Enhanced tonic inhibition influences the hypnotic and amnestic actions of the intravenous anesthetics etomidate and propofol

Preventing Phosphorylation of the GABAAR β3 Subunit Compromises the Behavioral Effects of Neuroactive Steroids

Neuroactive steroids (NASs) have potent anxiolytic, anticonvulsant, sedative, and hypnotic actions, that reflect in part their efficacy as GABA_AR positive allosteric modulators (PAM). In addition to this, NAS exert metabotropic effects on GABAergic inhibition via the activation of membrane progesterone receptors (mPRs), which are G-protein coupled receptors. mPR activation enhances the phosphorylation of residues serine 408 and 409 (S408/9) in the β3 subunit of GABA_ARs, increasing their accumulation in the plasma membrane leading to a sustained increase in tonic inhibition. To explore the significance of NAS-induced phosphorylation of GABA_ARs, we used mice in which S408/9 in the β3 subunit have been mutated to alanines, mutations that prevent the metabotropic actions of NASs on GABA_AR function while preserving NAS allosteric potentiation of GABAergic current. While the sedative actions of NAS were comparable to WT, their anxiolytic actions were reduced in S408/9A mice. Although the induction of hypnosis by NAS were maintained in the mutant mice the duration of the loss of righting reflex was significantly shortened. Finally, ability of NAS to terminate diazepam pharmacoresistant seizures was abolished in S408/9A mice. In conclusion, our results suggest that S408/9 in the GABA_AR β3 subunit contribute to the anxiolytic and anticonvulsant efficacy of NAS, in addition to their ability to regulate the loss of righting reflex.

Propofol modulates inhibitory inputs in paraventricular thalamic nucleus of mice

The mechanisms of general anaesthetics such as propofol have drawn substantial attention. The effects of propofol on inhibitory postsynaptic currents are not exactly the same in different brain nuclei. Recent studies revealed that the paraventricular thalamic nucleus (PVT) is a critical nucleus modulating wakefulness. However, the effects of propofol on PVT neurons and the mechanisms underlying such effects remain unknown. Here, we performed the whole-cell recording of the PVT neurons in acute brain slices and bath application of propofol. We found that propofol hyperpolarized the membrane potentials of the PVT neurons and suppressed the action potentials induced by step-current injection. Propofol did not affect the spontaneous inhibitory postsynaptic currents (sIPSCs) amplitude or frequency, but prolonged the sIPSCs half-width. Besides, propofol increased miniature inhibitory synaptic currents (mIPSCs) frequency and half-width. Furthermore, propofol could induce GABA_A receptors-mediated tonic inhibitory currents dose-dependently. Thus, our results demonstrate that propofol hyperpolarizes PVT neurons by modulating inhibitory currents via GABA_A receptors in mice.

Identification of a Core Amino Acid Motif within the α Subunit of GABAARs that Promotes Inhibitory Synaptogenesis and Resilience to Seizures

SUMMARY

The fidelity of inhibitory neurotransmission is dependent on the accumulation of γ-aminobutyric acid type A receptors (GABA_ARs) at the appropriate synaptic sites. Synaptic GABA_ARs are constructed from α(1–3), β(1–3), and γ2 subunits, and neurons can target these subtypes to specific synapses. Here, we identify a 15-amino acid inhibitory synapse targeting motif (ISTM) within the α2 subunit that promotes the association between GABA_ARs and the inhibitory scaffold proteins collybistin and gephyrin. Using mice in which the ISTM has been introduced into the α1 subunit (Gabra1–2 mice), we show that the ISTM is critical for axo-axonic synapse formation, the efficacy of GABAergic neurotransmission, and seizure sensitivity. The Gabra1–2 mutation rescues seizure-induced lethality in Gabra2–1 mice, which lack axo-axonic synapses due to the deletion of the ISTM from the α2 subunit. Taken together, our data demonstrate that the ISTM plays a critical role in promoting inhibitory synapse formation, both in the axonic and somatodendritic compartments.

In Brief

Molecular mechanisms regulating specific synaptic GABA_AR accumulation are critical for the fidelity of inhibitory neurotransmission. Nathanson et al. show that strengthening the interaction between α1-GABA_ARs and collybistin via genetic manipulation results in augmented synaptic targeting of these receptors, enhanced inhibitory neurotransmission, and seizure resilience.

Accidental awareness while under general anaesthesia

Accidental awareness during general anaesthesia may cause many intraoperative discomforts and bring further moderate to severe long-term symptoms including flashbacks, nightmares, hyperarousal or post-traumatic stress disorder. The incidence of awareness varied from 0.017% to 4% among studies. The relatively reliable incidence of intraoperative awareness with postoperative recall is 0.02%. The reason causing awareness was unclear. Insufficient anaesthetic dosing was thought as the principal cause. Even awareness was not comprehensively understood, some endeavors have been raised to prevent or reduce it, including i) Reducing the insufficient anaesthetic dosing induced by negligence; ii) Providing close clinical observation and clinical parameters from the monitor such as bispectral index or electroencephalogram, as well as isolated forearm technique and passive brain-computer interface may bring some effects sometimes. Because current studies still have some flaws, further trials with new detecting approach, superior methodology and underlying aetiology are needed to unfasten the possible factors causing awareness.

