The interaction of the general anesthetic etomidate with the gamma-aminobutyric acid type A receptor is influenced by a single amino acid

Structural insights into GABAA receptor potentiation by Quaalude

Methaqualone, a quinazolinone marketed commercially as Quaalude, is a central nervous system depressant that was used clinically as a sedative-hypnotic, then became a notorious recreational drug in the 1960s-80s. Due to its high abuse potential, medical use of methaqualone was eventually prohibited, yet it persists as a globally abused substance. Methaqualone principally targets GABAA receptors, which are the major inhibitory neurotransmitter-gated ion channels in the brain. The restricted status and limited accessibility of methaqualone have contributed to its pharmacology being understudied. Here, we use cryo-EM to localize the GABAA receptor binding sites of methaqualone and its more potent derivative, PPTQ, to the same intersubunit transmembrane sites targeted by the general anesthetics propofol and etomidate. Both methaqualone and PPTQ insert more deeply into subunit interfaces than the previously-characterized modulators. Binding of quinazolinones to this site results in widening of the extracellular half of the ion-conducting pore, following a trend among positive allosteric modulators in destabilizing the hydrophobic activation gate in the pore as a mechanism for receptor potentiation. These insights shed light on the underexplored pharmacology of quinazolinones and further elucidate the molecular mechanisms of allosteric GABAA receptor modulation through transmembrane binding sites.

Neurobiological basis of emergence from anesthesia

The suppression of consciousness by anesthetics and the emergence of the brain from anesthesia are complex and elusive processes. Anesthetics may exert their inhibitory effects by binding to specific protein targets or through membrane-mediated targets, disrupting neural activity and the integrity and function of neural circuits responsible for signal transmission and conscious perception/subjective experience. Emergence from anesthesia was generally thought to depend on the elimination of the anesthetic from the body. Recently, studies have suggested that emergence from anesthesia is a dynamic and active process that can be partially controlled and is independent of the specific molecular targets of anesthetics. This article summarizes the fundamentals of anesthetics' actions in the brain and the mechanisms of emergence from anesthesia that have been recently revealed in animal studies.

Schisandrin B, a dual positive allosteric modulator of GABAA and glycine receptors, alleviates seizures in multiple mouse models

Epilepsy is a prevalent and severe neurological disorder and approximately 30% of patients are resistant to existing medications. It is of utmost importance to develop alternative therapies to treat epilepsy. Schisandrin B (SchB) is a major bioactive constituent of Schisandra chinensis (Turcz.) Baill and has multiple neuroprotective effects, sedative and hypnotic activities. In this study, we investigated the antiseizure effect of SchB in various mouse models of seizure and explored the underlying mechanisms. Pentylenetetrazole (PTZ), strychnine (STR), and pilocarpine-induced mouse seizure models were established. We showed that injection of SchB (10, 30, 60 mg/kg, i.p.) dose-dependently delayed the onset of generalized tonic-clonic seizures (GTCS), reduced the incidence of GTCS and mortality in PTZ and STR models. Meanwhile, injection of SchB (30 mg/kg, i.p.) exhibited therapeutic potential in pilocarpine-induced status epilepticus model, which was considered as a drug-resistant model. In whole-cell recording from CHO/HEK-239 cells stably expressing recombinant human GABAA receptors (GABAARs) and glycine receptors (GlyRs) and cultured hippocampal neurons, co-application of SchB dose-dependently enhanced GABA or glycine-induced current with EC50 values at around 5 μM, and application of SchB (10 μM) alone did not activate the channels in the absence of GABA or glycine. Furthermore, SchB (10 μM) eliminated both PTZ-induced inhibition on GABA-induced current (IGABA) and strychnine (STR)-induced inhibition on glycine-induced current (Iglycine). Moreover, SchB (10 μM) efficiently rescued the impaired GABAARs associated with genetic epilepsies. In addition, the homologous mutants in both GlyRs-α1(S267Q) and GABAARs-α1(S297Q)β2(N289S)γ2L receptors by site-directed mutagenesis tests abolished SchB-induced potentiation of IGABA and Iglycine. In conclusion, we have identified SchB as a natural positive allosteric modulator of GABAARs and GlyRs, supporting its potential as alternative therapies for epilepsy.

Changes in Memory, Sedation, and Receptor Kinetics Imparted by the β2-N265M and β3-N265M GABAA Receptor Point Mutations

Point mutations in the β2 (N265S) and β3 (N265M) subunits of γ-amino butyric acid type A receptors (GABAARs) that render them insensitive to the general anesthetics etomidate and propofol have been used to link modulation of β2-GABAARs to sedation and β3-GABAARs to surgical immobility. These mutations also alter GABA sensitivity, and mice carrying the β3-N265M mutation have been reported to have impaired baseline memory. Here, we tested the effects of the β2-N265M and β3-N265M mutations on memory, movement, hotplate sensitivity, anxiety, etomidate-induced sedation, and intrinsic kinetics. We found that both β2-N265M and β3-N265M mice exhibited baseline deficits in the Context Preexposure Facilitation Effect learning paradigm. Exploratory activity was slightly greater in β2-N265M mice, but there were no changes in either genotype in anxiety or hotplate sensitivity. β2-N265M mice were highly resistant to etomidate-induced sedation, and heterozygous mice were partially resistant. In rapid solution exchange experiments, both mutations accelerated deactivation two- to three-fold compared to wild type receptors and prevented modulation by etomidate. This degree of change in the receptor deactivation rate is comparable to that produced by an amnestic dose of etomidate but in the opposite direction, indicating that intrinsic characteristics of GABAARs are optimally tuned under baseline conditions to support mnemonic function.

The difference in mean arterial pressure induced by remimazolam compared to etomidate in the presence of fentanyl at tracheal intubation: A randomized controlled trial

Background: Combined use of hypnotic and opioids during anesthesia inductions decreases blood pressure. Post-induction hypotension (PIHO) is the most common side effect of anesthesia induction. We aimed to compare the difference in mean arterial pressure (MAP) induced by remimazolam with that induced by etomidate in the presence of fentanyl at tracheal intubation. Methods: We assessed 138 adult patients with American Society of Anesthesiologists physical status I-II who underwent elective urological surgery. Patients were randomly allocated to receive either remimazolam or etomidate as alterative hypnotic in the presence of fentanyl during anesthesia induction. Comparable BIS values were achieved in both groups. The primary outcome was the difference in the MAP at tracheal intubation. The secondary outcomes included the characteristics of anesthesia, surgery, and adverse effects. Results: The MAP was higher in the etomidate group than in the remimazolam group at tracheal intubation (108 [22] mmHg vs. 83 [16] mmHg; mean difference, -26; 95% confidence interval [CI], -33 to -19; p < 0.0001). Heart rate was significantly higher in the etomidate group than in the remimazolam group at tracheal intubation. The patients' condition warranted the administration of ephedrine more frequently in the remimazolam group (22%) than in the etomidate group (5%) (p = 0.0042) during anesthesia induction. The remimazolam group had a lower incidence of hypertension (0% vs. 9%, p = 0.0133), myoclonus (0% vs. 47%, p < 0.001), and tachycardia (16% vs. 35%, p = 0.0148), and a higher incidence of PIHO (42% vs. 5%, p = 0.001) than the etomidate group during anesthesia induction. Conclusion: Remimazolam was associated with lower MAP and lower heart rate compared to etomidate in the presence of fentanyl at tracheal intubation. Patients in the remimazolam group had a higher incidence of PIHO, and their condition warranted the administration of ephedrine more frequently than in the etomidate group during anesthesia induction.

