonic pattern resulted in a more depolarized and less fluctuating membrane potential (Vm), a lower input resistance (Rm) and steeper relations of firing frequency versus injected current (F-I). Whatever their temporal dynamics, suppression of background synaptic activities increased mean Vm, without affecting Rm, and induced a rightward shift of F-I curves. Both types of synaptic drive generated a high variability in current-induced firing rate and patterns in cortical neurons, which was much reduced after removal of spontaneous activity. These findings suggest that oscillatory and tonic synaptic patterns differentially facilitate the input-output function of cortical neurons but result in a similar moment-to-moment variability in spike responses to incoming depolarizing inputs.

A study of subunit selectivity, mechanism and site of action of the delta selective compound 2 (DS2) at human recombinant and rodent native GABA(A) receptors

BACKGROUND AND PURPOSE

Most GABA(A) receptor subtypes comprise 2α, 2β and 1γ subunit, although for some isoforms, a δ replaces a γ-subunit. Extrasynaptic δ-GABA(A) receptors are important therapeutic targets, but there are few suitable pharmacological tools. We profiled DS2, the purported positive allosteric modulator (PAM) of δ-GABA(A) receptors to better understand subtype selectivity, mechanism/site of action and activity at native δ-GABA(A) receptors.

EXPERIMENTAL APPROACH

Subunit specificity of DS2 was determined using electrophysiological recordings of Xenopus laevis oocytes expressing human recombinant GABA(A) receptor isoforms. Effects of DS2 on GABA concentration-response curves were assessed to define mechanisms of action. Radioligand binding and electrophysiology utilising mutant receptors and pharmacology were used to define site of action. Using brain-slice electrophysiology, we assessed the influence of DS2 on thalamic inhibition in wild-type and δ(0/0) mice.

KEY RESULTS

Actions of DS2 were primarily determined by the δ-subunit but were additionally influenced by the α, but not the β, subunit (α4/6βxδ > α1βxδ >> γ2-GABA(A) receptors > α4β3). For δ-GABA(A) receptors, DS2 enhanced maximum responses to GABA, with minimal influence on GABA potency. (iii) DS2 did not act via the orthosteric, or known modulatory sites on GABA(A) receptors. (iv) DS2 enhanced tonic currents of thalamocortical neurones from wild-type but not δ(0/0) mice.

CONCLUSIONS AND IMPLICATIONS

DS2 is the first PAM selective for α4/6βxδ receptors, providing a novel tool to investigate extrasynaptic δ-GABA(A) receptors. The effects of DS2 are mediated by an unknown site leading to GABA(A) receptor isoform selectivity.

The benzodiazepine diazepam potentiates responses of α1β2γ2L γ-aminobutyric acid type A receptors activated by either γ-aminobutyric acid or allosteric agonists

BACKGROUND

The γ-aminobutyric acid (GABA) type A receptor is a target for several anesthetics, anticonvulsants, anxiolytics, and sedatives. Neurosteroids, barbiturates, and etomidate both potentiate responses to GABA and allosterically activate the receptor. We examined the ability of a benzodiazepine, diazepam, to potentiate responses to allosteric agonists.

METHODS

The GABA type A receptors were expressed in human embryonic kidney 293 cells and studied using whole-cell and single-channel patch clamp. The receptors were activated by the orthosteric agonist GABA and allosteric agonists pentobarbital, etomidate, and alfaxalone.

RESULTS

Diazepam is equally potent at enhancing responses to orthosteric and allosteric agonists. Diazepam EC50s were 25 ± 4, 26 ± 6, 33 ± 6, and 26 ± 3 nm for receptors activated by GABA, pentobarbital, etomidate, and alfaxalone, respectively (mean ± SD, 5-6 cells at each condition). Mutations to the benzodiazepine-binding site (α1(H101C), γ2(R144C), γ2(R197C)) reduced or removed potentiation for all agonists, and an inverse agonist at the benzodiazepine site reduced responses to all agonists. Single-channel data elicited by GABA demonstrate that in the presence of 1 μm diazepam the prevalence of the longest open-time component is increased from 13 ± 7 (mean ± SD, n = 5 patches) to 27 ± 8% (n = 3 patches) and the rate of channel closing is decreased from 129 ± 28 s(-1) to 47 ± 6 s(-1) (mean ± SD) CONCLUSIONS: We conclude that benzodiazepines do not act by enhancing affinity of the orthosteric site for GABA but rather by increasing channel gating efficacy. The results also demonstrate the presence of interactions between allosteric activators and potentiators, raising a possibility of effects on dosage requirements or changes in side effects.