Metabotropic, but not allosteric, effects of neurosteroids on GABAergic inhibition depend on the phosphorylation of GABAA receptors

Neuroactive steroids (NASs) are synthesized within the brain and exert profound effects on behavior. These effects are primarily believed to arise from the activities of NASs as positive allosteric modulators (PAMs) of the GABA-type A receptor (GABAAR). NASs also activate a family of G protein-coupled receptors known as membrane progesterone receptors (mPRs). Here, using surface-biotinylation assays and electrophysiology techniques, we examined mPRs' role in mediating the effects of NAS on the efficacy of GABAergic inhibition. Selective mPR activation enhanced phosphorylation of Ser-408 and Ser-409 (Ser-408/9) within the GABAAR β3 subunit, which depended on the activity of cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC). mPR activation did not directly modify GABAAR activity and had no acute effects on phasic or tonic inhibition. Instead, mPR activation induced a sustained elevation in tonic current, which was blocked by PKA and PKC inhibition. Substitution of Ser-408/9 to alanine residues also prevented the effects of mPR activation on tonic current. Furthermore, this substitution abolished the effects of sustained NAS exposure on tonic inhibition. Interestingly, the allosteric effects of NAS on GABAergic inhibition were independent of Ser-408/9 in the β3 subunit. Additionally, although allosteric effects of NAS on GABAergic inhibition were sensitive to a recently developed "NAS antagonist," the sustained effects of NAS on tonic inhibition were not. We conclude that metabotropic effects of NAS on GABAergic inhibition are mediated by mPR-dependent modulation of GABAAR phosphorylation. We propose that this mechanism may contribute to the varying behavioral effects of NAS.

Neuroactive Steroids Reverse Tonic Inhibitory Deficits in Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. A reduction in neuronal inhibition mediated by γ-aminobutyric acid type A receptors (GABA_ARs) has been implicated in the pathophysiology of FXS. Neuroactive steroids (NASs) are known allosteric modulators of GABA_AR channel function, but recent studies from our laboratory have revealed that NASs also exert persistent metabotropic effects on the efficacy of tonic inhibition by increasing the protein kinase C (PKC)-mediated phosphorylation of the α4 and β3 subunits which increase the membrane expression and boosts tonic inhibition. We have assessed the GABAergic signaling in the hippocampus of fragile X mental retardation protein (FMRP) knock-out (Fmr1KO) mouse. The GABAergic tonic current in dentate gyrus granule cells (DGGCs) from 3- to 5-week-old (p21–35) Fmr1KO mice was significantly reduced compared to WT mice. Additionally, spontaneous inhibitory post synaptic inhibitory current (sIPSC) amplitudes were increased in DGGCs from Fmr1 KO mice. While sIPSCs decay in both genotypes was prolonged by the prototypic benzodiazepine diazepam, those in Frm1-KO mice were selectively potentiated by RO15-4513. Consistent with this altered pharmacology, modifications in the expression levels and phosphorylation of receptor GABA_AR subtypes that mediate tonic inhibition were seen in Fmr1 KO mice. Significantly, exposure to NASs induced a sustained elevation in tonic current in Fmr1 KO mice which was prevented with PKC inhibition. Likewise, exposure reduced elevated membrane excitability seen in the mutant mice. Collectively, our results suggest that NAS act to reverse the deficits of tonic inhibition seen in FXS, and thereby reduce aberrant neuronal hyperexcitability seen in this disorder.

Etomidate Effects on Desensitization and Deactivation of α4β3δ GABAA Receptors Inducibly Expressed in HEK293 TetR Cells

Central α4βδ receptors are the most abundant isoform of δ subunit-containing extrasynaptic GABAA receptors that mediate tonic inhibition. Although the amplitude of GABA-activated currents through α4βδ receptors is modulated by multiple general anesthetics, the effects of general anesthetics on desensitization and deactivation of α4βδ receptors remain unknown. In the current study, we investigated the effect of etomidate, a potent general anesthetic, on the kinetics and the pseudo steady-state current amplitude of α4β3δ receptors inducibly expressed in human embryonic kidney 293 TetR cells. Etomidate directly activates α4β3δ receptors in a concentration-dependent manner. Etomidate at a clinically relevant concentration (3.2 μM) enhances maximal response without altering the EC50 of GABA concentration response. Etomidate also increases the extent of desensitization and prolongs the deactivation of α4β3δ receptors in the presence of maximally activating concentrations of GABA (1 mM). To mimic the modulatory effect of etomidate on tonic currents, long pulses (30-60 seconds) of a low GABA concentration (1 μM) were applied to activate α4β3δ receptors in the absence and presence of etomidate. Although etomidate increases the desensitization of α4β3δ receptors, the pseudo steady-state current amplitude at 1 μM GABA is augmented by etomidate. Our data demonstrate that etomidate enhances the pseudo steady-state current of α4β3δ receptors evoked by a GABA concentration comparable to an ambient GABA level, suggesting that α4β3δ receptors may mediate etomidate's anesthetic effect in the brain.