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.

Evaluation of site-selective drug effects on GABA receptors using nanovesicle-carbon nanotube hybrid devices

Site-selective drug effects on the ion-channel activities of γ-aminobutyric acid type A (GABA_A) receptors are evaluated by using a nanovesicle-carbon nanotube hybrid device. Here, nanovesicles containing GABA_A receptors are immobilized on the channel region of a carbon nanotube field-effect transistor. The receptor responses of this hybrid device to GABA are detected with a high sensitivity down to ∼1 aM even in the presence of other neurotransmitters. Further, sensitivity differences between two GABA_A-receptor-subunit compositions of α5β2γ2 and α1β2γ2 are assessed by normalizing the dose-dependent responses obtained from these hybrid devices. Specifically, the GABA concentration that produces 50% of maximal response (EC₅₀) is obtained as ∼10 pM for α5β2γ2 subunits and ∼1 nM for α1β2γ2 subunits of GABA_A receptor. Significantly, the potency profiles of both antagonist and agonist of GABA_A receptor can be evaluated by analyzing EC₅₀ values in the presence and absence of those drugs. A competitive antagonist increases the EC₅₀ value of GABA by binding to the same site as GABA, while an allosteric agonist reduces it by binding to a different site. These results indicate that this hybrid device can be a powerful tool for the evaluation of candidate drug substances modulating GABA-mediated neurotransmission.

GABAA Receptors in Astrocytes Are Targets for Commonly Used Intravenous and Inhalational General Anesthetic Drugs

Background: Perioperative neurocognitive disorders (PNDs) occur commonly in older patients after anesthesia and surgery. Treating astrocytes with general anesthetic drugs stimulates the release of soluble factors that increase the cell-surface expression and function of GABA_A receptors in neurons. Such crosstalk may contribute to PNDs; however, the receptor targets in astrocytes for anesthetic drugs have not been identified. GABA_A receptors, which are the major targets of general anesthetic drugs in neurons, are also expressed in astrocytes, raising the possibility that these drugs act on GABA_A receptors in astrocytes to trigger the release of soluble factors. To date, no study has directly examined the sensitivity of GABA_A receptors in astrocytes to general anesthetic drugs that are frequently used in clinical practice. Thus, the goal of this study was to determine whether the function of GABA_A receptors in astrocytes was modulated by the intravenous anesthetic etomidate and the inhaled anesthetic sevoflurane.

Methods: Whole-cell voltage-clamp recordings were performed in astrocytes in the stratum radiatum of the CA1 region of hippocampal slices isolated from C57BL/6 male mice. Astrocytes were identified by their morphologic and electrophysiologic properties. Focal puff application of GABA (300 μM) was applied with a Picospritzer system to evoke GABA responses. Currents were studied before and during the application of the non-competitive GABA_A receptor antagonist picrotoxin (0.5 mM), or etomidate (100 μM) or sevoflurane (532 μM).

Results: GABA consistently evoked inward currents that were inhibited by picrotoxin. Etomidate increased the amplitude of the peak current by 35.0 ± 24.4% and prolonged the decay time by 27.2 ± 24.3% (n = 7, P < 0.05). Sevoflurane prolonged current decay by 28.3 ± 23.1% (n = 7, P < 0.05) but did not alter the peak amplitude. Etomidate and sevoflurane increased charge transfer (area) by 71.2 ± 45.9% and 51.8 ± 48.9% (n = 7, P < 0.05), respectively.

Conclusion: The function of astrocytic GABA_A receptors in the hippocampus was increased by etomidate and sevoflurane. Future studies will determine whether these general anesthetic drugs act on astrocytic GABA_A receptors to stimulate the release of soluble factors that may contribute to PNDs.

The effect of obstructive jaundice on the sensitivity of intravenous anesthetic of remimazolam: study protocol for a controlled multicenter trial

BACKGROUND

It is well known that obstructive jaundice could affect the pharmacodynamics of some anesthetics, and the sensitivity of some anesthetics would increase among icteric patients. Remimazolam is a new ultra-short-acting intravenous benzodiazepine sedative/anesthetic, which is a high-selective and affinity ligand for the benzodiazepine site on the GABAA receptor. However, no study has reported the pharmacodynamics of remimazolam in patients with obstructive jaundice. We hypothesize that obstructive jaundice affects the pharmacodynamics of remimazolam, and the sensitivity of remimazolam increases among icteric patients.

METHODS/DESIGN

The study will be performed as a prospective, controlled, multicenter trial. The study design is a comparison of remimazolam requirements to reach a bispectral index of 50 in patients with obstructive jaundice versus non-jaundiced patients with chronic cholecystitisor intrahepatic bile duct stones. Remimazolam was infused at 6 mg/kg/h until this endpoint was reached.

DISCUSSION

Remimazolam could be suitable for anesthesia of patients with obstructive jaundice, because remimazolam is not biotransformed in the liver. Hyperbilirubinemia has been well-described to have toxic effects on the brain, which causes the increasing of sensitivity to some anesthetics, such as desflurane, isoflurane, and etomidate. Furthermore, remimazolam and etomidate have the same mechanism of action when exerting an anesthetic effect. We aim to demonstrate that obstructive jaundice affects the pharmacodynamics of remimazolam, and the dose of remimazolam when administered to patients with obstructive jaundice should be modified.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2100043585 . Registered on 23 February 2021.

α6-Containing GABAA Receptors: Functional Roles and Therapeutic Potentials

GABAA receptors containing the α6 subunit are highly expressed in cerebellar granule cells and less abundantly in many other neuronal and peripheral tissues. Here, we for the first time summarize their importance for the functions of the cerebellum and the nervous system. The cerebellum is not only involved in motor control but also in cognitive, emotional, and social behaviors. α6βγ2 GABAA receptors located at cerebellar Golgi cell/granule cell synapses enhance the precision of inputs required for cerebellar timing of motor activity and are thus involved in cognitive processing and adequate responses to our environment. Extrasynaptic α6βδ GABAA receptors regulate the amount of information entering the cerebellum by their tonic inhibition of granule cells, and their optimal functioning enhances input filtering or contrast. The complex roles of the cerebellum in multiple brain functions can be compromised by genetic or neurodevelopmental causes that lead to a hypofunction of cerebellar α6-containing GABAA receptors. Animal models mimicking neuropsychiatric phenotypes suggest that compounds selectively activating or positively modulating cerebellar α6-containing GABAA receptors can alleviate essential tremor and motor disturbances in Angelman and Down syndrome as well as impaired prepulse inhibition in neuropsychiatric disorders and reduce migraine and trigeminal-related pain via α6-containing GABAA receptors in trigeminal ganglia. Genetic studies in humans suggest an association of the human GABAA receptor α6 subunit gene with stress-associated disorders. Animal studies support this conclusion. Neuroimaging and post-mortem studies in humans further support an involvement of α6-containing GABAA receptors in various neuropsychiatric disorders, pointing to a broad therapeutic potential of drugs modulating α6-containing GABAA receptors. SIGNIFICANCE STATEMENT: α6-Containing GABAA receptors are abundantly expressed in cerebellar granule cells, but their pathophysiological roles are widely unknown, and they are thus out of the mainstream of GABAA receptor research. Anatomical and electrophysiological evidence indicates that these receptors have a crucial function in neuronal circuits of the cerebellum and the nervous system, and experimental, genetic, post-mortem, and pharmacological studies indicate that selective modulation of these receptors offers therapeutic prospects for a variety of neuropsychiatric disorders and for stress and its consequences.

