Sedation and anesthesia mediated by distinct GABA(A) receptor isoforms

Altered GABA(A) receptor subunit expression and pharmacology in human Angelman syndrome cortex

The neurodevelopmental disorder Angelman syndrome is most frequently caused by deletion of the maternally derived chromosome 15q11-q13 region, which includes not only the causative UBE3A gene, but also the beta(3)-alpha(5)-gamma(3) GABA(A) receptor subunit gene cluster. GABAergic dysfunction has been hypothesized to contribute to the occurrence of epilepsy and cognitive and behavioral impairments in this condition. In the present study, analysis of GABA(A) receptor subunit expression and pharmacology was performed in cerebral cortex from four subjects with Angelman syndrome and compared to that from control tissue. The membrane fraction of frozen postmortem neocortical tissue was isolated and subjected to quantitative Western blot analysis. The ratios of beta(3)/beta(2) and alpha(5)/alpha(1) subunit protein expression in Angelman syndrome cortex were significantly decreased when compared with controls. An additional membrane fraction was injected into Xenopus oocytes, resulting in incorporation of the brain membrane vesicles with their associated receptors into the oocyte cellular membrane. Two-electrode voltage-clamp analysis of GABA(A) receptor currents was then performed. Studies of GABA(A) receptor pharmacology in Angelman syndrome cortex revealed increased current enhancement by the alpha(1)-selective benzodiazepine-site agonist zolpidem and by the barbiturate phenobarbital, while sensitivity to current inhibition by zinc was decreased. GABA(A) receptor affinity and modulation by neurosteroids were unchanged. This shift in GABA(A) receptor subunit expression and pharmacology in Angelman syndrome is consistent with impaired extrasynaptic but intact to augmented synaptic cortical GABAergic inhibition, which could contribute to the epileptic, behavioral, and cognitive phenotypes of the disorder.

Molecular mechanisms of general anesthesia

General anesthetics produce a widespread neurodepression in the central nervous system by enhancing inhibitory neurotransmission and reducing excitatory neurotransmission. However, the action mechanisms of general anesthetics are not completely understood. Moreover, the general anesthetic state comprises multiple components (amnesia, unconsciousness, analgesia, and immobility), each of which is mediated by different receptors and neuronal pathways. Recently, neurotransmitter- and voltage-gated ion channels have emerged as the most likely molecular targets for general anesthetics. The γ-aminobutyric acid type A (GABA_A) receptors are leading candidates as a primary target of general anesthetics. This review summarizes current knowledge on how anesthetics modify GABA_A receptor function.

Numerous classes of general anesthetics inhibit etomidate binding to gamma-aminobutyric acid type A (GABAA) receptors

Enhancement of gamma-aminobutyric acid type A receptor (GABA(A)R)-mediated inhibition is a property of most general anesthetics and a candidate for a molecular mechanism of anesthesia. Intravenous anesthetics, including etomidate, propofol, barbiturates, and neuroactive steroids, as well as volatile anesthetics and long-chain alcohols, all enhance GABA(A)R function at anesthetic concentrations. The implied existence of a receptor site for anesthetics on the GABA(A)R protein was supported by identification, using photoaffinity labeling, of a binding site for etomidate within the GABA(A)R transmembrane domain at the beta-alpha subunit interface; the etomidate analog [(3)H]azietomidate photolabeled in a pharmacologically specific manner two amino acids, alpha1Met-236 in the M1 helix and betaMet-286 in the M3 helix (Li, G. D., Chiara, D. C., Sawyer, G. W., Husain, S. S., Olsen, R. W., and Cohen, J. B. (2006) J. Neurosci. 26, 11599-11605). Here, we use [(3)H]azietomidate photolabeling of bovine brain GABA(A)Rs to determine whether other structural classes of anesthetics interact with the etomidate binding site. Photolabeling was inhibited by anesthetic concentrations of propofol, barbiturates, and the volatile agent isoflurane, at low millimolar concentrations, but not by octanol or ethanol. Inhibition by barbiturates, which was pharmacologically specific and stereospecific, and by propofol was only partial, consistent with allosteric interactions, whereas isoflurane inhibition was nearly complete, apparently competitive. Protein sequencing showed that propofol inhibited to the same extent the photolabeling of alpha1Met-236 and betaMet-286. These results indicate that several classes of general anesthetics modulate etomidate binding to the GABA(A)R: isoflurane binds directly to the site with millimolar affinity, whereas propofol and barbiturates inhibit binding but do not bind in a mutually exclusive manner with etomidate.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Neurosteroids: endogenous allosteric modulators of GABA(A) receptors

In the mammalian central nervous system activation of the ionotropic GABA(A) receptor by the neurotransmitter GABA plays a crucial role in controlling neuronal excitability. This essential form of neuronal regulation may be subject to "fine tuning" by particular metabolites of progesterone and deoxycorticosterone, which bind directly to the GABA(A) receptor to enhance the actions of GABA. Originally such steroids were considered to act as endocrine messengers, being synthesised in peripheral glands such as the adrenals and ovaries and crossing the blood brain barrier to influence neuronal signalling. However, it is now evident that certain neurons and glia may produce such "neurosteroids" and that these locally synthesised modulators may act in a paracrine, or indeed an autocrine manner to influence neuronal activity. Neurosteroid synthesis may change dynamically in a variety of physiological situations (e.g. stress, pregnancy) and perturbations in their levels are implicated in a variety of neurological and psychiatric disorders. Here we will consider (1) evidence supporting the concept that neurosteroids act as local regulators of neuronal inhibition, (2) that extrasynaptic GABA(A) receptors appear to be a particularly important neurosteroid target and (3) recent advances in defining the neurosteroid binding site(s) on the GABA(A) receptor.