Zebrafish: A Pharmacogenetic Model for Anesthesia

General anesthetics are small molecules that interact with and effect the function of many different proteins to promote loss of consciousness, amnesia, and sometimes, analgesia. Owing to the complexity of this state transition and the transient nature of these drug/protein interactions, anesthetics can be difficult to study. The zebrafish is an emerging model for the discovery of both new genes required for the response to and side effects of anesthesia. Here we discuss the tools available to manipulate the zebrafish genome, including both genetic screens and genome engineering approaches. Additionally, there are various robust behavior assays available to study anesthetic and other drug responses. These assays are available for single-gene study or high throughput for genetic or drug discovery. Finally, we present a case study of using propofol as an anesthetic in the zebrafish. These techniques and protocols make the zebrafish a powerful model to study anesthetic mechanisms and drug discovery.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

The Influence of Regional Distribution and Pharmacologic Specificity of GABAAR Subtype Expression on Anesthesia and Emergence

Anesthetics produce unconsciousness by modulating ion channels that control neuronal excitability. Research has shown that specific GABA_A receptor (GABA_AR) subtypes in particular regions of the central nervous system contribute to different hyperpolarizing conductances, and behaviorally to distinct components of the anesthetized state. The expression of these receptors on the neuron cell surface, and thus the strength of inhibitory neurotransmission, is dynamically regulated by intracellular trafficking mechanisms. Pharmacologic or activity-based perturbations to these regulatory systems have been implicated in pathology of several neurological conditions, and can alter the individual response to anesthesia. Furthermore, studies are beginning to uncover how anesthetic exposure itself elicits enduring changes in subcellular physiology, including the processes that regulate ion channel trafficking. Here, we review the mechanisms that determine GABA_AR surface expression, and elaborate on influences germane to anesthesia and emergence. We address known trafficking differences between the intrasynaptic receptors that mediate phasic current and the extra-synaptic receptors mediating tonic current. We also describe neurophysiologic consequences and network-level abnormalities in brain function that result from receptor trafficking aberrations. We hypothesize that the relationship between commonly used anesthetic agents and GABA_AR surface expression has direct consequences on mature functioning neural networks and by extension ultimately influence the outcome of patients that undergo general anesthesia. Rational design of new anesthetics, anesthetic techniques, EEG-based monitoring strategies, or emergence treatments will need to take these effects into consideration.

Regulating the Efficacy of Inhibition Through Trafficking of γ-Aminobutyric Acid Type A Receptors

Trafficking of anesthetic-sensitive receptors within the plasma membrane, or from one cellular component to another, occurs continuously. Changes in receptor trafficking have implications in altering anesthetic sensitivity. γ-Aminobutyric acid type A receptors (GABAARs) are anion-permeable ion channels and are the major class of receptor in the adult mammalian central nervous system that mediates inhibition. GABAergic signaling allows for precise synchronized firing of action potentials within brain circuits that is critical for cognition, behavior, and consciousness. This precision depends upon tightly controlled trafficking of GABAARs into the membrane. General anesthetics bind to and allosterically enhance GABAARs by prolonging the open state of the receptor and thereby altering neuronal and brain circuit activity. Subunit composition and GABAAR localization strongly influence anesthetic end points; therefore, changes in GABAAR trafficking could have significant consequences to anesthetic sensitivity. GABAARs are not static membrane structures but are in a constant state of flux between extrasynaptic and synaptic locations and are continually endocytosed and recycled from and to the membrane. Neuronal activity, posttranslational modifications, and some naturally occurring and synthetic compounds can influence the expression and trafficking of GABAARs. In this article, we review GABAARs, their trafficking, and how phosphorylation of GABAAR subunits can influence the surface expression and function of the receptor. Ultimately, alterations of GABAAR trafficking could modify anesthetic end points, both unintentionally through pathologic processes but potentially as a therapeutic target to adjust anesthetic-sensitive GABAARs.

Neonatal Propofol and Etomidate Exposure Enhance Inhibitory Synaptic Transmission in Hippocampal Cornus Ammonis 1 Pyramidal Neurons

Background:

Propofol and etomidate are the most important intravenous general anesthetics in the current clinical use and that mediate gamma-aminobutyric acid's (GABAergic) synaptic transmission. However, their long-term effects on GABAergic synaptic transmission induced by neonatal propofol or etomidate exposure remain unclear. We investigated the long-term GABAergic neurotransmission alterations, following neonatal propofol and etomidate administration.

Methods:

Sprague-Dawley rat pups at postnatal days 4–6 were underwent 6-h-long propofol-induced or 5-h-long etomidate-induced anesthesia. We performed whole-cell patch-clamp recording from pyramidal cells in the cornus ammonis 1 area of acute hippocampal slices of postnatal 80–90 days. Spontaneous and miniature inhibitory GABAergic currents (spontaneous inhibitory postsynaptic currents [sIPSCs] and miniature inhibitory postsynaptic currents [mIPSCs]) and their kinetic characters were measured. The glutamatergic tonic effect on inhibitory transmission and the effect of bumetanide on neonatal propofol exposure were also examined.