Hippocampal β2-GABAA receptors mediate LTP suppression by etomidate and contribute to long-lasting feedback but not feedforward inhibition of pyramidal neurons

The general anesthetic etomidate, which acts through γ-aminobutyric acid type A (GABA_A) receptors, impairs the formation of new memories under anesthesia. This study addresses the molecular and cellular mechanisms by which this occurs. Here, using a new line of genetically engineered mice carrying the GABA_A receptor (GABA_AR) β2-N265M mutation, we tested the roles of receptors that incorporate GABA_A receptor β2 versus β3 subunits to suppression of long-term potentiation (LTP), a cellular model of learning and memory. We found that brain slices from β2-N265M mice resisted etomidate suppression of LTP, indicating that the β2-GABA_ARs are an essential target in this model. As these receptors are most heavily expressed by interneurons in the hippocampus, this finding supports a role for interneuron modulation in etomidate control of synaptic plasticity. Nevertheless, β2 subunits are also expressed by pyramidal neurons, so they might also contribute. Therefore, using a previously established line of β3-N265M mice, we also examined the contributions of β2- versus β3-GABA_ARs to GABA_A,slow dendritic inhibition, because dendritic inhibition is particularly well suited to controlling synaptic plasticity. We also examined their roles in long-lasting suppression of population activity through feedforward and feedback inhibition. We found that both β2- and β3-GABA_ARs contribute to GABA_A,slow inhibition and that both β2- and β3-GABA_ARs contribute to feedback inhibition, whereas only β3-GABA_ARs contribute to feedforward inhibition. We conclude that modulation of β2-GABA_ARs is essential to etomidate suppression of LTP. Furthermore, to the extent that this occurs through GABA_ARs on pyramidal neurons, it is through modulation of feedback inhibition.NEW & NOTEWORTHY Etomidate exerts its anesthetic actions through GABA_A receptors. However, the mechanism remains unknown. Here, using a hippocampal brain slice model, we show that β2-GABA_ARs are essential to this effect. We also show that these receptors contribute to long-lasting dendritic inhibition in feedback but not feedforward inhibition of pyramidal neurons. These findings hold implications for understanding how anesthetics block memory formation and, more generally, how inhibitory circuits control learning and memory.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A 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 [198].

Anesthesiology: Resetting Our Sights on Long-term Outcomes: The 2020 John W. Severinghaus Lecture on Translational Science

Anesthesiologists have worked relentlessly to improve intraoperative anesthesia care. They are now well positioned to expand their horizons and address many of the longer-term adverse consequences of anesthesia and surgery. Perioperative neurocognitive disorders, chronic postoperative pain, and opioid misuse are not inevitable adverse outcomes; rather, they are preventable and treatable conditions that deserve attention. The author's research team has investigated why patients experience new cognitive deficits after anesthesia and surgery. Their animal studies have shown that anesthetic drugs trigger overactivity of "memory-blocking receptors" that persists after the drugs are eliminated, and they have discovered new strategies to preserve brain function by repurposing available drugs and developing novel therapeutics that inhibit these receptors. Clinical trials are in progress to examine the cognitive outcomes of such strategies. This work is just one example of how anesthesiologists are advancing science with the goal of improving the lives of patients.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Electroencephalographic dynamics of etomidate-induced loss of consciousness

BACKGROUND

Highly structured electroencephalography (EEG) oscillations can occur in adults during etomidate-induced general anesthesia, but the link between these two phenomena is poorly understood. Therefore, in the present study, we investigated the electroencephalogram dynamics of etomidate-induced loss of consciousness (LOC) in order to understand the neurological mechanism of etomidate-induced LOC.

METHODS

This study is a prospective observational study. Etomidate-induced anesthesia was performed on eligible patients undergoing elective surgery. We analyzed EEG data from 20 patients who received etomidate for the induction of general anesthesia. We used power spectra and coherence methods to process and analyze the EEG data. Our study was based on 4-channel EEG recordings.

RESULTS

Compared with the baseline (awake period), etomidate induced an increase in power in delta, theta, alpha and beta waves during LOC. Compared with the awake period, the delta-wave (1-4 Hz), alpha-wave(8-13 Hz), and theta-wave(4-8 Hz) coherence increased significantly during LOC, while the slow-wave (< 1 Hz) coherence decreased. However, the delta wave (1.0-4.0 Hz) during etomidate-induced LOC was more coherent than during the awake period (1.86-3.17 Hz, two-group test for coherence, p < 0.001).

CONCLUSIONS

The neural circuit mechanism of etomidate-induced LOC is closely related to the induction of oscillation in delta, theta, alpha and beta waves and the enhancement of delta-wave coherence.

TRIAL REGISTRATION

ChiCTR1800017110.

Direct Structural Insights into GABAA Receptor Pharmacology

GABA_A receptors are pentameric ligand-gated ion channels that mediate most fast neuronal inhibition in the brain. In addition to their important physiological roles, they are noteworthy in their rich pharmacology; prominent drugs used for anxiety, insomnia, and general anesthesia act through positive modulation of GABA_A receptors. Direct structural information for how these drugs work was absent until recently. Efforts in structural biology over the past few years have revealed how important drug classes and natural products interact with the GABA_A receptor, providing a foundation for studies in dynamics and structure-guided drug design. Here, we review recent developments in GABA_A receptor structural pharmacology, focusing on subunit assemblies of the receptor found at synapses.

From Methaqualone and Beyond: Structure-Activity Relationship of 6-, 7-, and 8-Substituted 2,3-Diphenyl-quinazolin-4(3H)-ones and in Silico Prediction of Putative Binding Modes of Quinazolin-4(3H)-ones as Positive Allosteric Modulators of GABAA Receptors

Methaqualone (2-methyl-3-(o-tolyl)-quinazolin-4(3H)-one, MTQ) is a moderately potent positive allosteric modulator (PAM) of GABA_A receptors (GABA_ARs). In a previous structure-activity relationship (SAR) study probing the importance of 2- and 3-substituents in the quinazolin-4(3H)-one scaffold, several potent GABA_AR PAMs were identified, including 2,3-diphenylquinazolin-4(3H)-one (PPQ) and 3-(2-chlorophenyl)-2-phenylquinazolin-4(3H)-one (Cl-PPQ). Here, PPQ was applied as lead in a SAR study of 6-, 7-, and 8-substituents in the quinazolin-4(3H)-one by synthesis and functional characterization of 36 PPQ analogs at various GABA_AR subtypes. While none of the new analogs were significantly more potent than PPQ or displayed pronounced subtype selectivity across the GABA_ARs tested, several interesting SAR observations were extracted from the study. In an in silico study, the putative binding modes of MTQ, PPQ, and Cl-PPQ in the transmembrane β₂⁽⁺⁾/α₁^(-) interface of the α₁β₂γ_2S GABA_AR were predicted. Several plausible binding modes were identified for the three PAMs, and rationalization of the molecular basis for their different modulatory potencies was attempted.