Distinct actions of etomidate and propofol at beta3-containing gamma-aminobutyric acid type A receptors

Etomidate and propofol have clearly distinguishable effects on the central nervous system. However, studies in knock-in mice provided evidence that these agents produce anesthesia via largely overlapping molecular targets, namely GABA(A) receptors containing beta3 subunits. Here the authors address the question as to whether etomidate and propofol are targeting different subpopulations of beta3 subunit containing GABA(A) receptors. The effects of etomidate and propofol (0.5 muM and 1.0 muM) on spontaneous activity of neocortical neurons were investigated in organotypic slice cultures from wild-type and beta3(N265M) knock-in mice. Firing patterns were characterized by mean burst length and number of action potentials per burst. Additionally, etomidate and propofol actions on GABA(A) receptor-mediated currents were investigated by whole-cell voltage clamp recordings. On the network level, the duration of spontaneously occurring bursts of action potentials was decreased by etomidate but increased by propofol in the wild-type. The effects of etomidate were abolished in beta3(N265M) mutant slices while those of propofol were qualitatively inverted. On the receptor level, GABA(A) receptor-mediated inhibition of cortical neurons was modulated by etomidate and propofol in different ways. Again, drug-specific actions of etomidate and propofol were largely attenuated by the beta3(N265M) mutation. Etomidate and propofol alter the firing patterns and GABA(A) receptor-mediated inhibition of neocortical neurons in different ways. This suggests that etomidate and propofol act via non-uniform molecular targets. Because the major effects induced by these anesthetics were attenuated by the beta3(N265M) mutation, different subpopulations of beta3-containing GABA(A) receptors are likely to be involved.

CaMKII phosphorylation of the GABA(A) receptor: receptor subtype- and synapse-specific modulation

As a major inhibitory neurotransmitter, GABA plays a vital role in the brain by controlling the extent of neuronal excitation. This widespread role is reflected by the ubiquitous distribution of GABA(A) receptors throughout the central nervous system. To regulate the level of neuronal inhibition requires some endogenous control over the release of GABA and/or its postsynaptic response. In this context, Ca(2+) ions are often used as primary or secondary messengers frequently resulting in the activation of protein kinases and phosphatases. One such kinase, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), can target the GABA(A) receptor to cause its phosphorylation. Evidence is now emerging, which is reviewed here, that GABA(A) receptors are indeed substrates for CaMKII and that this covalent modification alters the expression of cell surface receptors and their function. This type of regulation can also feature at inhibitory synapses leading to long-term inhibitory synaptic plasticity. Most recently, CaMKII has now been proposed to differentially phosphorylate particular isoforms of GABA(A) receptors in a synapse-specific context.

Neurosteroids allosterically modulate binding of the anesthetic etomidate to gamma-aminobutyric acid type A receptors

Photoaffinity labeling of gamma-aminobutyric acid type A (GABA(A))-receptors (GABA(A)R) with an etomidate analog and mutational analyses of direct activation of GABA(A)R by neurosteroids have each led to the proposal that these structurally distinct general anesthetics bind to sites in GABA(A)Rs in the transmembrane domain at the interface between the beta and alpha subunits. We tested whether the two ligand binding sites might overlap by examining whether neuroactive steroids inhibited etomidate analog photolabeling. We previously identified (Li, G. D., Chiara, D. C., Sawyer, G. W., Husain, S. S., Olsen, R. W., and Cohen, J. B. (2006) J. Neurosci. 26, 11599-11605) azietomidate photolabeling of GABA(A)R alpha1Met-236 and betaMet-286 (in alphaM1 and betaM3). Positioning these two photolabeled amino acids in a single type of binding site at the interface of beta and alpha subunits (two copies per pentamer) is consistent with a GABA(A)R homology model based upon the structure of the nicotinic acetylcholine receptor and with recent alphaM1 to betaM3 cross-linking data. Biologically active neurosteroids enhance rather than inhibit azietomidate photolabeling, as assayed at the level of GABA(A)R subunits on analytical SDS-PAGE, and protein microsequencing establishes that the GABA(A)R-modulating neurosteroids do not inhibit photolabeling of GABA(A)R alpha1Met-236 or betaMet-286 but enhance labeling of alpha1Met-236. Thus modulatory steroids do not bind at the same site as etomidate, and neither of the amino acids identified as neurosteroid activation determinants (Hosie, A. M., Wilkins, M. E., da Silva, H. M., and Smart, T. G. (2006) Nature 444, 486-489) are located at the subunit interface defined by our etomidate site model.

Role of GABAA receptors in the physiology and pharmacology of sleep

Most sedative-hypnotics used in insomnia treatment target the gamma-aminobutyric acid (GABA)(A) receptors. A vast repertoire of GABA(A) receptor subtypes has been identified and displays specific electrophysiological and functional properties. GABA(A)-mediated inhibition traditionally refers to 'phasic' inhibition, arising from synaptic GABA(A) receptors which transiently inhibit neurons. However, there is growing evidence that peri- or extra-synaptic GABA(A) receptors are continuously activated by low GABA concentrations and mediate a 'tonic' conductance. This slower type of signaling appears to play a key role in controlling cell excitability. This review aims at summarizing recent knowledge on GABA transmission, including the emergence of tonic conductance, and highlighting the importance of GABA(A) receptor heterogeneity. The mechanism of action of sedative-hypnotic drugs and their effects on sleep and the electroencephalogram will be reported. Furthermore, studies using genetically engineered mice will be emphasized, providing insights into the role of GABA(A) receptors in mechanisms underlying physiological and pharmacological sleep. Finally, we will address the potential of GABA(A) receptor pharmacology for the treatment of insomnia.