Results:

Neonatal propofol exposure significantly increased the frequency of mIPSCs (from 1.87 ± 0.35 Hz to 3.43 ± 0.51 Hz, P < 0.05) and did not affect the amplitude of mIPSCs and sIPSCs. Both propofol and etomidate slowed the decay time of mIPSCs kinetics (168.39 ± 27.91 ms and 267.02 ± 100.08 ms vs. 68.18 ± 12.43 ms; P < 0.05). Bumetanide significantly blocked the frequency increase and reversed the kinetic alteration of mIPSCs induced by neonatal propofol exposure (3.01 ± 0.45 Hz and 94.30 ± 32.56 ms).

Conclusions:

Neonatal propofol and etomidate exposure has long-term effects on inhibitory GABAergic transmission. Propofol might act at pre- and post-synaptic GABA receptor A (GABA_A) receptors within GABAergic synapses and impairs the glutamatergic tonic input to GABAergic synapses; etomidate might act at the postsynaptic site.

General Anesthetic Binding Sites in Human α4β3δ γ-Aminobutyric Acid Type A Receptors (GABAARs)

Extrasynaptic γ-aminobutyric acid type A receptors (GABAARs),which contribute generalized inhibitory tone to the mammalian brain, are major targets for general anesthetics. To identify anesthetic binding sites in an extrasynaptic GABAAR, we photolabeled human α4β3δ GABAARs purified in detergent with [3H]azietomidate and a barbiturate, [3H]R-mTFD-MPAB, photoreactive anesthetics that bind with high selectivity to distinct but homologous intersubunit binding sites in the transmembrane domain of synaptic α1β3γ2 GABAARs. Based upon 3H incorporation into receptor subunits resolved by SDS-PAGE, there was etomidate-inhibitable labeling by [3H]azietomidate in the α4 and β3 subunits and barbiturate-inhibitable labeling by [3H]R-mTFD-MPAB in the β3 subunit. These sites did not bind the anesthetic steroid alphaxalone, which enhanced photolabeling, or DS-2, a δ subunit-selective positive allosteric modulator, which neither enhanced nor inhibited photolabeling. The amino acids labeled by [3H]azietomidate or [3H]R-mTFD-MPAB were identified by N-terminal sequencing of fragments isolated by HPLC fractionation of enzymatically digested subunits. No evidence was found for a δ subunit contribution to an anesthetic binding site. [3H]azietomidate photolabeling of β3Met-286 in βM3 and α4Met-269 in αM1 that was inhibited by etomidate but not by R-mTFD-MPAB established that etomidate binds to a site at the β3+-α4- interface equivalent to its site in α1β3γ2 GABAARs. [3H]Azietomidate and [3H]R-mTFD-MPAB photolabeling of β3Met-227 in βM1 established that these anesthetics also bind to a homologous site, most likely at the β3+-β3- interface, which suggests a subunit arrangement of β3α4β3δβ3.

Clarithromycin increases neuronal excitability in CA3 pyramidal neurons through a reduction in GABAergic signaling

Antibiotics are used in the treatment and prevention of bacterial infections, but effects on neuron excitability have been documented. A recent study demonstrated that clarithromycin alleviates daytime sleepiness in hypersomnia patients (Trotti LM, Saini P, Freeman AA, Bliwise DL, García PS, Jenkins A, Rye DB. J Psychopharmacol 28: 697-702, 2014). To explore the potential application of clarithromycin as a stimulant, we performed whole cell patch-clamp recordings in rat pyramidal cells from the CA3 region of hippocampus. In the presence of the antibiotic, rheobase current was reduced by 50%, F-I relationship (number of action potentials as a function of injected current) was shifted to the left, and the resting membrane potential was more depolarized. Clarithromycin-induced hyperexcitability was dose dependent; doses of 30 and 300 μM clarithromycin significantly increased the firing frequency and membrane potential compared with controls (P = 0.003, P < 0.0001). We hypothesized that clarithromycin enhanced excitability by reducing GABA_A receptor activation. Clarithromycin at 30 μM significantly reduced (P = 0.001) the amplitude of spontaneous miniature inhibitory GABAergic currents and at 300 μM had a minor effect on action potential width. Additionally, we tested the effect of clarithromycin in an ex vivo seizure model by evaluating its effect on spontaneous local field potentials. Bath application of 300 μM clarithromycin enhanced burst frequency twofold compared with controls (P = 0.0006). Taken together, these results suggest that blocking GABAergic signaling with clarithromycin increases cellular excitability and potentially serves as a stimulant, facilitating emergence from anesthesia or normalizing vigilance in hypersomnia and narcolepsy. However, the administration of clarithromycin should be carefully considered in patients with seizure disorders.

NEW & NOTEWORTHY

Clinical administration of the macrolide antibiotic clarithromycin has been associated with side effects such as mania, agitation, and delirium. Here, we investigated the adverse effects of this antibiotic on CA3 pyramidal cell excitability. Clarithromycin induces hyperexcitability in single neurons and is related to a reduction in GABAergic signaling. Our results support a potentially new application of clarithromycin as a stimulant to facilitate emergence from anesthesia or to normalize vigilance.