The general anesthetic etomidate and fenamate mefenamic acid oppositely affect GABAAR and GlyR: a structural explanation

GABA and glycine act as inhibitory neurotransmitters in the CNS. Inhibitory neurotransmission is mediated via activation of ionotropic GABA_A and glycine receptors. We used a modeling approach to explain the opposite effects of the general anesthetic etomidate (ETM) and fenamate mefenamic acid (MFA) on GABA- and glycine-activated currents recorded in isolated cerebellar Purkinje cells and hippocampal pyramidal neurons, respectively. These drugs potentiated GABA_ARs but blocked GlyRs. We built a homology model of α1β GlyR based on the cryo-EM structure of open α1 GlyR, used the α1β3γ2 GABA_AR structure from the PDB, and applied Monte-Carlo energy minimization to optimize models of receptors and ligand-receptor complexes. In silico docking suggests that ETM/MFA bind at the transmembrane β( +)/α( -) intersubunit interface in GABA_AR. Our models predict that the bulky side chain of the highly conserved Arg^19' residue at the plus interface side wedges the interface and maintains the conducting receptor state. We hypothesized that MFA/ETM binding at the β( +)/α( -) interface leads to prolongation of receptor life-time in the open state. Having analyzed different GABA_AR and GlyR structures available in the PDB, we found that mutual arrangement of the Arg^19' and Gln^-26' side chains at the plus and minus interface sides, respectively, plays an important role when the receptor switches from the open to closed state. We show that this process is accompanied by narrowing of the intersubunit interfaces, leading to extrusion of the Arg^19' side chain from the interface. Our models allow us to explain the lack of GlyR potentiation in our electrophysiological experiments.

Shared structural mechanisms of general anaesthetics and benzodiazepines

Most general anaesthetics and classical benzodiazepine drugs act through positive modulation of γ-aminobutyric acid type A (GABAA) receptors to dampen neuronal activity in the brain1-5. 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 GABAA 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 GABAA 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 GABAA 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.

A potent photoreactive general anesthetic with novel binding site selectivity for GABAA receptors

The pentameric γ-aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory ligand-gated ion channels in the central nervous system. They mediate diverse physiological functions, mutations in them are associated with mental disorders and they are the target of many drugs such as general anesthetics, anxiolytics and anti-convulsants. The five subunits of synaptic GABA_ARs are arranged around a central pore in the order β-α-β-α-γ. In the outer third of the transmembrane domain (TMD) drugs may bind to five homologous intersubunit binding sites. Etomidate binds between the pair of β - α subunit interfaces (designated as β⁺/α^-) and R-mTFD-MPAB binds to an α⁺/β^- and an γ⁺/β^- subunit interface (a β^- selective ligand). Ligands that bind selectively to other homologous sites have not been characterized. We have synthesized a novel photolabel, (2,6-diisopropyl-4-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenyl)methanol or pTFD-di-iPr-BnOH). It is a potent general anesthetic that positively modulates agonist and benzodiazepine binding. It enhances GABA-induced currents, shifting the GABA concentration-response curve to lower concentrations. Photolabeling-protection studies show that it has negligible affinity for the etomidate sites and high affinity for only one of the two R-mTFD-MPAB sites. Exploratory site-directed mutagenesis studies confirm the latter conclusions and hint that pTFD-di-iPr-BnOH may bind between the α⁺/β^- and α⁺/γ^- subunits in the TMD, making it an α⁺ ligand. The latter α⁺/γ^- site has not previously been implicated in ligand binding. Thus, pTFD-di-iPr-BnOH is a promising new photolabel that may open up a new pharmacology for synaptic GABA_ARs.

Drug-selective Anesthetic Insensitivity of Zebrafish Lacking γ-Aminobutyric Acid Type A Receptor β3 Subunits

BACKGROUND

Transgenic mouse studies suggest that γ-aminobutyric acid type A (GABAA) receptors containing β3 subunits mediate important effects of etomidate, propofol, and pentobarbital. Zebrafish, recently introduced for rapid discovery and characterization of sedative-hypnotics, could also accelerate pharmacogenetic studies if their transgenic phenotypes reflect those of mammals. The authors hypothesized that, relative to wild-type, GABAA-β3 functional knock-out (β3) zebrafish would show anesthetic sensitivity changes similar to those of β3 mice.

METHODS

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 mutagenesis was used to create a β3 zebrafish line. Wild-type and β3 zebrafish were compared for fertility, growth, and craniofacial development. Sedative and hypnotic effects of etomidate, propofol, pentobarbital, alphaxalone, ketamine, tricaine, dexmedetomidine, butanol, and ethanol, along with overall activity and thigmotaxis were quantified in 7-day postfertilization larvae using video motion analysis of up to 96 animals simultaneously.

RESULTS

Xenopus oocyte electrophysiology showed that the wild-type zebrafish β3 gene encodes ion channels activated by propofol and etomidate, while the β3 zebrafish transgene does not. Compared to wild-type, β3 zebrafish showed similar morphology and growth, but more rapid swimming. Hypnotic EC50s (mean [95% CI]) were significantly higher for β3 versus wild-type larvae with etomidate (1.3 [1.0 to 1.6] vs. 0.6 [0.5 to 0.7] µM; P < 0.0001), propofol (1.1 [1.0 to 1.4] vs. 0.7 [0.6 to 0.8] µM; P = 0.0005), and pentobarbital (220 [190 to 240] vs. 130 [94 to 179] μM; P = 0.0009), but lower with ethanol (150 [106 to 213] vs. 380 [340 to 420] mM; P < 0.0001) and equivalent with other tested drugs. Comparing β3 versus wild-type sedative EC50s revealed a pattern similar to hypnosis.

CONCLUSIONS

Global β3 zebrafish are selectively insensitive to the same few sedative-hypnotics previously reported in β3 transgenic mice, indicating phylogenetic conservation of β3-containing GABAA receptors as anesthetic targets. Transgenic zebrafish are potentially valuable models for sedative-hypnotic mechanisms research.

The effects of a competitive antagonist on GABA-evoked currents in the presence of sedative-hypnotic agents

BACKGROUND

Many sedative-hypnotic agents are thought to act by positively modulating γ-aminobutyric acid type A (GABA_A) receptors. However, for many agents, the location(s) of the binding site(s) responsible for such receptor modulation is uncertain. We previously developed a low efficacy ligand (naphthalene-etomidate) that binds within a homologous set of hydrophobic cavities located at GABA_A receptor subunit interfaces in the transmembrane domain, and thus acts as a competitive antagonist for higher efficacy sedative-hypnotics that also bind to these sites. In this report, we describe studies using this compound as a pharmacological screening tool to test whether sedative-hypnotics representing a range of chemical classes can modulate GABA_A receptors by binding within these receptor cavities.

METHODS

The impact of naphthalene-etomidate on GABA-evoked currents that were mediated by oocyte-expressed α₁β₃γ_2L GABA_A receptors and potentiated by muscimol, alphaxalone, 2,2,2-trichloroethanol, isoflurane, AA29504, loreclezole, or diazepam was quantified using electrophysiological techniques.