[The GABA(A) receptor family: possibilities for the development of better anesthetics]

Clinically used anesthetics show amnestic, sedative, hypnotic and immobilizing properties. On a molecular level these drugs affect several receptors in the cell membrane of neurons. By using genetically engineered mice a linkage can now be made between actions on certain receptors and clinically desired and undesired effects. Experiments show that a certain GABA(A) receptor subtype mediates hypnosis and immobility, whereas another subtype is involved in side-effects like sedation and hypothermia. These findings form the basis for the development of new drugs, acting highly specific and with fewer side-effects.

Modeling the GABAergic action of etomidate on the thalamocortical system

BACKGROUND

We have used a computational model of the thalamocortical system to investigate the effects of a GABAergic anesthetic (etomidate) on cerebral cortical and thalamic neuronal function. We examined the effects of phasic and tonic inhibition, as well as the relative importance of anesthetic action in the thalamus and cortex.

METHODS

The amount of phasic GABAergic inhibition was adjusted in the model to simulate etomidate concentrations of between 0.25 and 2 microM, with the concentration range producing unconsciousness assumed to be between 0.25 and 0.5 microM. In addition, we modeled tonic inhibition separately, and then phasic and tonic inhibition together. We also introduced phasic and tonic inhibition into the cerebral cortex and thalamus separately to determine the relative importance of each of these structures to anesthetic-induced depression of the thalamocortical system.

RESULTS

Phasic inhibition decreased cortical neuronal firing by 11%-18% in the 0.25-0.5 microM range and by 38% at 2 microM. Tonic inhibition produced similar depression (11%-21%) in the 0.25-0.5 microM range but 65% depression at 2 microM; phasic and tonic inhibition combined produced the most inhibition (76% depression at 2 microM). When the thalamus and cortex were separately subjected to phasic and tonic inhibition, cortical firing rates decreased less compared to when both structures were targeted. In the 0.25-0.5 microM range, cortical firing rate was minimally affected when etomidate action was simulated in the thalamus only.

CONCLUSIONS

This computational model of the thalamocortical system indicated that tonic GABAergic inhibition seems to be more important than phasic GABAergic inhibition (especially at larger etomidate concentrations), although both combined had the most effect on cerebral cortical firing rates. Furthermore, etomidate action in the thalamus, by itself, does not likely explain etomidate-induced unconsciousness.

GABA A receptors: subtypes provide diversity of function and pharmacology

This mini-review attempts to update experimental evidence on the existence of GABA(A) receptor pharmacological subtypes and to produce a list of those native receptors that exist. GABA(A) receptors are chloride channels that mediate inhibitory neurotransmission. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. They are assembled from a family of 19 homologous subunit gene products and form numerous receptor subtypes with properties that depend upon subunit composition, mostly hetero-oligomeric. These vary in their regulation and developmental expression, and importantly, in brain regional, cellular, and subcellular localization, and thus their role in brain circuits and behaviors. We propose several criteria for including a receptor hetero-oligomeric subtype candidate on a list of native subtypes, and a working GABA(A) receptor list. These criteria can be applied to all the members of the LGIC superfamily. The list is divided into three categories of native receptor subtypes: "Identified", "Existence with High Probability", and "Tentative", and currently includes 26 members, but will undoubtedly grow, with future information. This list was first presented by Olsen & Sieghart (in press).

A transmembrane residue influences the interaction of propofol with the strychnine-sensitive glycine alpha1 and alpha1beta receptor

BACKGROUND

Propofol, well known for its anesthetic effects, acts as a positive allosteric modulator of the alpha-aminobutyric acid type A (GABA(A)) receptor but also enhances the function of the glycine receptor. The GABA modulatory effects of propofol are influenced by an amino acid residue located within the second transmembrane domain (TM2) of the GABA(A) receptor beta subunit. In glycine alpha(1) subunits, the homologous residue (serine 267) affects the glycine modulatory actions of alcohols and alkane anesthetics. In the present study we investigated the role of this residue on the interaction of propofol with the glycine alpha(1) and alpha(1)beta receptor.

METHODS

The influence of propofol on wild type and mutant (alpha(1)S267M, alpha(1)S267I, alpha(1)S267Mbeta, alpha(1)S267Ibeta) glycine receptors expressed in human embryonic kidney 293 cells was investigated by using the whole-cell clamp technique.

RESULTS

Mutation of the alpha(1) subunit TM2 serine residue to either isoleucine or methionine decreased the sensitivity of the receptor to glycine, and abolished the direct activation of the glycine receptor by propofol. Additionally, the methionine and particularly the isoleucine mutation decreased the glycine-enhancing actions of propofol.

CONCLUSIONS

The nature of the TM2 residue (267) of the glycine alpha(1) subunit influences the glycine modulatory effect of propofol and direct activation of the receptor by this anesthetic. A comparison of the impact of such complementary mutations on the interaction of propofol with glycine and GABA(A) receptors should permit a better understanding of the molecular determinants of action of propofol on these structurally related receptors and may aid in the development of selective glycine receptor modulators.

Alcohol-induced tolerance and physical dependence in mice with ethanol insensitive alpha1 GABA A receptors

BACKGROUND

Although many people consume alcohol (ethanol), it remains unknown why some become addicted. Elucidating the molecular mechanisms of tolerance and physical dependence (withdrawal) may provide insight into alcohol addiction. While the exact molecular mechanisms of ethanol action are unclear, gamma-aminobutyric acid type A receptors (GABA(A)-Rs) have been extensively implicated in ethanol action. The alpha1 GABA(A)-R subunit is associated with tolerance and physical dependence, but its exact role remains unknown. In this report, we tested the hypothesis that alpha1-GABA(A)-Rs mediate in part these effects of ethanol.