How theories evolved concerning the mechanism of action of barbiturates

The barbiturate phenobarbital has been in use in the treatment of epilepsy for 100 years. It has long been recognized that barbiturates act by prolonging and potentiating the action of γ-aminobutyric acid (GABA) on GABA(A) receptors and at higher concentrations directly activating the receptors. A large body of data supports the concept that GABA(A) receptors are the primary central nervous system target for barbiturates, including the finding that transgenic mice with a point mutation in the β3 GABA(A) -receptor subunit exhibit diminished sensitivity to the sedative and immobilizing actions of the anesthetic barbiturate pentobarbital. Although phenobarbital is only modestly less potent as a GABA(A) -receptor modulator than pentobarbital, phenobarbital is minimally sedating at effective anticonvulsant doses. Possible explanations for the reduced sedative effect of phenobarbital include more regionally restricted action; partial agonist activity; reduced propensity to directly activate GABA(A) receptors (possibly including extrasynaptic receptors containing δ subunits); and reduced activity at other ion channel targets, including voltage-gated calcium channels. In recent years, substantial progress has been made in defining the structural features of GABA(A) receptors responsible for gating and allosteric modulation by drugs. Although the precise sites of action of barbiturates have not yet been defined, the second and third transmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunit methionine 286 as well as α-subunit methionine 236. In addition to effects on GABA(A) receptors, barbiturates block AMPA/kainate receptors, and they inhibit glutamate release through an effect on P/Q-type high-voltage activated calcium channels. The combination of these various actions likely accounts for their diverse clinical activities. Despite the remarkable progress of the last century, there is still much to learn about the actions of barbiturates that can be applied to the discovery of new, more therapeutically useful agents.

Robust photoregulation of GABA(A) receptors by allosteric modulation with a propofol analogue

Photochemical switches represent a powerful method for improving pharmacological therapies and controlling cellular physiology. Here we report the photo-regulation of GABA_A receptors (GABA_ARs) by a derivative of propofol (2,6-diisopropylphenol), a GABA_AR allosteric modulator, that we have modified to contain photo-isomerizable azobenzene. Using α₁β₂γ₂ GABA_ARs expressed in Xenopus laevis oocytes and native GABA_ARs of isolated retinal ganglion cells, we show that the trans-azobenzene isomer of the new compound (trans-MPC088), generated by visible light (wavelengths ~440 nm), potentiates the GABA-elicited response and at higher concentrations directly activates the receptors. cis-MPC088, generated from trans-MPC088 by UV light (~365 nm), produces little if any receptor potentiation/activation. In cerebellar slices, MPC088 co-applied with GABA affords bidirectional photo-modulation of Purkinje cell membrane current and spike-firing rate. The findings demonstrate photo-control of GABA_ARs by an allosteric ligand and open new avenues for fundamental and clinically oriented research on GABA_ARs, a major class of neurotransmitter receptors in the central nervous system.

Agonist-specific conformational changes in the α1-γ2 subunit interface of the GABA A receptor

The GABA(A) receptor undergoes conformational changes upon the binding of agonist that lead to the opening of the channel gate and a flow of small anions across the cell membrane. Besides the transmitter GABA, allosteric ligands such as the general anesthetics pentobarbital and etomidate can activate the receptor. Here, we have investigated the agonist specificity of structural changes in the extracellular domain of the receptor. We used the substituted cysteine accessibility method and focused on the γ2(S195C) site (loop F). We show that modification of the site with (2-sulfonatoethyl)methanethiosulfonate (MTSES) results in an enhanced response to GABA, indicating accessibility of the resting receptor to the modifying agent. Coapplication of GABA or muscimol, but not of gabazine, with MTSES prevented the effect, suggesting that GABA and muscimol elicit a conformational change that reduces access to the γ2(S195C) site. Exposure of the receptors to MTSES in the presence of the allosteric activators pentobarbital and etomidate resulted in an enhanced current response indicating accessibility and labeling of the γ2(S195C) site. However, comparison of the rates of modification indicated that labeling in the presence of etomidate was significantly faster than that in the presence of pentobarbital or gabazine or in resting receptors. We infer from the data that the structure of the α1-γ2 subunit interface undergoes agonist-specific conformational changes.