RESULTS

Naphthalene-etomidate (300 µM) significantly reduced GABA_A receptor currents potentiated by alphaxalone (by 22 ± 11%), 2,2,2-trichloroethanol (by 23 ± 6%), isoflurane (by 32 ± 10%), AA29504 (by 41 ± 6%), loreclezole (by 43 ± 9%), but significantly increased those potentiated by muscimol (by 26 ± 11%). Naphthalene-etomidate significantly increased currents potentiated by a low (1 µM) diazepam concentration (by 56 ± 14%) while reducing those potentiated by a high (100 µM) diazepam concentration (by 11 ± 7%).

CONCLUSIONS

Our results suggest that many (but not all) sedative-hypnotics are capable of positively modulating the GABA_A receptor by binding within a common set of hydrophobic cavities.

GABAA Receptor Ligands Often Interact with Binding Sites in the Transmembrane Domain and in the Extracellular Domain-Can the Promiscuity Code Be Cracked?

Many allosteric binding sites that modulate gamma aminobutyric acid (GABA) effects have been described in heteropentameric GABA type A (GABA_A) receptors, among them sites for benzodiazepines, pyrazoloquinolinones and etomidate. Diazepam not only binds at the high affinity extracellular “canonical” site, but also at sites in the transmembrane domain. Many ligands of the benzodiazepine binding site interact also with homologous sites in the extracellular domain, among them the pyrazoloquinolinones that exert modulation at extracellular α+/β− sites. Additional interaction of this chemotype with the sites for etomidate has also been described. We have recently described a new indole-based scaffold with pharmacophore features highly similar to pyrazoloquinolinones as a novel class of GABA_A receptor modulators. Contrary to what the pharmacophore overlap suggests, the ligand presented here behaves very differently from the identically substituted pyrazoloquinolinone. Structural evidence demonstrates that small changes in pharmacophore features can induce radical changes in ligand binding properties. Analysis of published data reveals that many chemotypes display a strong tendency to interact promiscuously with binding sites in the transmembrane domain and others in the extracellular domain of the same receptor. Further structural investigations of this phenomenon should enable a more targeted path to less promiscuous ligands, potentially reducing side effect liabilities.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Binding site location on GABAA receptors determines whether mixtures of intravenous general anaesthetics interact synergistically or additively in vivo

BACKGROUND AND PURPOSE

General anaesthetics can act on synaptic GABA_A receptors by binding to one of three classes of general anaesthetic sites. Canonical drugs that bind selectively to only one class of site are etomidate, alphaxalone, and the mephobarbital derivative, R-mTFD-MPAB. We tested the hypothesis that the general anaesthetic potencies of mixtures of such site-selective agents binding to the same or to different sites would combine additively or synergistically respectively.

EXPERIMENTAL APPROACH

The potency of general anaesthetics individually or in combinations to cause loss of righting reflexes in tadpoles was determined, and the results were analysed using isobolographic methods.

KEY RESULTS

The potencies of combinations of two or three site-selective anaesthetics that all acted on a single class of site were strictly additive, regardless of which single site was involved. Combinations of two or three site-selective anaesthetics that all bound selectively to different sites always interacted synergistically. The strength of the synergy increased with the number of separate sites involved such that the percentage of each agent's EC₅₀ required to cause anaesthesia was just 35% and 14% for two or three sites respectively. Propofol, which binds non-selectively to the etomidate and R-mTFD-MPAB sites, interacted synergistically with each of these agents.

CONCLUSIONS AND IMPLICATIONS

The established pharmacology of the three anaesthetic binding sites on synaptic GABA_A receptors was sufficient to predict whether a mixture of anaesthetics interacted additively or synergistically to cause loss of righting reflexes in vivo. The principles established here have implications for clinical practice.

The Locus Coeruleus Modulates Intravenous General Anesthesia of Zebrafish via a Cooperative Mechanism

How general anesthesia causes loss of consciousness has been a mystery for decades. It is generally thought that arousal-related brain nuclei, including the locus coeruleus (LC), are involved. Here, by monitoring locomotion behaviors and neural activities, we developed a larval zebrafish model for studying general anesthesia induced by propofol and etomidate, two commonly used intravenous anesthetics. Local lesion of LC neurons via two-photon laser-based ablation or genetic depletion of norepinephrine (NE; a neuromodulator released by LC neurons) via CRISPR/Cas9-based mutation of dopamine-β-hydroxylase (dbh) accelerates induction into and retards emergence from general anesthesia. Mechanistically, in vivo whole-cell recording revealed that both anesthetics suppress LC neurons' activity through a cooperative mechanism, inhibiting presynaptic excitatory inputs and inducing GABA_A receptor-mediated hyperpolarization of these neurons. Thus, our study indicates that the LC-NE system plays a modulatory role in both induction of and emergence from intravenous general anesthesia.

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABA_AR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABA_AR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane β(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and β subunit specificity of the fenamate potentiation.

Etomidate and Etomidate Analog Binding and Positive Modulation of γ-Aminobutyric Acid Type A Receptors: Evidence for a State-dependent Cutoff Effect

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Naphthalene-etomidate, an etomidate analog containing a bulky phenyl ring substituent group, possesses very low γ-aminobutyric acid type A (GABAA) receptor efficacy and acts as an anesthetic-selective competitive antagonist. Using etomidate analogs containing phenyl ring substituents groups that range in volume, we tested the hypothesis that this unusual pharmacology is caused by steric hindrance that reduces binding to the receptor's open state.

METHODS

The positive modulatory potencies and efficacies of etomidate and phenyl ring-substituted etomidate analogs were electrophysiology defined in oocyte-expressed α1β3γ2L GABAA receptors. Their binding affinities to the GABAA receptor's two classes of transmembrane anesthetic binding sites were assessed from their abilities to inhibit receptor labeling by the site-selective photolabels [H]azi-etomidate and tritiated R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid.

RESULTS

The positive modulatory activities of etomidate and phenyl ring-substituted etomidate analogs progressively decreased with substituent group volume, reflecting significant decreases in both potency (P = 0.005) and efficacy (P < 0.0001). Affinity for the GABAA receptor's two β - α anesthetic binding sites similarly decreased with substituent group volume (P = 0.003), whereas affinity for the receptor's α - β/γ - β sites did not (P = 0.804). Introduction of the N265M mutation, which is located at the β - α binding sites and renders GABAA receptors etomidate-insensitive, completely abolished positive modulation by naphthalene-etomidate.

CONCLUSIONS

Steric hindrance selectively reduces phenyl ring-substituted etomidate analog binding affinity to the two β - α anesthetic binding sites on the GABAA receptor's open state, suggesting that the binding pocket where etomidate's phenyl ring lies becomes smaller as the receptor isomerizes from closed to open.

Toxicologic and Inhibitory Receptor Actions of the Etomidate Analog ABP-700 and Its Metabolite CPM-Acid

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

The etomidate analog ABP-700 produces involuntary muscle movements that could be manifestations of seizures. To define the relationship (if any) between involuntary muscle movements and seizures, electroencephalographic studies were performed in Beagle dogs receiving supra-therapeutic (~10× clinical) ABP-700 doses. γ-aminobutyric acid type A (GABAA) and glycine receptor studies were undertaken to test receptor inhibition as the potential mechanism for ABP-700 seizures.