Endogenous and synthetic neuroactive steroids evoke sustained increases in the efficacy of GABAergic inhibition via a protein kinase C-dependent mechanism

The neuroactive steroid (NAS) tetrahydrodeoxycorticosterone (THDOC) increases protein kinase C (PKC) mediated phosphorylation of extrasynaptic GABA_A receptor (GABA_AR) subunits leading to increased surface expression of α4/β3 subunit-containing extrasynaptic GABA_ARs, leading to a sustained increase in GABA_AR tonic current density. Whether other naturally occurring and synthetic NASs share both an allosteric and metabotropic action on GABA_ARs is unknown. Here, we examine the allosteric and metabotropic properties of allopregnanolone (ALLO), and synthetic NASs SGE-516 and ganaxolone. ALLO, SGE-516, and ganaxolone all allosterically enhanced prototypical synaptic and extrasynaptic recombinant GABA_ARs. In dentate gyrus granule cells (DGGCs) all three NASs, when applied acutely, allosterically enhanced tonic and phasic GABAergic currents. In separate experiments, slices were exposed to NASs for 15 min, and then transferred to a steroid naïve recording chamber followed by ≥ 30 min wash before tonic currents were measured. A sustained increase in tonic current was observed following exposure to ALLO, or SGE-516 and was prevented by inhibiting PKC with GF 109203X. No increase in tonic current was observed with exposure to ganaxolone. In agreement with the observations of an increased tonic current, the NASs ALLO and SGE-516 increased the phosphorylation and surface expression of the β3 subunit-containing GABA_ARs. Our studies demonstrate that neuroactive steroids have differential abilities to induce sustained increases in the efficacy of tonic inhibition by promoting GABA_AR phosphorylation and membrane trafficking dependent on PKC activity.

Genetic Deletion of the Clathrin Adaptor GGA3 Reduces Anxiety and Alters GABAergic Transmission

Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) is a monomeric clathrin adaptor that has been shown to regulate the trafficking of the Beta-site APP-cleaving enzyme (BACE1), which is required for production of the Alzheimer’s disease (AD)-associated amyloid βpeptide. Our previous studies have shown that BACE1 is degraded via the lysosomal pathway and that depletion of GGA3 results in increased BACE1 levels and activity owing to impaired lysosomal trafficking and degradation. We further demonstrated the role of GGA3 in the regulation of BACE1 in vivo by showing that BACE1 levels are increased in the brain of GGA3 null mice. We report here that GGA3 deletion results in novelty-induced hyperactivity and decreased anxiety-like behaviors. Given the pivotal role of GABAergic transmission in the regulation of anxiety-like behaviors, we performed electrophysiological recordings in hippocampal slices and found increased phasic and decreased tonic inhibition in the dentate gyrus granule cells (DGGC). Moreover, we found that the number of inhibitory synapses is increased in the dentate gyrus of GGA3 null mice in further support of the electrophysiological data. Thus, the increased GABAergic transmission is a leading candidate mechanism underlying the reduced anxiety-like behaviors observed in GGA3 null mice. All together these findings suggest that GGA3 plays a key role in GABAergic transmission. Since BACE1 levels are elevated in the brain of GGA3 null mice, it is possible that at least some of these phenotypes are a consequence of increased processing of BACE1 substrates.

Astrocytes contribute to the effects of etomidate on synaptic transmission in rat primary somatosensory cortex

Little is known about the mechanisms of unconsciousness induced by general anesthetics. Previous studies have shown that the primary somatosensory cortex (S1) is a sensitive region to a variety of intravenous general anesthetics. Etomidate is a widely used intravenous anesthetic that can influence synaptic transmission. Recently, there are some evidences suggesting that astrocytes, a type of glia cell, also contribute to information transmission in the brain, and modulate synaptic function by releasing neuroactive substances. However, it is unknown whether astrocytes influence the effects of etomidate on information transmission in S1 pyramidal neurons. In the present study, the role of astrocytes in etomidate-induced unconsciousness was investigated by using the whole-cell patch clamp technique. We observed etomidate at clinically relevant concentrations inhibited the spontaneous postsynaptic currents (sPSCs) of rat S1 pyramidal neurons in a concentration-dependent manner, and the EC50 value of etomidate for inhibiting sPSCs from the concentration-effect curve was 6.9μM. Furthermore, in the presence of fluorocitrate, a glia-selective metabolism inhibitor that blocks the aconitase enzyme, both the amplitude and frequency of sPSCs in rat S1 pyramidal neurons were reduced, and the inhibitory effects of etomidate on sPSCs amplitude was strengthened without affecting the effects of etomidate on frequency. From these data, we deduce that etomidate suppresses synaptic activity via presynaptic and postsynaptic components. Furthermore, astrocytes participate in synaptic transmission and influence the effects of etomidate on postsynaptic receptors. This study provides new insight into the role of astrocytes in etomidate-induced unconsciousness.

