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Nature Jan 2019Type-A γ-aminobutyric (GABA) receptors are ligand-gated chloride channels with a very rich pharmacology. Some of their modulators, including benzodiazepines and general...
Type-A γ-aminobutyric (GABA) receptors are ligand-gated chloride channels with a very rich pharmacology. Some of their modulators, including benzodiazepines and general anaesthetics, are among the most successful drugs in clinical use and are common substances of abuse. Without reliable structural data, the mechanistic basis for the pharmacological modulation of GABA receptors remains largely unknown. Here we report several high-resolution cryo-electron microscopy structures in which the full-length human α1β3γ2L GABA receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam. We describe the binding modes and mechanistic effects of these ligands, the closed and desensitized states of the GABA receptor gating cycle, and the basis for allosteric coupling between the extracellular, agonist-binding region and the transmembrane, pore-forming region. This work provides a structural framework in which to integrate previous physiology and pharmacology research and a rational basis for the development of GABA receptor modulators.
Topics: Allosteric Regulation; Alprazolam; Benzodiazepines; Bicuculline; Binding, Competitive; Cryoelectron Microscopy; Diazepam; GABA Modulators; Humans; Ligands; Models, Molecular; Nanostructures; Picrotoxin; Receptors, GABA-A; Signal Transduction
PubMed: 30602790
DOI: 10.1038/s41586-018-0832-5 -
Nature Sep 2020Most general anaesthetics and classical benzodiazepine drugs act through positive modulation of γ-aminobutyric acid type A (GABA) receptors to dampen neuronal activity...
Most general anaesthetics and classical benzodiazepine drugs act through positive modulation of γ-aminobutyric acid type A (GABA) receptors to dampen neuronal activity in the brain. However, direct structural information on the mechanisms of general anaesthetics at their physiological receptor sites is lacking. Here we present cryo-electron microscopy structures of GABA receptors bound to intravenous anaesthetics, benzodiazepines and inhibitory modulators. These structures were solved in a lipidic environment and are complemented by electrophysiology and molecular dynamics simulations. Structures of GABA receptors in complex with the anaesthetics phenobarbital, etomidate and propofol reveal both distinct and common transmembrane binding sites, which are shared in part by the benzodiazepine drug diazepam. Structures in which GABA receptors are bound by benzodiazepine-site ligands identify an additional membrane binding site for diazepam and suggest an allosteric mechanism for anaesthetic reversal by flumazenil. This study provides a foundation for understanding how pharmacologically diverse and clinically essential drugs act through overlapping and distinct mechanisms to potentiate inhibitory signalling in the brain.
Topics: Allosteric Regulation; Anesthetics, General; Barbiturates; Benzodiazepines; Bicuculline; Binding Sites; Binding, Competitive; Cryoelectron Microscopy; Diazepam; Electrophysiology; Etomidate; Flumazenil; GABA-A Receptor Antagonists; Humans; Ligands; Models, Molecular; Molecular Conformation; Molecular Dynamics Simulation; Phenobarbital; Picrotoxin; Propofol; Receptors, GABA-A; gamma-Aminobutyric Acid
PubMed: 32879488
DOI: 10.1038/s41586-020-2654-5 -
Neuron Nov 2023The neurotransmitter γ-aminobutyric acid (GABA) drives critical inhibitory processes in and beyond the nervous system, partly via ionotropic type-A receptors (GABARs)....
The neurotransmitter γ-aminobutyric acid (GABA) drives critical inhibitory processes in and beyond the nervous system, partly via ionotropic type-A receptors (GABARs). Pharmacological properties of ρ-type GABARs are particularly distinctive, yet the structural basis for their specialization remains unclear. Here, we present cryo-EM structures of a lipid-embedded human ρ1 GABAR, including a partial intracellular domain, under apo, inhibited, and desensitized conditions. An apparent resting state, determined first in the absence of modulators, was recapitulated with the specific inhibitor (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid and blocker picrotoxin and provided a rationale for bicuculline insensitivity. Comparative structures, mutant recordings, and molecular simulations with and without GABA further explained the sensitized but slower activation of ρ1 relative to canonical subtypes. Combining GABA with picrotoxin also captured an apparent uncoupled intermediate state. This work reveals structural mechanisms of gating and modulation with applications to ρ-specific pharmaceutical design and to our biophysical understanding of ligand-gated ion channels.