Methods

ABP-700 was administered to 14 dogs (6 mg/kg bolus followed by a 2-h infusion at 1 mg · kg-1 · min-1, 1.5 mg · kg-1 · min-1, or 2.3 mg · kg-1 · min-1). Involuntary muscle movements were documented, electroencephalograph was recorded, and plasma ABP-700 and CPM-acid concentrations were measured during and after ABP-700 administration. The concentration-dependent modulatory actions of ABP-700 and CPM-acid were defined in oocyte-expressed α1β3γ2L GABAA and α1β glycine receptors (n = 5 oocytes/concentration) using electrophysiologic techniques.

Results

ABP-700 produced both involuntary muscle movements (14 of 14 dogs) and seizures (5 of 14 dogs). However, these phenomena were temporally and electroencephalographically distinct. Mean peak plasma concentrations were (from lowest to highest dosed groups) 35 μM, 45 μM, and 102 μM (ABP-700) and 282 μM, 478 μM, and 1,110 μM (CPM-acid). ABP-700 and CPM-acid concentration–GABAA receptor response curves defined using 6 μM γ-aminobutyric acid exhibited potentiation at low and/or intermediate concentrations and inhibition at high ones. The half-maximal inhibitory concentrations of ABP-700 and CPM-acid defined using 1 mM γ-aminobutyric acid were 770 μM (95% CI, 590 to 1,010 μM) and 1,450 μM (95% CI, 1,340 to 1,560 μM), respectively. CPM-acid similarly inhibited glycine receptors activated by 1 mM glycine with a half-maximal inhibitory concentration of 1,290 μM (95% CI, 1,240 to 1,330 μM).

Conclusions

High dose ABP-700 infusions produce involuntary muscle movements and seizures in Beagle dogs via distinct mechanisms. CPM-acid inhibits both GABAA and glycine receptors at the high (~100× clinical) plasma concentrations achieved during the dog studies, providing a plausible mechanism for the seizures.

Anesthetics: from modes of action to unconsciousness and neurotoxicity

Modern anesthetic compounds and advanced monitoring tools have revolutionized the field of medicine, allowing for complex surgical procedures to occur safely and effectively. Faster induction times and quicker recovery periods of current anesthetic agents have also helped reduce health care costs significantly. Moreover, extensive research has allowed for a better understanding of anesthetic modes of action, thus facilitating the development of more effective and safer compounds. Notwithstanding the realization that anesthetics are a prerequisite to all surgical procedures, evidence is emerging to support the notion that exposure of the developing brain to certain anesthetics may impact future brain development and function. Whereas the data in support of this postulate from human studies is equivocal, the vast majority of animal research strongly suggests that anesthetics are indeed cytotoxic at multiple brain structure and function levels. In this review, we first highlight various modes of anesthetic action and then debate the evidence of harm from both basic science and clinical studies perspectives. We present evidence from animal and human studies vis-à-vis the possible detrimental effects of anesthetic agents on both the young developing and the elderly aging brain while discussing potential ways to mitigate these effects. We hope that this review will, on the one hand, invoke debate vis-à-vis the evidence of anesthetic harm in young children and the elderly, and on the other hand, incentivize the search for better and less toxic anesthetic compounds.

Identifying Drugs that Bind Selectively to Intersubunit General Anesthetic Sites in the α1β3γ2 GABAAR Transmembrane Domain

GABA_A receptors (GABA_ARs) are targets for important classes of clinical agents (e.g., anxiolytics, anticonvulsants, and general anesthetics) that act as positive allosteric modulators (PAMs). Previously, using photoreactive analogs of etomidate ([³H]azietomidate) and mephobarbital [[³H]1-methyl-5-allyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid ([³H]R-mTFD-MPAB)], we identified two homologous but pharmacologically distinct classes of general anesthetic binding sites in the α1β3γ2 GABA_AR transmembrane domain at β ⁺-α ^- (β ⁺ sites) and α ⁺-β ^-/γ ⁺-β ^- (β ^- sites) subunit interfaces. We now use competition photolabeling with [³H]azietomidate and [³H]R-mTFD-MPAB to identify para-substituted propofol analogs and other drugs that bind selectively to intersubunit anesthetic sites. Propofol and 4-chloro-propofol bind with 5-fold selectivity to β ⁺, while derivatives with bulkier lipophilic substitutions [4-(tert-butyl)-propofol and 4-(hydroxyl(phenyl)methyl)-propofol] bind with ∼10-fold higher affinity to β ^- sites. Similar to R-mTFD-MPAB and propofol, these drugs bind in the presence of GABA with similar affinity to the α ⁺-β ^- and γ ⁺-β ^- sites. However, we discovered four compounds that bind with different affinities to the two β ^- interface sites. Two of these bind with higher affinity to one of the β ^- sites than to the β ⁺ sites. We deduce that 4-benzoyl-propofol binds with >100-fold higher affinity to the γ ⁺-β ^- site than to the α ⁺-β ^- or β ⁺-α ^- sites, whereas loreclezole, an anticonvulsant, binds with 5- and 100-fold higher affinity to the α ⁺-β ^- site than to the β ⁺ and γ ⁺-β ^- sites. These studies provide a first identification of PAMs that bind selectively to a single intersubunit site in the GABA_AR transmembrane domain, a property that may facilitate the development of subtype selective GABA_AR PAMs.

Monod-Wyman-Changeux Allosteric Shift Analysis in Mutant α1β3γ2L GABAA Receptors Indicates Selectivity and Crosstalk among Intersubunit Transmembrane Anesthetic Sites

Propofol, etomidate, and barbiturate anesthetics are allosteric coagonists at pentameric α1β3γ2 GABA_A receptors, modulating channel activation via four biochemically established intersubunit transmembrane pockets. Etomidate selectively occupies the two β ⁺/α ^- pockets, the barbiturate photolabel R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) occupies homologous α ⁺/β ^- and γ ⁺/β ^- pockets, and propofol occupies all four. Functional studies of mutations at M2-15' or M3-36' loci abutting these pockets provide conflicting results regarding their relative contributions to propofol modulation. We electrophysiologically measured GABA-dependent channel activation in α1β3γ2L or receptors with single M2-15' (α1S270I, β3N265M, and γ2S280W) or M3-36' (α1A291W, β3M286W, and γ2S301W) mutations, in the absence and presence of equipotent clinical range concentrations of etomidate, R-mTFD-MPAB, and propofol. Estimated open probabilities were calculated and analyzed using global two-state Monod-Wyman-Changeux models to derive log(d) parameters proportional to anesthetic-induced channel modulating energies (where d is the allosteric anesthetic shift factor). All mutations reduced the log(d) values for anesthetics occupying both abutting and nonabutting pockets. The Δlog(d) values [log(d, mutant) - log(d, wild type)] for M2-15' mutations abutting an anesthetic's biochemically established binding sites were consistently larger than the Δlog(d) values for nonabutting mutations, although this was not true for the M3-36' mutant Δlog(d) values. The sums of the anesthetic-associated Δlog(d) values for sets of M2-15' or M3-36' mutations were all much larger than the wild-type log(d) values. Mutant Δlog(d) values qualitatively reflect anesthetic site occupancy patterns. However, the lack of Δlog(d) additivity undermines quantitative comparisons of distinct site contributions to anesthetic modulation because the mutations impaired both abutting anesthetic binding effects and positive cooperativity between anesthetic binding sites.