Structural studies of the actions of anesthetic drugs on the γ-aminobutyric acid type A receptor

The γ-aminobutyric acid type A receptor is the major transmitter-gated inhibitory channel in the central nervous system. The receptor is a target for anesthetics, anticonvulsants, anxiolytics, and sedatives whose actions facilitate the flow of chloride ions through the channel and enhance the inhibitory tone in the brain. Both the kinetic and structural aspects of the actions of modulators of the γ-aminobutyric acid type A receptor are of great importance to understanding the molecular mechanisms of general anesthesia. In this review, the structural rearrangements that take place in the γ-aminobutyric acid type A receptor during channel activation and modulation are described, focusing on data obtained using voltage-clamp fluorometry. Voltage-clamp fluorometry entails the binding of an environmentally sensitive fluorophore molecule to a site of interest in the receptor, and measurement of changes in the fluorescence signal resulting from activation- or modulation-elicited structural changes. Detailed investigations can provide a map of structural changes that underlie or accompany the functional effects of modulators.

A comparison of glycine- and ivermectin-mediated conformational changes in the glycine receptor ligand-binding domain

Glycine receptor chloride channels are Cys-loop receptor proteins that isomerize between a low affinity closed state and a high affinity ion-conducting state. There is currently much interest in understanding the mechanisms that link affinity changes with conductance changes. This essentially involves an agonist binding in the glycine receptor ligand-binding site initiating local conformational changes that propagate in a wave towards the channel gate. However, it has proved difficult to convincingly distinguish those agonist-induced domain movements that are critical for triggering activation from those that are simply local deformations to accommodate ligands in the site. We employed voltage-clamp fluorometry to compare conformational changes in the ligand-binding site in response to activation by glycine, which binds locally, and ivermectin, which binds in the transmembrane domain. We reasoned that ivermectin-mediated activation should initiate a conformational wave that propagates from the pore-lining domain towards the ligand-binding domain, eliciting conformational changes in those extracellular domains that are allosterically linked to the gate. We found that ivermectin indeed elicited conformational changes in ligand-binding domain loops C, D and F. This implies that conformational changes in these domains are important for activation. This result also provides a mechanism to explain how ivermectin potentiates glycine-induced channel activation.

Insights into the structure and pharmacology of GABA(A) receptors

GABA is the major inhibitory neurotransmitter in the adult mammalian CNS. The ionotropic GABA type A receptors (GABA(A)Rs) belong to the Cys-loop family of receptors. Each member of the family is a large pentameric protein in which each subunit traverses the cell membrane four times. Within this family, the GABA type A receptors are particularly important for their rich pharmacology as they are targets for a range of therapeutically important drugs, including the benzodiazepines, barbiturates, neuroactive steroids and anesthetics. This review discusses new insights into receptor properties that allow us to begin to relate the structure of an individual receptor to its functional and pharmacological properties.

Pharmacology of structural changes at the GABA(A) receptor transmitter binding site

BACKGROUND AND PURPOSE

The binding of transmitter to specialized binding pockets leads to rearrangements in the structure of the receptor eventually resulting in channel opening. We used voltage-clamp fluorometry to investigate the pharmacological basis and biophysical processes that underlie structural changes at the transmitter binding site of the rat α1β2γ2L GABA(A) receptor.

EXPERIMENTAL APPROACH

Simultaneous electrophysiological and site-specific fluorescence measurements were conducted on receptors expressed in Xenopus oocytes and labelled with an environmentally-sensitive fluorophore, Alexa 546 maleimide, at the α1L127C site.