Topics: Humans; Receptors, GABA-A; Picrotoxin; Ligands; gamma-Aminobutyric Acid; Bicuculline; Binding Sites
PubMed: 37659407
DOI: 10.1016/j.neuron.2023.08.006 -
The phosphorylation status of eukaryotic elongation factor-2 indicates neural activity in the brain.Molecular Brain Sep 2021Assessment of neural activity in the specific brain area is critical for understanding the circuit mechanisms underlying altered brain function and behaviors. A number...
Assessment of neural activity in the specific brain area is critical for understanding the circuit mechanisms underlying altered brain function and behaviors. A number of immediate early genes (IEGs) that are rapidly transcribed in neuronal cells in response to synaptic activity have been used as markers for neuronal activity. However, protein detection of IEGs requires translation, and the amount of newly synthesized gene product is usually insufficient to detect using western blotting, limiting their utility in western blot analysis of brain tissues for comparison of basal activity between control and genetically modified animals. Here, we show that the phosphorylation status of eukaryotic elongation factor-2 (eEF2) rapidly changes in response to synaptic and neural activities. Intraperitoneal injections of the GABA A receptor (GABAR) antagonist picrotoxin and the glycine receptor antagonist brucine rapidly dephosphorylated eEF2. Conversely, potentiation of GABARs or inhibition of AMPA receptors (AMPARs) induced rapid phosphorylation of eEF2 in both the hippocampus and forebrain of mice. Chemogenetic suppression of hippocampal principal neuron activity promoted eEF2 phosphorylation. Novel context exploration and acute restraint stress rapidly modified the phosphorylation status of hippocampal eEF2. Furthermore, the hippocampal eEF2 phosphorylation levels under basal conditions were reduced in mice exhibiting epilepsy and abnormally enhanced excitability in CA3 pyramidal neurons. Collectively, the results indicated that eEF2 phosphorylation status is sensitive to neural activity and the ratio of phosphorylated eEF2 to total eEF2 could be a molecular signature for estimating neural activity in a specific brain area.
Topics: Animals; Brain; CA3 Region, Hippocampal; Eukaryotic Initiation Factor-2; Genes, Reporter; Mice; Muscimol; Nerve Tissue Proteins; Phosphorylation; Picrotoxin; Prosencephalon; Protein Processing, Post-Translational; Pyramidal Cells; Quinoxalines; Restraint, Physical; Stress, Physiological; Strychnine
PubMed: 34526091
DOI: 10.1186/s13041-021-00852-0 -
Cells Mar 2023The functional deficiency of the inhibitory system typically appears during development and can progress to psychiatric disorders or epilepsy, depending on its severity,...
The functional deficiency of the inhibitory system typically appears during development and can progress to psychiatric disorders or epilepsy, depending on its severity, in later years. It is known that interneurons, the major source of GABAergic inhibition in the cerebral cortex, can make direct connections with arterioles and participate in the regulation of vasomotion. The goal of this study was to mimic the functional deficiency of interneurons through the use of localized microinjections of the GABA antagonist, picrotoxin, in such a concentration that it did not elicit epileptiform neuronal activity. First, we recorded the dynamics of resting-state neuronal activity in response to picrotoxin injections in the somatosensory cortex of an awake rabbit; second, we assessed the altered neuronal and hemodynamic responses to whisker stimulation using BOLD fMRI and electrophysiology recordings; third, we evaluated brain tissue oxygen levels before and after picrotoxin injection. Our results showed that neuronal activity typically increased after picrotoxin administration, the BOLD responses to stimulation became negative, and the oxygen response was nearly abolished. Vasoconstriction during the resting baseline was not observed. These results indicate that picrotoxin provoked imbalanced hemodynamics either due to increased neuronal activity, decreased vascular response, or a combination of both.