Compromising the phosphodependent regulation of the GABAAR β3 subunit reproduces the core phenotypes of autism spectrum disorders

Alterations in the efficacy of neuronal inhibition mediated by GABAA receptors (GABAARs) containing β3 subunits are continually implicated in autism spectrum disorders (ASDs). In vitro, the plasma membrane stability of GABAARs is potentiated via phosphorylation of serine residues 408 and 409 (S408/9) in the β3 subunit, an effect that is mimicked by their mutation to alanines. To assess if modifications in β3 subunit expression contribute to ASDs, we have created a mouse in which S408/9 have been mutated to alanines (S408/9A). S408/9A homozygotes exhibited increased phasic, but decreased tonic, inhibition, events that correlated with alterations in the membrane stability and synaptic accumulation of the receptor subtypes that mediate these distinct forms of inhibition. S408/9A mice exhibited alterations in dendritic spine structure, increased repetitive behavior, and decreased social interaction, hallmarks of ASDs. ASDs are frequently comorbid with epilepsy, and consistent with this comorbidity, S408/9A mice exhibited a marked increase in sensitivity to seizures induced by the convulsant kainic acid. To assess the relevance of our studies using S408/9A mice for the pathophysiology of ASDs, we measured S408/9 phosphorylation in Fmr1 KO mice, a model of fragile X syndrome, the most common monogenetic cause of ASDs. Phosphorylation of S408/9 was selectively and significantly enhanced in Fmr1 KO mice. Collectively, our results suggest that alterations in phosphorylation and/or activity of β3-containing GABAARs may directly contribute to the pathophysiology of ASDs.

Intraoperative awareness: controversies and non-controversies

Intraoperative awareness, with or without recall, continues to be a topic of clinical significance and neurobiological interest. In this article, we review evidence pertaining to the incidence, sequelae, and prevention of intraoperative awareness. We also assess which aspects of the complication are well understood (i.e. non-controversial) and which require further research for clarification (i.e. controversial).

Propofol inhibits SIRT2 deacetylase through a conformation-specific, allosteric site

meta-Azi-propofol (AziPm) is a photoactive analog of the general anesthetic propofol. We photolabeled a myelin-enriched fraction from rat brain with [(3)H]AziPm and identified the sirtuin deacetylase SIRT2 as a target of the anesthetic. AziPm photolabeled three SIRT2 residues (Tyr(139), Phe(190), and Met(206)) that are located in a single allosteric protein site, and propofol inhibited [(3)H]AziPm photolabeling of this site in myelin SIRT2. Structural modeling and in vitro experiments with recombinant human SIRT2 determined that propofol and [(3)H]AziPm only bind specifically and competitively to the enzyme when co-equilibrated with other substrates, which suggests that the anesthetic site is either created or stabilized in enzymatic conformations that are induced by substrate binding. In contrast to SIRT2, specific binding of [(3)H]AziPm or propofol to recombinant human SIRT1 was not observed. Residues that line the propofol binding site on SIRT2 contact the sirtuin co-substrate NAD(+) during enzymatic catalysis, and assays that measured SIRT2 deacetylation of acetylated α-tubulin revealed that propofol inhibits enzymatic function. We conclude that propofol inhibits the mammalian deacetylase SIRT2 through a conformation-specific, allosteric protein site that is unique from the previously described binding sites of other inhibitors. This suggests that propofol might influence cellular events that are regulated by protein acetylation state.

Thalamic δ-subunit containing GABAA receptors promote electrocortical signatures of deep non-REM sleep but do not mediate the effects of etomidate at the thalamus in vivo

Extrasynaptic δ-subunits containing GABAA receptors (δGABAARs) are sensitive targets for several commonly used hypnotic agents and mediate tonic neuronal inhibition. δGABAARs are highly expressed within the thalamus and their activation promotes a switch from tonic to burst firing in vitro. Here we test two hypotheses in vivo. (1) Activation of thalamic δGABAARs will elicit electrocortical signatures consistent with widespread thalamocortical burst firing such as increased delta oscillations (1-4 Hz) and reciprocal changes in spindle-like oscillations (7-14 Hz). (2) These signatures will be recapitulated by the general anesthetic etomidate, if the electrocortical effects of etomidate at the thalamus are mediated by δGABAARs. Microperfusion of the δGABAAR-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; 10 and 50 μM) into the ventrobasal complex produced significant effects on electrocortical activity in wild-type mice, but not in mice lacking δGABAARs (Gabrd(-/-)), i.e., the effects with THIP were dependent on δGABAARs. THIP (1) increased 1-4 Hz power in wakefulness and nonrapid-eye movement (NREM) sleep; (2) reduced spindle-like oscillations in NREM sleep; and (3) increased the speed of stable transitions into NREM sleep, indicating effects on state-space dynamics. In contrast, microperfusion of etomidate (10 and 30 μM) into the ventrobasal complex produced effects on electrocortical activity that were independent of δGABAARs, i.e., effects occurred in wild-type and Gabrd(-/-) mice. Etomidate (1) decreased 1-4 Hz power, increased 8-12 Hz, and/or 12-30 Hz power in all sleep-wake states; (2) increased spindle-like oscillations; and (3) increased REM sleep expression. These results indicate that thalamic δGABAARs promote electrocortical signatures of deep NREM sleep, but do not mediate the effects of etomidate at the thalamus in vivo.

Anesthetic action on extra-synaptic receptors: effects in neural population models of EEG activity

The role of extra-synaptic receptors in the regulation of excitation and inhibition in the brain has attracted increasing attention. Because activity in the extra-synaptic receptors plays a role in regulating the level of excitation and inhibition in the brain, they may be important in determining the level of consciousness. This paper reviews briefly the literature on extra-synaptic GABA and NMDA receptors and their affinity to anesthetic drugs. We propose a neural population model that illustrates how the effect of the anesthetic drug propofol on GABAergic extra-synaptic receptors results in changes in neural population activity and the electroencephalogram (EEG). Our results show that increased tonic inhibition in inhibitory cortical neurons cause a dramatic increase in the power of both δ− and α− bands. Conversely, the effects of increased tonic inhibition in cortical excitatory neurons and thalamic relay neurons have the opposite effect and decrease the power in these bands. The increased δ-activity is in accord with observed data for deepening propofol anesthesia; but is absolutely dependent on the inclusion of extrasynaptic (tonic) GABA action in the model.