KEY RESULTS

Receptors activated by GABA demonstrate a concentration-dependent increase in fluorescence intensity, indicating that the environment surrounding the fluorophore becomes less polar upon activation. Qualitatively similar responses were observed with other GABA site ligands such as piperidine-4-sulphonic acid, muscimol, β-alanine and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. Fluorescence changes were not affected by the direction of current flow. During long applications of GABA significant desensitization developed, which was not accompanied by additional changes in fluorescence. Pentobarbital was an efficacious agonist of the labelled mutant receptor but did not cause changes in fluorescence. Direct activation by etomidate or the steroid allopregnanolone also did not result in fluorescence changes. Functional potentiation of GABA-activated receptors by allopregnanolone or etomidate enhanced both the GABA-elicited functional response and the fluorescence change. In contrast, potentiation by pentobarbital was not accompanied by an enhanced fluorescence response.

CONCLUSIONS AND IMPLICATIONS

The data indicate that there is no direct correlation between current flow or position of the activation gate and the structural changes as detected by Alexa 546-labelled α1L127Cβ2γ2L GABA(A) receptors. Channel potentiation by pentobarbital qualitatively differs from potentiation by etomidate or allopregnanolone.

Occupation of either site for the neurosteroid allopregnanolone potentiates the opening of the GABAA receptor induced from either transmitter binding site

Potentiating neuroactive steroids are potent and efficacious modulators of the GABA(A) receptor that act by allosterically enhancing channel activation elicited by GABA. Steroids interact with the membrane-spanning domains of the α subunits of the receptor, whereas GABA binds to pockets in the interfaces between β and α subunits. Steroid interaction with a single site is known to be sufficient to produce potentiation, but it is not clear whether effects within the same β-α pair mediate potentiation. Here, we have investigated whether the sites for GABA and steroids are functionally linked (i.e., whether the occupancy of a steroid site selectively affects activation elicited by GABA binding to the transmitter binding site within the same β-α pair). For that, we used receptors formed of mutated concatenated subunits to selectively eliminate one of the two GABA sites and one of the two steroid sites. The data demonstrate that receptors containing a single functional GABA site are potentiated by the neurosteroid allopregnanolone regardless of whether the steroid interacts with the α subunit from the same or the other β-α pair. We conclude that steroids potentiate the opening of the GABA(A) receptor induced by either agonist binding site.

Ligand- and subunit-specific conformational changes in the ligand-binding domain and the TM2-TM3 linker of {alpha}1 {beta}2 {gamma}2 GABAA receptors

Cys-loop receptor ligand binding sites are located at subunit interfaces where they are lined by loops A-C from one subunit and loops D-F from the adjacent subunit. Agonist binding induces large conformational changes in loops C and F. However, it is controversial as to whether these conformational changes are essential for gating. Here we used voltage clamp fluorometry to investigate the roles of loops C and F in gating the α1 β2 γ2 GABA(A) receptor. Voltage clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. Previous attempts to define the roles of loops C and F using this technique have focused on homomeric Cys-loop receptors. However, the problem with studying homomeric receptors is that it is difficult to eliminate the possibility of bound ligands interacting directly with attached fluorophores at the same site. Here we show that ligands binding to the β2-α1 interface GABA binding site produce conformational changes at the adjacent subunit interface. This is most likely due to agonist-induced loop C closure directly altering loop F conformation at the adjacent α1-β2 subunit interface. However, as antagonists and agonists produce identical α1 subunit loop F conformational changes, these conformational changes appear unimportant for gating. Finally, we demonstrate that TM2-TM3 loops from adjacent β2 subunits in α1 β2 receptors can dimerize via K24'C disulfides in the closed state. This result implies unexpected conformational mobility in this crucial part of the gating machinery. Together, this information provides new insights into the activation mechanisms of Cys-loop receptors.

New insights into the molecular mechanisms of general anaesthetics

This paper provides new insights of how general anaesthetic research should be carried out in the future by an analysis of what we know, what we do not know and what we would like to know. I describe previous hypotheses on the mechanism of action of general anaesthetics (GAs) involving membranes and protein receptors. I provide the reasons why the GABA type A receptor, the NMDA receptor and the glycine receptor are strong candidates for the sites of action of GAs. I follow with a review on attempts to provide a mechanism of action, and how future research should be conducted with the help of physical and chemical methods.