Topics: Animals; Rabbits; Picrotoxin; Magnetic Resonance Imaging; Interneurons; Neurons; Oxygen
PubMed: 36899947
DOI: 10.3390/cells12050811 -
Nature Nov 2021Glycine receptors (GlyRs) are pentameric, 'Cys-loop' receptors that form chloride-permeable channels and mediate fast inhibitory signalling throughout the central...
Glycine receptors (GlyRs) are pentameric, 'Cys-loop' receptors that form chloride-permeable channels and mediate fast inhibitory signalling throughout the central nervous system. In the spinal cord and brainstem, GlyRs regulate locomotion and cause movement disorders when mutated. However, the stoichiometry of native GlyRs and the mechanism by which they are assembled remain unclear, despite extensive investigation. Here we report cryo-electron microscopy structures of native GlyRs from pig spinal cord and brainstem, revealing structural insights into heteromeric receptors and their predominant subunit stoichiometry of 4α:1β. Within the heteromeric pentamer, the β(+)-α(-) interface adopts a structure that is distinct from the α(+)-α(-) and α(+)-β(-) interfaces. Furthermore, the β-subunit contains a unique phenylalanine residue that resides within the pore and disrupts the canonical picrotoxin site. These results explain why inclusion of the β-subunit breaks receptor symmetry and alters ion channel pharmacology. We also find incomplete receptor complexes and, by elucidating their structures, reveal the architectures of partially assembled α-trimers and α-tetramers.
Topics: Animals; Brain Stem; Cryoelectron Microscopy; Models, Molecular; Phenylalanine; Picrotoxin; Protein Subunits; Receptors, Glycine; Spinal Cord; Swine
PubMed: 34555840
DOI: 10.1038/s41586-021-04022-z -
Neuropharmacology Jul 2018γ-aminobutyric acid type A receptors (GABARs) are important components of the central nervous system and they are functionally tasked with controlling neuronal...
γ-aminobutyric acid type A receptors (GABARs) are important components of the central nervous system and they are functionally tasked with controlling neuronal excitability. These receptors are subject to post-translational modification and also to modulation by endogenous regulators, such as the neurosteroids. These modulators can either potentiate or inhibit GABAR function. Whilst the former class of neurosteroids are considered to bind to and act from the transmembrane domain of the receptor, the domains that are important for the inhibitory neurosteroids remain less clear. In this study, we systematically compare a panel of recombinant synaptic-type and extrasynaptic-type GABARs expressed in heterologous cell systems for their sensitivity to inhibition by the classic inhibitory neurosteroid, pregnenolone sulphate. Generally, peak GABA current responses were inhibited less compared to steady-state currents, implicating the desensitised state in inhibition. Moreover, pregnenolone sulphate inhibition increased with GABA concentration, but showed minimal voltage dependence. There was no strong dependence of inhibition on receptor subunit composition, the exception being the ρ1 receptor, which is markedly less sensitive. By using competition experiments with pregnenolone sulphate and the GABA channel blocker picrotoxinin, discrete binding sites are proposed. Furthermore, by assessing inhibition using site-directed mutagenesis and receptor chimeras comprising α, β or γ subunits with ρ1 subunits, the receptor transmembrane domains are strongly implicated in mediating inhibition and most likely the binding location for pregnenolone sulphate in GABARs. This article is part of the "Special Issue Dedicated to Norman G. Bowery".
Topics: Animals; Binding Sites; Cell Line; Humans; Membrane Potentials; Mice; Neurotransmitter Agents; Pentobarbital; Picrotoxin; Pregnenolone; Protein Domains; Receptors, GABA-A; Recombinant Proteins
PubMed: 29447845
DOI: 10.1016/j.neuropharm.2018.02.008 -
Angewandte Chemie (International Ed. in... Aug 2021The plant metabolite picrotoxinin (PXN) is a widely used tool in neuroscience for the identification of GABAergic signaling. Its hydrolysis in weakly alkaline media has...