Phosphorylation of GABAA receptors influences receptor trafficking and neurosteroid actions

RATIONALE

Gamma-aminobutyric acid type A receptors (GABAARs) are the principal mediators of inhibitory transmission in the mammalian central nervous system. GABAARs can be localized at post-synaptic inhibitory specializations or at extrasynaptic sites. While synaptic GABAARs are activated transiently following the release of GABA from presynaptic vesicles, extrasynaptic GABAARs are typically activated continuously by ambient GABA concentrations and thus mediate tonic inhibition. The tonic inhibitory currents mediated by extrasynaptic GABAARs control neuronal excitability and the strength of synaptic transmission. However, the mechanisms by which neurons control the functional properties of extrasynaptic GABAARs had not yet been explored.

OBJECTIVES

We review GABAARs, how they are assembled and trafficked, and the role phosphorylation has on receptor insertion and membrane stabilization. Finally, we review the modulation of GABAARs by neurosteroids and how GABAAR phosphorylation can influence the actions of neurosteroids.

CONCLUSIONS

Trafficking and stability of functional channels to the membrane surface are critical for inhibitory efficacy. Phosphorylation of residues within GABAAR subunits plays an essential role in the assembly, trafficking, and cell surface stability of GABAARs. Neurosteroids are produced in the brain and are highly efficacious allosteric modulators of GABAAR-mediated current. This allosteric modulation by neurosteroids is influenced by the phosphorylated state of the GABAAR which is subunit dependent, adding temporal and regional variability to the neurosteroid response. Possible links between neurosteroid actions, phosphorylation, and GABAAR trafficking remain to be explored, but potential novel therapeutic targets may exist for numerous neurological and psychological disorders which are linked to fluctuations in neurosteroid levels and GABAA subunit expression.

Propofol modulates phasic and tonic GABAergic currents in spinal ventral horn interneurones

BACKGROUND

Surgical interventions like skin incisions trigger withdrawal reflexes which require motor neurones and local circuit interneurones in the spinal ventral horn. This region plays a key role in mediating immobilizing properties of the GABAergic anaesthetic propofol. However, it is unclear how propofol modulates GABA(A) receptors in the spinal ventral horn and whether tonic or phasic inhibition is involved.

METHODS

Organotypic spinal cord tissue slices were prepared from mice. Whole-cell recordings were performed for quantifying effects of propofol on GABA(A) receptor-mediated phasic transmission and tonic conductance.

RESULTS

Propofol increased GABAergic phasic transmission by a prolongation of the decay time constant in a concentration-dependent manner. The amount of the charge transferred per inhibitory post-synaptic current, described by the area under the curve, was significantly augmented by 1 µM propofol (P<0.01). A GABA(A) receptor-mediated tonic current was not induced by 1 µM propofol but at a concentration of 5 µM (P<0.05).

CONCLUSIONS

Propofol depresses ventral horn interneurones predominantly by phasic rather than by tonic GABA(A) receptor-mediated inhibition. However, the present results suggest that the involvement of a tonic inhibition might contribute to the efficacy of propofol to depress nociceptive reflexes at high concentrations of the anaesthetic.

Study of GABAergic extra-synaptic tonic inhibition in single neurons and neural populations by traversing neural scales: application to propofol-induced anaesthesia

Anaesthetic agents are known to affect extra-synaptic GABAergic receptors, which induce tonic inhibitory currents. Since these receptors are very sensitive to small concentrations of agents, they are supposed to play an important role in the underlying neural mechanism of general anaesthesia. Moreover anaesthetic agents modulate the encephalographic activity (EEG) of subjects and hence show an effect on neural populations. To understand better the tonic inhibition effect in single neurons on neural populations and hence how it affects the EEG, the work considers single neurons and neural populations in a steady-state and studies numerically and analytically the modulation of their firing rate and nonlinear gain with respect to different levels of tonic inhibition. We consider populations of both type-I (Leaky Integrate-and-Fire model) and type-II (Morris-Lecar model) neurons. To bridge the single neuron description to the population description analytically, a recently proposed statistical approach is employed which allows to derive new analytical expressions for the population firing rate for type-I neurons. In addition, the work shows the derivation of a novel transfer function for type-I neurons as considered in neural mass models and studies briefly the interaction of synaptic and extra-synaptic inhibition. We reveal a strong subtractive and divisive effect of tonic inhibition in type-I neurons, i.e. a shift of the firing rate to higher excitation levels accompanied by a change of the nonlinear gain. Tonic inhibition shortens the excitation window of type-II neurons and their populations while maintaining the nonlinear gain. The gained results are interpreted in the context of recent experimental findings under propofol-induced anaesthesia.