METHODS

Ethanol-induced behavioral responses related to tolerance and physical dependence were investigated in knockin (KI) mice that have ethanol-insensitive alpha1 GABA(A)-Rs and wildtype (WT) controls. Acute functional tolerance (AFT) was assessed using the stationary dowel and loss of righting reflex (LORR) assays. Chronic tolerance was assessed on the LORR, fixed speed rotarod, hypothermia, and radiant tail-flick assays following 10 consecutive days of ethanol exposure. Withdrawal-related hyperexcitability was assessed by handling-induced convulsions following 3 cycles of ethanol vapor exposure/withdrawal. Immunoblots were used to assess alpha1 protein levels.

RESULTS

Compared with controls, KI mice displayed decreased AFT and chronic tolerance to ethanol-induced motor ataxia, and also displayed heightened ethanol-withdrawal hyperexcitability. No differences between WT and KI mice were seen in other ethanol-induced behavioral measures. Following chronic exposure to ethanol, control mice displayed reductions in alpha1 protein levels, but KIs did not.

CONCLUSIONS

We conclude that alpha1-GABA(A)-Rs play a role in tolerance to ethanol-induced motor ataxia and withdrawal-related hyperexcitability. However, other aspects of behavioral tolerance and physical dependence do not rely on alpha1-containing GABA(A)-Rs.

Population based models of cortical drug response: insights from anaesthesia

A great explanatory gap lies between the molecular pharmacology of psychoactive agents and the neurophysiological changes they induce, as recorded by neuroimaging modalities. Causally relating the cellular actions of psychoactive compounds to their influence on population activity is experimentally challenging. Recent developments in the dynamical modelling of neural tissue have attempted to span this explanatory gap between microscopic targets and their macroscopic neurophysiological effects via a range of biologically plausible dynamical models of cortical tissue. Such theoretical models allow exploration of neural dynamics, in particular their modification by drug action. The ability to theoretically bridge scales is due to a biologically plausible averaging of cortical tissue properties. In the resulting macroscopic neural field, individual neurons need not be explicitly represented (as in neural networks). The following paper aims to provide a non-technical introduction to the mean field population modelling of drug action and its recent successes in modelling anaesthesia.

Tryptophan mutations at azi-etomidate photo-incorporation sites on alpha1 or beta2 subunits enhance GABAA receptor gating and reduce etomidate modulation

The potent general anesthetic etomidate produces its effects by enhancing GABA(A) receptor activation. Its photolabel analog [(3)H]azi-etomidate labels residues within transmembrane domains on alpha and beta subunits: alphaMet236 and betaMet286. We hypothesized that these methionines contribute to etomidate sites formed at alpha-beta subunit interfaces and that increasing side-chain bulk and hydrophobicity at either locus would mimic etomidate binding and block etomidate effects. Channel activity was electrophysiologically quantified in alpha(1)beta(2)gamma(2L) receptors with alpha(1)M236W or beta(2)M286W mutations, in both the absence and the presence of etomidate. Measurements included spontaneous activation, GABA EC(50), etomidate agonist potentiation, etomidate direct activation, and rapid macrocurrent kinetics. Both alpha(1)M236W and beta(2)M286W mutations induced spontaneous channel opening, lowered GABA EC(50), increased maximal GABA efficacy, and slowed current deactivation, mimicking effects of etomidate on alpha(1)beta(2)gamma(2L) channels. These changes were larger with alpha(1)M236W than with beta(2)M286W. Etomidate (3.2 muM) reduced GABA EC(50) much less in alpha(1)M236Wbeta(2)gamma(2L) receptors (2-fold) than in wild type (23-fold). However, etomidate was more potent and efficacious in directly activating alpha(1)M236Wbeta(2)gamma(2L) compared with wild type. In alpha(1)beta(2)M286Wgamma(2L) receptors, etomidate induced neither agonist-potentiation nor direct channel activation. These results support the hypothesis that alpha(1)Met236 and beta(2)Met286 are within etomidate sites that allosterically link to channel gating. Although alpha(1)M236W produced the larger impact on channel gating, beta(2)M286W produced more profound changes in etomidate sensitivity, suggesting a dominant role in drug binding. Furthermore, quantitative mechanistic analysis demonstrated that wild-type and mutant results are consistent with the presence of only one class of etomidate sites mediating both agonist potentiation and direct activation.

International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update

In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."

Etomidate, propofol and the neurosteroid THDOC increase the GABA efficacy of recombinant alpha4beta3delta and alpha4beta3 GABA A receptors expressed in HEK cells

General anesthetics, once thought to exert their effects through non-specific membrane effects, have highly specific ion channel targets that can silence neuronal populations in the nervous system, thereby causing unconsciousness and immobility, characteristic of general anesthesia. Inhibitory GABA(A) receptors (GABA(A)Rs), particularly highly GABA-sensitive extrasynaptic receptor subtypes that give rise to sustained inhibitory currents, are uniquely sensitive to GABA(A)R-active anesthetics. A prominent population of extrasynaptic GABA(A)Rs is made up of alpha4, beta2 or beta3, and delta subunits. Considering the demonstrated importance of GABA receptor beta3 subunits for in vivo anesthetic effects of etomidate and propofol, we decided to investigate the effects of GABA anesthetics on "extrasynaptic" alpha4beta3delta and also binary alpha4beta3 receptors expressed in human embryonic kidney (HEK) cells. Consistent with previous work on similar receptor subtypes we show that maximal GABA currents through "extrasynaptic" alpha4beta3delta receptors, receptors defined by sensitivity to EtOH (30mM) and the beta-carboline beta-CCE (1microM), are enhanced by the GABA(A)R-active anesthetics etomidate, propofol, and the neurosteroid anesthetic THDOC. Furthermore, we show that receptors formed by alpha4beta3 subunits alone also show high GABA sensitivity and that saturating GABA responses of alpha4beta3 receptors are increased to the same extent by etomidate, propofol, and THDOC as are alpha4beta3delta receptors. Therefore, both alpha4beta3 and alpha4beta3delta receptors show low GABA efficacy, and GABA is also a partial agonist on certain binary alphabeta receptor subtypes. Increasing GABA efficacy on alpha4/6beta3delta and alpha4beta3 receptors is likely to make an important contribution to the anesthetic effects of etomidate, propofol and the neurosteroid THDOC.