Photochemical proteolysis of an unstructured linker of the GABAAR extracellular domain prevents GABA but not pentobarbital activation

The GABA type A receptor (GABA(A)R) is the major inhibitory receptor in the mammalian central nervous system and the target of numerous pharmaceuticals. The alpha-subunit of these pentameric Cys-loop neurotransmitter-gated ion channels contributes to the binding of both GABA and allosteric modulators such as the benzodiazepines, suggesting a role for this subunit in the conformational changes associated with activation of the receptor. Herein we use the nonsense suppression methodology to incorporate a photoactivatable unnatural amino acid and photochemically cleave the backbone of the alpha subunit of the alpha(1)beta(2) GABA(A)R in a linker region that is believed to span the subunit. Proteolytic cleavage impairs GABA but not pentobarbital activation, strongly suggesting that conformational changes involving this linker region are critical to the GABA activation pathway.

Site-specific fluorescence reveals distinct structural changes induced in the human rho 1 GABA receptor by inhibitory neurosteroids

The rho 1 GABA receptor is inhibited by a number of neuroactive steroids. A previous study (J Pharmacol Exp Ther 323:236-247, 2007) focusing on the electrophysiological effects of inhibitory steroids on the rho 1 receptor found that steroid inhibitors could be divided into three major groups based on how mutations to residues in the M2 transmembrane domain modified inhibition. It was proposed that the steroids act through distinct mechanisms. We selected representatives of the three groups (pregnanolone, tetrahydrodeoxycorticosterone, pregnanolone sulfate, allopregnanolone sulfate, and beta-estradiol) and probed how these steroids, as well as the nonsteroidal inhibitor picrotoxinin, modify GABA-elicited fluorescence changes from the Alexa 546 C5 maleimide fluorophore attached to residues in the extracellular region of the receptor. The fluorophore responds with changes in quantum yield to changes in the environment, allowing it to probe for structural changes taking place during channel activation or modulation. The results indicate that the modulators have specific effects on fluorescence changes suggesting that distinct conformational changes accompany inhibition. The findings are consistent with the steroids acting as allosteric inhibitors of the rho 1 GABA receptor and support the hypothesis that divergent mechanisms underlie the action of inhibitory steroids on the rho 1 GABA receptor.

Activation of alpha6-containing GABAA receptors by pentobarbital occurs through a different mechanism than activation by GABA

The GABA(A) receptors are ligand-gated chloride channels which are the targets for many clinically used sedatives, including the barbiturates. The barbiturate pentobarbital acts through multiple sites on the GABA(A) receptor. At low concentrations (muM), it acts as a positive allosteric modulator while at higher concentrations it can directly activate the receptor. This agonist action is influenced by the subunit composition of the receptor, and pentobarbital is a more effective agonist than GABA only at receptors containing an alpha6 subunit. The conformational change that translates GABA binding into channel opening is known to involve a lysine residue located in an extracellular domain between the 2nd and 3rd transmembrane domains. Mutations of this residue disrupt activation of the channel by GABA and have been linked to inherited epilepsy. Pentobarbital binds to the receptor at a different agonist site than GABA, but could use a common signal transduction mechanism to gate the channel. To address this question, we compared the effect of a mutating the homologous lysine residue in the alpha1 or alpha6 subunits (K278 or K277, respectively) to methionine on direct activation of recombinant GABA(A) receptors by GABA or pentobarbital. We found that this mutation reduced GABA sensitivity for both alpha1 and alpha6 subunits, but affected pentobarbital sensitivity only for the alpha1 subunit. This suggests that pentobarbital acts through a distinct signal transduction pathway at the alpha6 subunit, which may account for its greater efficacy compared to GABA at receptors containing this subunit.