Molecular and behavioral characterization of adolescent protein kinase C following high dose ethanol exposure

RATIONALE

Ethanol is commonly used and abused during adolescence. Although adolescents display differential behavioral responses to ethanol, the mechanisms by which this occurs are not known. The protein kinase C (PKC) pathway has been implicated in mediating many ethanol-related effects in adults, as well as gamma-aminobutyric acid (GABA(A)) receptor regulation.

OBJECTIVES

The present study was designed to characterize cortical PKC isoform and GABA(A) receptor subunit expression during adolescence relative to adults as well as assess PKC involvement in ethanol action.

RESULTS

Novel PKC isoforms were elevated, while PKCγ was lower during mid-adolescence relative to adults. Whole-cell lysate and synaptosomal preparations correlated for all isoforms except PKCδ. In parallel, synaptosomal GABAA receptor subunit expression was also developmentally regulated, with GABA(A)R δ and α4 being lower while α1 and γ2 were higher or similar, respectively, in adolescents compared to adults. Following acute ethanol exposure, synaptosomal novel and atypical PKC isoform expression was decreased only in adolescents. Behaviorally, inhibiting PKC with calphostin C, significantly increased ethanol-induced loss of righting reflex (LORR) in adolescents but not adults, whereas activating PKC with phorbol dibutyrate was ineffective in adolescents but decreased LORR duration in adults. Further investigation revealed that inhibiting the cytosolic phospholipase A2/arachidonic acid (cPLA2/AA) pathway increased LORR duration in adolescents, but was ineffective in adults.

CONCLUSIONS

These data indicate that PKC isoforms are variably regulated during adolescence and may contribute to adolescent ethanol-related behavior. Furthermore, age-related differences in the cPLA2/AA pathway may contribute to ethanol's age-related effects on novel and atypical PKC isoform expression and behavior.

Altered localization of the δ subunit of the GABAA receptor in the thalamus of α4 subunit knockout mice

The α4 subunit of the GABAA receptor (GABAAR) is highly expressed in the thalamus where receptors containing the α4 and δ subunits are major mediators of tonic inhibition. The α4 subunit also exhibits considerable plasticity in a number of physiological and pathological conditions, raising questions about the expression of remaining GABAAR subunits when the α4 subunit is absent. Immunohistochemical studies of an α4 subunit knockout (KO) mouse revealed a substantial decrease in δ subunit expression in the ventrobasal nucleus of the thalamus as well as other forebrain regions where the α4 subunit is normally expressed. In contrast, several subunits associated primarily with phasic inhibition, including the α1 and γ2 subunits, were moderately increased. Intracellular localization of the δ subunit was also altered. While δ subunit labeling was decreased within the neuropil, some labeling remained in the cell bodies of many neurons in the ventrobasal nucleus. Confocal microscopy demonstrated co-localization of this labeling with an endoplasmic reticulum marker, and electron microscopy demonstrated increased immunogold labeling near the endoplasmic reticulum in the α4 KO mouse. These results emphasize the strong partnership of the δ and α4 subunit in the thalamus and suggest that the α4 subunit of the GABAAR plays a critical role in trafficking of the δ subunit to the neuronal surface. The findings also suggest that previously observed reductions in tonic inhibition in the α4 subunit KO mouse are likely to be related to alterations in δ subunit expression, in addition to loss of the α4 subunit.

Mechanisms revealed through general anesthetic photolabeling

General anesthetic photolabels are used to reveal molecular targets and molecular binding sites of anesthetic ligands. After identification, the relevance of anesthetic substrates or binding sites can be tested in biological systems. Halothane and photoactive analogs of isoflurane, propofol, etomidate, neurosteroids, anthracene, and long chain alcohols have been used in anesthetic photolabeling experiments. Interrogated protein targets include the nicotinic acetylcholine receptor, GABA_A receptor, tubulin, leukocyte function-associated antigen-1, and protein kinase C. In this review, we summarize insights revealed by photolabeling these targets, as well as general features of anesthetics, such as their propensity to partition to mitochondria and bind voltage-dependent anion channels. The theory of anesthetic photolabel design and the experimental application of photoactive ligands are also discussed.

Disrupting the clustering of GABAA receptor α2 subunits in the frontal cortex leads to reduced γ-power and cognitive deficits

In schizophrenia, cognitive dysfunction is highly predictive of poor patient outcomes and is not responsive to current medications. Postmortem studies have suggested that cognitive deficits in schizophrenia are correlated with modifications in the number and size of inhibitory synapses. To test if these modifications lead to cognitive deficits, we have created a dominant-negative virus [adeno-associated (AAV)-DN1] that disrupts the clustering of γ-aminobutyric acid type A receptors (GABA(A)Rs) at postsynaptic inhibitory specializations. When injected into the frontal cortex of mice, AAV-DN1 impairs GABA(A)R α2 subunit and GABA transporter 1 (GAT-1) clustering, but increases GABA(A)R α1 subunit clustering on the perisomatic region, with no influence on axon-initial segment clustering. Mice expressing AAV-DN1 have prepulse inhibition deficits and impairments in working memory. Significantly, these behavioral deficits are paralleled by a reduction in electroencephalography γ-power. Collectively, our study provides functional evidence revealing that GABAergic synapses in the prefrontal cortex directly contribute to cognition and γ-power.