The plant metabolite picrotoxinin (PXN) is a widely used tool in neuroscience for the identification of GABAergic signaling. Its hydrolysis in weakly alkaline media has been observed for over a century and the structure of the unstable hydrolysis intermediate was assigned by analogy to the degradation product picrotoxic acid. Here we show this assignment to be in error and we revise the structure of the hydrolysis product by spectroscopic characterization in situ. Counterintuitively, hydrolysis occurs at a lactone that remains closed in the major isolable degradation product, which accounts for the longstanding mistake in the literature.
Topics: Carbon-13 Magnetic Resonance Spectroscopy; Carboxylic Acids; Hydrogen-Ion Concentration; Hydrolysis; Picrotoxin; Proton Magnetic Resonance Spectroscopy; Sesterterpenes; Sodium Hydroxide
PubMed: 34236745
DOI: 10.1002/anie.202107785 -
Neurotoxicology Jul 2018Neurotoxicity testing still relies on ethically debated, expensive and time consuming in vivo experiments, which are unsuitable for high-throughput toxicity screening....
Neurotoxicity testing still relies on ethically debated, expensive and time consuming in vivo experiments, which are unsuitable for high-throughput toxicity screening. There is thus a clear need for a rapid in vitro screening strategy that is preferably based on human-derived neurons to circumvent interspecies translation. Recent availability of commercially obtainable human induced pluripotent stem cell (hiPSC)-derived neurons and astrocytes holds great promise in assisting the transition from the current standard of rat primary cortical cultures to an animal-free alternative. We therefore composed several hiPSC-derived neuronal models with different ratios of excitatory and inhibitory neurons in the presence or absence of astrocytes. Using immunofluorescent stainings and multi-well micro-electrode array (mwMEA) recordings we demonstrate that these models form functional neuronal networks that become spontaneously active. The differences in development of spontaneous neuronal activity and bursting behavior as well as spiking patterns between our models confirm the importance of the presence of astrocytes. Preliminary neurotoxicity assessment demonstrates that these cultures can be modulated with known seizurogenic compounds, such as picrotoxin (PTX) and endosulfan, and the neurotoxicant methylmercury (MeHg). However, the chemical-induced effects on different parameters for neuronal activity, such as mean spike rate (MSR) and mean burst rate (MBR), may depend on the ratio of inhibitory and excitatory neurons. Our results thus indicate that hiPSC-derived neuronal models must be carefully designed and characterized prior to large-scale use in neurotoxicity screening.
Topics: Action Potentials; Astrocytes; Cells, Cultured; Coculture Techniques; Endosulfan; Humans; Induced Pluripotent Stem Cells; Methylmercury Compounds; Neurons; Picrotoxin
PubMed: 29909083
DOI: 10.1016/j.neuro.2018.06.007 -
ELife May 2020Synaptic transmission is initiated via spontaneous or action-potential evoked fusion of synaptic vesicles. At excitatory synapses, glutamatergic receptors activated by...
Synaptic transmission is initiated via spontaneous or action-potential evoked fusion of synaptic vesicles. At excitatory synapses, glutamatergic receptors activated by spontaneous and evoked neurotransmission are segregated. Although inhibitory synapses also transmit signals spontaneously or in response to action potentials, they differ from excitatory synapses in both structure and function. Therefore, we hypothesized that inhibitory synapses may have different organizing principles. We report picrotoxin, a GABAR antagonist, blocks neurotransmission in a use-dependent manner at rat hippocampal synapses and therefore can be used to interrogate synaptic properties. Using this tool, we uncovered partial segregation of inhibitory spontaneous and evoked neurotransmission. We found up to 40% of the evoked response is mediated through GABARs which are only activated by evoked neurotransmission. These data indicate GABAergic spontaneous and evoked neurotransmission processes are partially non-overlapping, suggesting they may serve divergent roles in neuronal signaling.
Topics: Animals; Cells, Cultured; Electric Stimulation; Female; GABA Antagonists; GABAergic Neurons; Hippocampus; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Neural Inhibition; Picrotoxin; Rats, Sprague-Dawley; Receptors, GABA-A; Synaptic Transmission
PubMed: 32401197
DOI: 10.7554/eLife.52852