Role of GABAA receptor subtypes in the behavioural effects of intravenous general anaesthetics

Since the introduction of general anaesthetics into clinical practice, researchers have been mystified as to how these chemically disparate drugs act to produce their dramatic effects on central nervous system function and behaviour. Scientific advances, particularly during the last 25 years, have now begun to reveal the molecular mechanisms underpinning their behavioural effects. For certain i.v. general anaesthetics, such as etomidate and propofol, a persuasive case can now be made that the GABAA receptor, a major inhibitory receptor in the mammalian central nervous system, is an important target. Advances in molecular pharmacology and in genetic manipulation of rodent genes reveal that different subtypes of the GABAA receptor are responsible for mediating particular aspects of the anaesthetic behavioural repertoire. Such studies provide a better understanding of the neuronal circuitry involved in the various anaesthetic-induced behaviours and, in the future, may result in the development of novel therapeutics with a reduced propensity for side-effects.

2-(4-methyl-thiazol-5-yl) ethyl nitrate maleate-potentiated GABAA receptor response in hippocampal neurons

AIMS

2-(4-methyl-thiazol-5-yl) ethyl nitrate maleate (NMZM), a derivative of clomethiazole (CMZ), had been investigated for the treatment of Alzheimer's disease (AD). The beneficial effects of NMZM in AD included reversing cognitive deficit, improving learning and memory as well as neuroprotection. The pharmacological effects of NMZM on GABA_A receptors were reported previously; however, the mechanisms were unclear and were explored therefore.

RESULTS

In this study, we demonstrated that NMZM improved learning and memory by alleviating scopolamine-induced long-term potentiation (LTP) suppression in the dentate gyrus of rats, indicating that NMZM had protective effects against scopolamine-induced depression of LTP. Next, we investigated the action of NMZM on GABA_A receptors in hippocampal neurons and the binding site of NMZM on GABA_A receptors. NMZM directly activated GABA_A receptors in hippocampal neurons in a weak manner. However, NMZM could potentiate the response of GABA_A receptors to GABA and NMZM positively modulated GABA_A receptors with an EC₅₀ value of 465 μmol/L at 3 μmol/L GABA while this potentiation at low concentration of GABA (1, 3 μmol/L) was more significant than that at high concentration (10, 30 μmol/L). In addition, NMZM could enhance GABA currents after using diazepam and pentobarbital, the positive modulators of GABA_A receptors. NMZM could not affect the etomidate-potentiated GABA_A current. It suggested that the binding site of NMZM on GABA_A receptors is the same as etomidate.

CONCLUSIONS

These results provided support for the neuroprotective effect of NMZM, which was partly dependent on the potentiation of GABA_A receptors. The etomidate binding site might be a new target for neuronal protection and for drug development.

Alphaxalone Binds in Inner Transmembrane β+-α- Interfaces of α1β3γ2 γ-Aminobutyric Acid Type A Receptors

BACKGROUND

Neurosteroids like alphaxalone are potent anxiolytics, anticonvulsants, amnestics, and sedative-hypnotics, with effects linked to enhancement of γ-aminobutyric acid type A (GABAA) receptor gating in the central nervous system. Data locating neurosteroid binding sites on synaptic αβγ GABAA receptors are sparse and inconsistent. Some evidence points to outer transmembrane β-α interfacial pockets, near sites that bind the anesthetics etomidate and propofol. Other evidence suggests that steroids bind more intracellularly in β-α interfaces.

METHODS

The authors created 12 single-residue β3 cysteine mutations: β3T262C and β3T266C in β3-M2; and β3M283C, β3Y284C, β3M286C, β3G287C, β3F289C, β3V290C, β3F293C, β3L297C, β3E298C, and β3F301C in β3-M3 helices. The authors coexpressed α1 and γ2L with each mutant β3 subunit in Xenopus oocytes and electrophysiologically tested each mutant for covalent sulfhydryl modification by the water-soluble reagent para-chloromercuribenzenesulfonate. Then, the authors assessed whether receptor-bound alphaxalone, etomidate, or propofol blocked cysteine modification, implying steric hindrance.

RESULTS

Eleven mutant β3 subunits, when coexpressed with α1 and γ2L, formed functional channels that displayed varied sensitivities to the three anesthetics. Exposure to para-chloromercuribenzenesulfonate produced irreversible functional changes in ten mutant receptors. Protection by alphaxalone was observed in receptors with β3V290C, β3F293C, β3L297C, or β3F301C mutations. Both etomidate and propofol protected receptors with β3M286C or β3V290C mutations. Etomidate also protected β3F289C. In α1β3γ2L structural homology models, all these protected residues are located in transmembrane β-α interfaces.

CONCLUSIONS

Alphaxalone binds in transmembrane β-α pockets of synaptic GABAA receptors that are adjacent and intracellular to sites for the potent anesthetics etomidate and propofol.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Aquaporin-4 knockout mice exhibit increased hypnotic susceptibility to ketamine

PURPOSE

This study examines anesthetic/hypnotic effects of ketamine in AQP4 knockout (KO) and wild-type (WT) mice with the particular focus on neurotransmission.

MATERIALS AND METHODS

Ketamine (100 mg/kg) was intraperitoneally injected in 16 WT and 16 KO mice. The hypnotic potencies were evaluated by the loss of the righting reflex (LORR). The amino acids neurotransmitter levels in prefrontal cortex were measured by microdialysis.

RESULTS

This study demonstrated that AQP4 knockout significantly shortened the latency compared with WT mice (98.0 ± 4.2 vs. 138.1 ± 15.0 s, p < .05) and prolonged duration of LORR (884.7 ± 58.6 vs. 562.0 ± 51.7 s, p < .05) compared with WT mice in LORR induced by ketamine. Microdialysis showed that lack of AQP4 markedly decreased glutamate level within 20 min (p < .05) and increased γ-aminobutyric acid (GABA) level within 30-40 min (p < .05) after use of ketamine. Moreover, the levels of taurine were remarkably higher in KO mice than in WT mice, but no obvious differences in aspartate were observed between two genotypes.

CONCLUSION

AQP4 deficiency led to more susceptibility of mice to ketamine, which is probably due to the modulation of specific neurotransmitters, hinting an essential maintenance of synaptic activity mediated by AQP4 in the action of ketamine.

Native System and Cultured Cell Electrophysiology for Investigating Anesthetic Mechanisms

Anesthetic agents interact with a variety of ion channels and membrane-bound receptors, often at agent-specific binding sites of a single protein. These molecular-level interactions are ultimately responsible for producing the clinically anesthetized state. Between these two scales of effect, anesthetic agents can be studied in terms of how they impact the physiology of neuronal circuits, individual neurons, and cells expressing individual receptor types. The acutely dissected hippocampal slice is one of the most extensively studied and characterized preparations of intact neural tissue and serves as a highly useful experimental model system to test hypotheses of anesthetic mechanisms. Specific agent-receptor interactions and their effect on excitable membranes can further be defined with molecular precision in cell-based expression systems. We highlight several approaches in these respective systems that we have used and that also have been used by many investigators worldwide. We emphasize economy and quality control, to allow an experimenter to carry out these types of studies in a rigorous and efficient manner.