Anaesthesia for Angelman syndrome

We describe the administration of anaesthesia to a patient with Angelman syndrome, which is characterised by an abnormality of chromosome 15, where a subunit of the GABA receptor is coded. This has far-reaching anaesthetic implications as many drugs used in anaesthesia are thought to act via GABA receptors. Our patient had an uneventful peri-operative period and was discharged home on the second postoperative day.

General anaesthesia: from molecular targets to neuronal pathways of sleep and arousal

The mechanisms through which general anaesthetics, an extremely diverse group of drugs, cause reversible loss of consciousness have been a long-standing mystery. Gradually, a relatively small number of important molecular targets have emerged, and how these drugs act at the molecular level is becoming clearer. Finding the link between these molecular studies and anaesthetic-induced loss of consciousness presents an enormous challenge, but comparisons with the features of natural sleep are helping us to understand how these drugs work and the neuronal pathways that they affect. Recent work suggests that the thalamus and the neuronal networks that regulate its activity are the key to understanding how anaesthetics cause loss of consciousness.

Comparison of the sedative, cognitive, and analgesic effects of nitrous oxide, sevoflurane, and ethanol

BACKGROUND

Anaesthetics which work by different mechanisms may have different patterns of effect. Measurement of these patterns thus may elucidate their mechanisms of action and allow therapeutic choices between the agents.

METHODS

We compared the effects of ethanol (approximately 80 mg per 100 ml), and different end-tidal concentrations of nitrous oxide (15% and 25%) and sevoflurane (0.3% and 0.5%) in volunteers. We measured speed and accuracy in psychomotor tests, reaction time and memory, touch and pain sensitivity to von Frey filaments, and subjective mood for a range of descriptors.

RESULTS

All treatments caused the same degree of overall abnormal feelings, but sevoflurane caused more obtunding (subjective drowsiness, slow reaction times, and loss of memory function) and nitrous oxide was more analgesic. Ethanol caused a marked feeling of drunkenness, but little drowsiness or analgesia.

CONCLUSIONS

In the same volunteer subjects, direct comparison of sub-anaesthetic doses of these agents showed a clear and characteristic pattern of effects. These support the possible mechanisms for these disparate agents and may help choose appropriate agents for specific desired anaesthetic outcomes such as sedation or analgesia.

GABA(A) receptor subtypes underlying general anesthesia

General anesthetics produce a constellation of behavioral responses and widespread neurodepression. Recent studies have implicated the gamma-aminobutyric acid (GABA) subtype A receptor as a primary anesthetic target. During the past decade, considerable progress has been made in dissecting the behavioral effects of anesthetics according to the subunit composition of GABA(A) receptors. In this review, we describe how particular GABA(A) receptor subtypes expressed in different brain regions are critical for the expression of behavioral endpoints, such as amnesia, sedation, and hypnosis.

The expression of GABAA beta subunit isoforms in synaptic and extrasynaptic receptor populations of mouse dentate gyrus granule cells

The subunit composition of GABA(A) receptors influences their biophysical and pharmacological properties, dictates neuronal location and the interaction with associated proteins, and markedly influences the impact of intracellular biochemistry. The focus has been on alpha and gamma subunits, with little attention given to beta subunits. Dentate gyrus granule cells (DGGCs) express all three beta subunit isoforms and exhibit both synaptic and extrasynaptic receptors that mediate 'phasic' and 'tonic' transmission, respectively. To investigate the subcellular distribution of the beta subunits we have utilized the patch-clamp technique to compare the properties of 'tonic' and miniature inhibitory postsynaptic currents (mIPSCs) recorded from DGGCs of hippocampal slices of P20-26 wild-type (WT), beta(2)(-/-), beta(2N265S) (etomidate-insensitive), alpha(1)(-/-) and delta(-/-) mice. Deletion of either the beta(2) or the delta subunit produced a significant reduction of the tonic current and attenuated the increase of this current induced by the delta subunit-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). By contrast, mIPSCs were not influenced by deletion of these genes. Enhancement of the tonic current by the beta(2/3) subunit-selective agent etomidate was significantly reduced for DGGCs derived from beta(2N265S) mice, whereas this manipulation had no effect on the prolongation of mIPSCs produced by this anaesthetic. Collectively, these observations, together with previous studies on alpha(4)(-/-) mice, identify a population of extrasynaptic alpha(4)beta(2)delta receptors, whereas synaptic GABA(A) receptors appear to primarily incorporate the beta(3) subunit. A component of the tonic current is diazepam sensitive and is mediated by extrasynaptic receptors incorporating alpha(5) and gamma(2) subunits. Deletion of the beta(2) subunit had no effect on the diazepam-induced current and therefore these extrasynaptic receptors do not contain this subunit. The unambiguous identification of these distinct pools of synaptic and extrasynaptic GABA(A) receptors should aid our understanding of how they act in harmony, to regulate hippocampal signalling in health and disease.