A residue in loop 9 of the beta2-subunit stabilizes the closed state of the GABAA receptor

In gamma-aminobutyric acid type A (GABA(A)) receptors, the structural elements that couple ligand binding to channel opening remain poorly defined. Here, site-directed mutagenesis was used to determine if Loop 9 on the non-GABA binding site interface of the beta2-subunit may be involved in GABA(A) receptor activation. Specifically, residues Gly(170)-Gln(185) of the beta2-subunit were mutated to alanine, co-expressed with wild-type alpha1- and gamma2S-subunits in human embryonic kidney (HEK) 293 cells and assayed for their activation by GABA, the intravenous anesthetic propofol and the endogenous neurosteroid pregnanolone using whole cell macroscopic recordings. Three mutants, G170A, V175A, and G177A, produced 2.5-, 6.7-, and 5.6-fold increases in GABA EC(50) whereas one mutant, Q185A, produced a 5.2-fold decrease in GABA EC(50). None of the mutations affected the ability of propofol or pregnanolone to potentiate a submaximal GABA response, but the Q185A mutant exhibited 8.3- and 3.5-fold increases in the percent direct activation by propofol and pregnanolone, respectively. Mutant Q185A receptors also had an increased leak current that was sensitive to picrotoxin, indicating an increased gating efficiency. Further Q185E, Q185L, and Q185W substitutions revealed a strong correlation between the hydropathy of the amino acid at this position and the GABA EC(50). Taken together, these results indicate that beta2 Loop 9 is involved in receptor activation by GABA, propofol, and pregnanolone and that beta2(Q185) participates in hydrophilic interactions that are important for stabilizing the closed state of the GABA(A) receptor.

Magnitude of a conformational change in the glycine receptor beta1-beta2 loop is correlated with agonist efficacy

The efficacy of agonists at Cys-loop ion channel receptors is determined by the rate they isomerize receptors to a pre-open flip state. Once the flip state is reached, the shut-open reaction is similar for low and high efficacy agonists. The present study sought to identify a conformational change associated with the closed-flip transition in the alpha1-glycine receptor. We employed voltage-clamp fluorometry to compare ligand-binding domain conformational changes induced by the following agonists, listed from highest to lowest affinity and efficacy: glycine > beta-alanine > taurine. Voltage-clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. Agonist affinity and efficacy correlated inversely with maximum fluorescence magnitudes at labeled residues in ligand-binding domain loops D and E, suggesting that large conformational changes in this region preclude efficacious gating. However, agonist affinity and efficacy correlated directly with maximum fluorescence magnitudes from a label attached to A52C in loop 2, near the transmembrane domain interface. Because glycine experiences the largest affinity increase between closed and flip states, we propose that the magnitude of this fluorescence signal is directly proportional to the agonist affinity increase. In contrast, labeled residues in loops C, F, and the pre-M1 domain yielded agonist-independent fluorescence responses. Our results support the conclusion that a closed-flip conformation change, with a magnitude proportional to the agonist affinity increase from closed to flip states, occurs in the microenvironment of Ala-52.

Ligand-specific conformational changes in the alpha1 glycine receptor ligand-binding domain

Understanding the activation mechanism of Cys loop ion channel receptors is key to understanding their physiological and pharmacological properties under normal and pathological conditions. The ligand-binding domains of these receptors comprise inner and outer beta-sheets and structural studies indicate that channel opening is accompanied by conformational rearrangements in both beta-sheets. In an attempt to resolve ligand-dependent movements in the ligand-binding domain, we employed voltage-clamp fluorometry on alpha1 glycine receptors to compare changes mediated by the agonist, glycine, and by the antagonist, strychnine. Voltage-clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. In the inner beta-sheet, we labeled residues in loop 2 and in binding domain loops D and E. At each position, strychnine and glycine induced distinct maximal fluorescence responses. The pre-M1 domain responded similarly; at each of four labeled positions glycine produced a strong fluorescence signal, whereas strychnine did not. This suggests that glycine induces conformational changes in the inner beta-sheet and pre-M1 domain that may be important for activation, desensitization, or both. In contrast, most labeled residues in loops C and F yielded fluorescence changes identical in magnitude for glycine and strychnine. A notable exception was H201C in loop C. This labeled residue responded differently to glycine and strychnine, thus underlining the importance of loop C in ligand discrimination. These results provide an important step toward mapping the domains crucial for ligand discrimination in the ligand-binding domain of glycine receptors and possibly other Cys loop receptors.