Combining Mutations and Electrophysiology to Map Anesthetic Sites on Ligand-Gated Ion Channels

General anesthetics are known to act in part by binding to and altering the function of pentameric ligand-gated ion channels such as nicotinic acetylcholine and γ-aminobutyric acid type A receptors. Combining heterologous expression of the subunits that assemble to form these ion channels, mutagenesis techniques and voltage-clamp electrophysiology have enabled a variety of "structure-function" approaches to questions of where anesthetic binds to these ion channels and how they enhance or inhibit channel function. Here, we review the evolution of concepts and experimental strategies during the last three decades, since molecular biological and electrophysiological tools became widely used. Topics covered include: (1) structural models as interpretive frameworks, (2) various electrophysiological approaches and their limitations, (3) Monod-Wyman-Changeux allosteric models as functional frameworks, (4) structural strategies including chimeras and point mutations, and (5) methods based on cysteine substitution and covalent modification. We discuss in particular depth the experimental design considerations for substituted cysteine modification-protection studies.

Comparison of αβδ and αβγ GABAA receptors: Allosteric modulation and identification of subunit arrangement by site-selective general anesthetics

GABA_A receptors play a dominant role in mediating inhibition in the mature mammalian brain, and defects of GABAergic neurotransmission contribute to the pathogenesis of a variety of neurological and psychiatric disorders. Two types of GABAergic inhibition have been described: αβγ receptors mediate phasic inhibition in response to transient high-concentrations of synaptic GABA release, and αβδ receptors produce tonic inhibitory currents activated by low-concentration extrasynaptic GABA. Both αβδ and αβγ receptors are important targets for general anesthetics, which induce apparently different changes both in GABA-dependent receptor activation and in desensitization in currents mediated by αβγ vs. αβδ receptors. Many of these differences are explained by correcting for the high agonist efficacy of GABA at most αβγ receptors vs. much lower efficacy at αβδ receptors. The stoichiometry and subunit arrangement of recombinant αβγ receptors are well established as β-α-γ-β-α, while those of αβδ receptors remain controversial. Importantly, some potent general anesthetics selectively bind in transmembrane inter-subunit pockets of αβγ receptors: etomidate acts at β⁺/α^- interfaces, and the barbiturate R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) acts at α⁺/β^- and γ⁺/β^- interfaces. Thus, these drugs are useful as structural probes in αβδ receptors formed from free subunits or concatenated subunit assemblies designed to constrain subunit arrangement. Although a definite conclusion cannot be drawn, studies using etomidate and R-mTFD-MPAB support the idea that recombinant α1β3δ receptors may share stoichiometry and subunit arrangement with α1β3γ2 receptors.

α subunits in GABAA receptors are dispensable for GABA and diazepam action

The major isoform of the GABA_A receptor is α₁β₂γ₂. The binding sites for the agonist GABA are located at the β₂+/α₁− subunit interfaces and the modulatory site for benzodiazepines at α₁+/γ₂−. In the absence of α₁ subunits, a receptor was formed that was gated by GABA and modulated by diazepam similarly. This indicates that alternative subunits can take over the role of the α₁ subunits. Point mutations were introduced in β₂ or γ₂ subunits at positions homologous to α₁− benzodiazepine binding and GABA binding positions, respectively. From this mutation work we conclude that the site for GABA is located at a β₂+/β₂− subunit interface and that the diazepam site is located at the β₂+/γ₂− subunit interface. Computational docking leads to a structural hypothesis attributing this non-canonical interaction to a binding mode nearly identical with the one at the α₁+/γ₂− interface. Thus, the β₂ subunit can take over the role of the α₁ subunit for the formation of both sites, its minus side for the GABA binding site and its plus side for the diazepam binding site.

Orthosteric- versus allosteric-dependent activation of the GABAA receptor requires numerically distinct subunit level rearrangements

Anaesthetic molecules act on synaptic transmission via the allosteric modulation of ligand-gated chloride channels, such as hetero-oligomeric α₁β₂γ₂ GABA_A receptors. To elucidate the overall activation paradigm via allosteric versus orthosteric sites, we used highly homologous, but homo-oligomeric, ρ₁ receptors that are contrastingly insensitive to anaesthetics and respond partially to several full GABA α₁β₂γ₂ receptor agonists. Here, we coexpressed varying ratios of RNAs encoding the wild-type and the mutated ρ₁ subunits, which are anaesthetic-sensitive and respond with full efficacy to partial GABA agonists, to generate distinct ensembles of receptors containing five, four, three, two, one, or zero mutated subunits. Using these experiments, we then demonstrate that, in the pentamer, three anaesthetic-sensitive ρ₁ subunits are needed to impart full efficacy to the partial GABA agonists. By contrast, five anaesthetic-sensitive subunits are required for direct activation by anaesthetics alone, and only one anaesthetic-sensitive subunit is sufficient to confer the anaesthetic-dependent potentiation to the GABA current. In conclusion, our data indicate that GABA and anaesthetics holistically activate the GABA_A ρ₁ receptor through distinct subunit level rearrangements and suggest that in contrast to the global impact of GABA via orthosteric sites, the force of anaesthetics through allosteric sites may not propagate to the neighbouring subunits and, thus, may have only a local and limited effect on the ρ₁ GABA_A receptor model system.

Novel Molecule Exhibiting Selective Affinity for GABAA Receptor Subtypes

Aminoquinoline derivatives were evaluated against a panel of receptors/channels/transporters in radioligand binding experiments. One of these derivatives (DCUK-OEt) displayed micromolar affinity for brain γ-aminobutyric acid type A (GABA_A) receptors. DCUK-OEt was shown to be a positive allosteric modulator (PAM) of GABA currents with α1β2γ2, α1β3γ2, α5β3γ2 and α1β3δ GABA_A receptors, while having no significant PAM effect on αβ receptors or α1β1γ2, α1β2γ1, α4β3γ2 or α4β3δ receptors. DCUK-OEt modulation of α1β2γ2 GABA_A receptors was not blocked by flumazenil. The subunit requirements for DCUK-OEt actions distinguished DCUK-OEt from other currently known modulators of GABA function (e.g., anesthetics, neurosteroids or ethanol). Simulated docking of DCUK-OEt at the GABA_A receptor suggested that its binding site may be at the α + β- subunit interface. In slices of the central amygdala, DCUK-OEt acted primarily on extrasynaptic GABA_A receptors containing the α1 subunit and generated increases in extrasynaptic “tonic” current with no significant effect on phasic responses to GABA. DCUK-OEt is a novel chemical structure acting as a PAM at particular GABA_A receptors. Given that neurons in the central amygdala responding to DCUK-OEt were recently identified as relevant for alcohol dependence, DCUK-OEt should be further evaluated for the treatment of alcoholism.

Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes

IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing γ-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity also show remarkably selective interactions with distinct interfacial sites. These results suggest strategies for developing new drugs that selectively modulate distinct GABAA receptor subtypes.