Neurosteroid modulation of GABAA receptors: molecular determinants and significance in health and disease

Over the past 20 years it has become apparent that certain steroids, synthesised de novo in the brain, hence named neurosteroids, produce immediate changes (within seconds) in neuronal excitability, a time scale that precludes a genomic locus of action. Identified molecular targets underlying modulation of brain excitability include both the inhibitory GABA(A) and the excitatory NMDA receptor. Of particular interest is the interaction of certain neurosteroids with the GABA(A) receptor, the major inhibitory receptor in mammalian brain. During the last decade, compelling evidence has accrued to reveal that locally produced neurosteroids may selectively "fine tune" neuronal inhibition. A range of molecular mechanisms including the subunit composition of the receptor(s), phosphorylation and local steroid metabolism, underpin the region- and neuronal selectivity of action of neurosteroids at synaptic and extrasynaptic GABA(A) receptors. The relative contribution played by each of these mechanisms in a variety of physiological and pathophysiological scenarios is currently being scrutinised at a cellular and molecular level. However, it is not known how such mechanisms may act in concert to influence behavioural profiles in health and disease. An important question concerns the identification of the anatomical substrates mediating the repertoire of behaviours produced by neurosteroids. "Knock-in" mice expressing mutant GABA(A) subunits engineered to be insensitive to benzodiazepines or general anaesthetics have proved invaluable in evaluating the role of GABA(A) receptor subtypes in complex behaviours such as sedation, cognition and anxiety [Rudolph, U., Mohler, H., 2006. GABA-based therapeutic approaches: GABA(A) receptor subtype functions. Curr. Opin. Pharmacol. 6, 18-23]. However, the development of a similar approach for neurosteroids has been hampered by the limited knowledge that, until recently, has surrounded the identity of the amino acid residues contributing to the neurosteroid binding pocket. Here, we will review recent progress in identifying the neurosteroid binding site on the GABA(A) receptor, and discuss how these discoveries will impact on our understanding of the role of neurosteroids in health and disease.

GABA(A) receptors: structure and function in the basal ganglia

gamma-Aminobutyric acid type A (GABA(A)) receptors, the major inhibitory neurotransmitter receptors responsible for fast inhibition in the basal ganglia, belong to the superfamily of "cys-cys loop" ligand-gated ion channels. GABA(A) receptors form as pentameric assemblies of subunits, with a central Cl(-) permeable pore. On binding of two GABA molecules to the extracellular receptor domain, a conformational change is induced in the oligomer and Cl(-), in most adult neurons, moves into the cell leading to an inhibitory hyperpolarization. Nineteen mammalian subunit genes have been identified, each showing distinct regional and cell-type-specific expression. The combinatorial assembly of the subunits generates considerable functional diversity. Here we place the focus on GABA(A) receptor expression in the basal ganglia: striatum, globus pallidus, substantia nigra and subthalamic nucleus, where, in addition to the standard alpha1beta2/3gamma2 receptor subtype, significant levels of other subunits (alpha2, alpha3, alpha4, gamma1, gamma3 and delta) are expressed in some nuclei.

Correlating the clinical actions and molecular mechanisms of general anesthetics

PURPOSE OF REVIEW

To summarize recent in-vitro and in-vivo research on molecular mechanisms of general anesthetics' actions.

RECENT FINDINGS

Classes of general anesthetics with distinct clinical profiles appear to induce amnesia, hypnosis, and immobility via different molecular targets. Propofol, etomidate, and barbiturates produce profound amnesia and hypnosis, but weak immobility, by enhancing the activity of specific gamma-aminobutyric acid typeA receptors. In contrast, nitrous oxide, xenon, and ketamine produce analgesia, but weak hypnosis and amnesia, by inhibiting glutamate and nicotinic receptors and activating potassium 'leak' channels such as TREK-1. Volatile halogenated anesthetics show little selectivity for molecular targets. They act on all the channels mentioned above, and other targets such as glycine receptors and mediators of neurotransmitter release.

SUMMARY

Several clinically distinct 'anesthetic states' are induced by different classes of drugs acting on neuronal circuits via different molecular targets. Understanding the mechanisms underlying the therapeutic and toxic actions of general anesthetics helps us reframe the 'art' of anesthesia into more of a 'science'. These studies also enhance efforts to develop new drugs with improved clinical utility.

Determinants of the Anesthetic Sensitivity of Two-pore Domain Acid-sensitive Potassium Channels

Certain two-pore domain K(+) channels are plausible targets for volatile general anesthetics, yet little is known at the molecular level about how these simple agents cause channel activation. The first anesthetic-activated K(+) current I(K(An)) that was characterized was discovered in the mollusk Lymnaea stagnalis and is remarkable for both its sensitivity to general anesthetics and its stereoselective responses to anesthetic enantiomers (Franks, N. P., and Lieb, W. R. (1988) Nature 333, 662-664 and Franks, N. P., and Lieb, W. R. (1991) Science 254, 427-430). Here we report the molecular cloning of a two-pore domain K(+) channel LyTASK from L. stagnalis and show that, when expressed in HEK-293 cells, it displays the same biophysical characteristics as the anesthetic-activated K(+) current I(K(An)). Sequence analysis and functional properties show it to be a member of the TASK family of channels with approximately 47% identity at the amino acid level when compared with human TASK-1 and TASK-3. By using chimeric channel constructs and site-directed mutagenesis we have identified the specific amino acid 159 to be a critical determinant of anesthetic sensitivity, which, when mutated to alanine, essentially eliminates anesthetic activation in the human channels and greatly reduces activation in LyTASK. The L159A mutation in LyTASK disrupts the stereoselective response to isoflurane while having no effect on the pH sensitivity of the channel, suggesting this critical amino acid may form part of an anesthetic binding site.

Modulation of presynaptic beta3-containing GABAA receptors limits the immobilizing actions of GABAergic anesthetics

Intravenous GABAergic anesthetics are potent hypnotics but are rather ineffective in depressing movements. Immobility is mediated, in part, by the ventral horn of the spinal cord. We hypothesized that the efficacy of these anesthetics in producing immobility is compromised by the activation of GABA(A) receptors located presynaptically, which modulate GABA release onto neurons in the ventral horn. Because anesthetics acting by modulation of GABA(A) receptor function require GABA to be present at its binding site, a decrease in GABA release would abate their efficacy in reducing neuronal excitability. Here we report that in organotypic spinal cord slices, the efficacy of the intravenous anesthetic etomidate to depress network activity of ventral horn neurons is limited to approximately 60% at concentrations greater than 1 microM that produce immobility. Depression of spinal network activity was almost abolished in spinal slices from beta3(N265M) knock-in mice. In the wild type, etomidate prolonged decay times of GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) and concomitantly reduced the frequency of action potential-dependent IPSCs. Etomidate prolonged the decay time of GABA(A) receptors at all tested concentrations. At concentrations greater than 1.0 microM, anesthetic-induced decrease of GABA release via modulation of presynaptic GABA(A) receptors and enhancement of postsynaptic GABA(A) receptor-function compensated for each other. The results suggest that the limited immobilizing efficacy of these agents is probably due to a presynaptic mechanism and that GABAergic agents with a specificity for post-versus presynaptic receptors would probably have much stronger immobilizing actions, pointing out novel avenues for drug development.

Alteration of behavior in mice by muscimol is associated with regional electroencephalogram synchronization

We tested the hypothesis that the effects of GABAergic agonists on behavior and the electroencephalogram (EEG) result from an increased regional synchronization in cortical circuits. The relationship between regional EEG topography, EEG synchronization and alteration of behavior was investigated by administering male C57BL/6 mice (n=7) a high, 3 mg/kg i.p. dose of muscimol, a selective GABA(A) agonist. Parietal and frontal cortical EEG, electromyogram, infrared and running wheel activity were recorded for 3 h before and 9 h after injection. Muscimol consistently elicited biphasic behavioral changes. Initially, it induced a catalepsy-like state lasting 96.0+/-12.4 min. This state was followed by a hyperactivity period of 49.7+/-5.4 min, during which the mice engaged in vigorous wheel running. During catalepsy, the EEG exhibited high amplitude waves which showed a consistent phase relationship between the frontal and parietal derivation. Moreover, the typical regional differences between the EEG spectra of the two derivations were abolished, and a redistribution of EEG power toward lower frequencies (<3 Hz) occurred in both derivations. In contrast, during hyperactivity the parietal EEG was dominated by theta-activity (7-9 Hz), which is typical for running behavior, while high amplitude slow waves, resembling the normal non-rapid eye movement sleep EEG pattern, predominated in the frontal EEG. The data indicate that the GABAergic system is involved in the regulation of cortical synchronization of neuronal activity and suggest a link between regional EEG synchronization and behavioral states.

Mechanisms of anesthetic actions and the brain

The neural mechanisms behind anesthetic-induced behavioral changes such as loss of consciousness, amnesia, and analgesia, are insufficiently understood, though general anesthesia has been of tremendous importance for the development of medicine. In this review, I summarize what is currently known about general anesthetic actions at different organizational levels and discuss current and future research, using systems neuroscience approaches such as functional neuroimaging and quantitative electrophysiology to understand anesthesia actions at the integrated brain level.

Isoflurane depression of spinal nociceptive processing and minimum alveolar anesthetic concentration are not attenuated in mice expressing isoflurane resistant gamma-aminobutyric acid type-A receptors

Anesthetics produce immobility and depress spinal nociceptive processing, but the exact sites and mechanisms of anesthetic action are unknown. The gamma-aminobutyric acid type-A (GABAA) receptor is thought to be important to anesthetic action. We studied knock-in mice that had mutations in the alpha1 subunit of the GABAA receptor that imparts resistance to isoflurane in in vitro systems. We determined the isoflurane minimum alveolar concentration (MAC) that produces immobility in 50% of subjects and responses of lumbar neurons (single-unit recordings) to noxious stimulation (5 s pinch) of the hindpaw. Isoflurane MAC did not differ between wild-type (1.1+/-0.1%) and knock-in (1.1+/-0.1%) mice. Isoflurane depressed neuronal responses to noxious stimulation (60 s period during and after pinch) similarly in both wild-type and knock-in mice (555+/-133 and 636+/-106 impulses/min, respectively, at 0.8 MAC and 374+/-81 and 409+/-85 impulses/min at 1.2 MAC). We conclude that isoflurane enhancement of alpha1-containing GABAA receptors is not required to produce immobility or depress spinal nociceptive processing.

Neurosteroid modulation of synaptic and extrasynaptic GABA(A) receptors

Certain naturally occurring pregnane steroids act in a nongenomic manner to potently and selectively enhance the interaction of the inhibitory neurotransmitter GABA with the GABA(A) receptor. Consequently such steroids exhibit anxiolytic, anticonvulsant, analgesic, sedative, hypnotic, and anesthetic properties. In both physiological and pathophysiological scenarios, the pregnane steroids may function as endocrine messengers (e.g., produced in the periphery and cross the blood-brain barrier) to influence behaviour. However, additionally "neurosteroids" can be synthesised in the brain and spinal cord to act in a paracrine or autocrine manner and thereby locally influence neuronal activity. Given the ubiquitous expression of the GABA(A) receptor throughout the mammalian central nervous system (CNS), physiological, pathophysiological, or drug-induced pertubations of neurosteroid levels may be expected to produce widespread changes in brain excitability. However, the neurosteroid/GABA(A) receptor interaction is brain region and indeed neuron specific. The molecular basis of this specificity will be reviewed here, including (1) the importance of the subunit composition of the GABA(A) receptor; (2) how protein phosphorylation may dynamically influence the sensitivity of GABA(A) receptors to neurosteroids; (3) the impact of local steroid metabolism; and (4) the emergence of extrasynaptic GABA(A) receptors as a neurosteroid target.

The lack of CB1 receptors prevents neuroadapatations of both NMDA and GABA(A) receptors after chronic ethanol exposure

As the contribution of cannabinoid (CB1) receptors in the neuroadaptations following chronic alcohol exposure is unknown, we investigated the neuroadaptations induced by chronic alcohol exposure on both NMDA and GABA(A) receptors in CB1-/- mice. Our results show that basal levels of hippocampal [(3)H]MK-801 ((1)-5-methyl-10,11-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-imine) binding sites were decreased in CB1-/- mice and that these mice were also less sensitive to the locomotor effects of MK-801. Basal level of both hippocampal and cerebellar [(3)H]muscimol binding was lower and sensitivity to the hypothermic effects of diazepam and pentobarbital was increased in CB1-/- mice. GABA(A)alpha1, beta2, and gamma2 and NMDA receptor (NR) 1 and 2B subunit mRNA levels were altered in striatum of CB1-/- mice. Our results also showed that [(3)H]MK-801 binding sites were increased in cerebral cortex and hippocampus after chronic ethanol ingestion only in wild-type mice. Chronic ethanol ingestion did not modify the sensitivity to the locomotor effects of MK-801 in both genotypes. Similarly, chronic ethanol ingestion reduced the number of [(3)H]muscimol binding sites in cerebral cortex, but not in cerebellum, only in CB1+/+ mice. We conclude that lifelong deletion of CB1 receptors impairs neuroadaptations of both NMDA and GABA(A) receptors after chronic ethanol exposure and that the endocannabinoid/CB1 receptor system is involved in alcohol dependence.

The EEG effects of THIP (Gaboxadol) on sleep and waking are mediated by the GABA(A)delta-subunit-containing receptors

THIP (4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridine-3-ol, Gaboxadol) is a selective gamma-aminobutyric acid (GABA)(A) agonist, acting in vitro with high potency and efficacy at the extrasynaptic GABA(A)delta-containing receptors. THIP was suggested to be a potential hypnotic to treat insomnia, and it is currently in clinical trial. Here we assessed whether the GABA(A)delta-containing receptors mediate in vivo the effect of THIP on sleep and the sleep electroencephalogram (EEG). We performed EEG recordings in a mouse model deficient in the GABA(A)delta-subunit gene (delta(-/-) mice) and in wild-type littermate controls. THIP (4 and 6 mg/kg intraperitoneally) induced an abnormal EEG pattern, resulting in dramatic changes in the waking and non-rapid eye movement (NREM) sleep EEG spectra in wild-type mice. Indeed, a massive increase in EEG power lasting 2-3 h occurred in both the frontal and parietal derivation, especially in frequencies below 6 Hz. All effects were more prominent in the frontal EEG. Furthermore, the highest dose of THIP lengthened REM sleep latency and suppressed REM sleep. In contrast, vigilance states and sleep latencies were not affected in delta(-/-) mice. Moreover, only minor changes were observed in the NREM sleep EEG spectrum after THIP injection in the delta-subunit-deficient mice. The present findings do not indicate a sleep-promoting effect of THIP in mice, which is in accordance with a previous report in this species. Moreover, our results in vivo demonstrate that THIP acts preferentially at GABA(A) receptors containing the delta-subunit.

Molecular regulation of cognitive functions and developmental plasticity: impact of GABAA receptors

By controlling spike timing and sculpting neuronal rhythms, inhibitory interneurons play a key role in regulating neuronal circuits and behavior. The pronounced diversity of GABAergic (gamma-aminobutyric acid) interneurons is paralleled by an extensive diversity of GABAA receptor subtypes. The region- and domain-specific location of these receptor subtypes offers the opportunity to gain functional insights into the role of defined neuronal circuits. These developments are reviewed with regard to the regulation of sleep, anxiety, memory, sensorimotor processing and post-natal developmental plasticity.

Mapping the contribution of beta3-containing GABAA receptors to volatile and intravenous general anesthetic actions

Background

Agents belonging to diverse chemical classes are used clinically as general anesthetics. The molecular targets mediating their actions are however still only poorly defined. Both chemical diversity and substantial differences in the clinical actions of general anesthetics suggest that general anesthetic agents may have distinct pharmacological targets. It was demonstrated previously that the immobilizing action of etomidate and propofol is completely, and the immobilizing action of isoflurane partly mediated, by β3-containing GABA_A receptors. This was determined by using the β3(N265M) mice, which carry a point mutation known to decrease the actions of general anesthetics at recombinant GABA_A receptors. In this communication, we analyzed the contribution of β3-containing GABA_A receptors to the pharmacological actions of isoflurane, etomidate and propofol by means of β3(N265M) mice.

Results

Isoflurane decreased core body temperature and heart rate to a smaller degree in β3(N265M) mice than in wild type mice, indicating a minor but significant role of β3-containing GABA_A receptors in these actions. Prolonged time intervals in the ECG and increased heart rate variability were indistinguishable between genotypes, suggesting no involvement of β3-containing GABA_A receptors. The anterograde amnesic action of propofol was indistinguishable in β3(N265M) and wild type mice, suggesting that it is independent of β3-containing GABA_A receptors. The increase of heart rate variability and prolongation of ECG intervals by etomidate and propofol were also less pronounced in β3(N265M) mice than in wild type mice, pointing to a limited involvement of β3-containing GABA_A receptors in these actions. The lack of etomidate- and propofol-induced immobilization in β3(N265M) mice was also observed in congenic 129X1/SvJ and C57BL/6J backgrounds, indicating that this phenotype is stable across different backgrounds.

Conclusion

Our results provide evidence for a defined role of β3-containing GABA_A receptors in mediating some, but not all, of the actions of general anesthetics, and confirm the multisite model of general anesthetic action. This pharmacological separation of anesthetic endpoints also suggests that subtype-selective substances with an improved side-effect profile may be developed.

Modulation of extrasynaptic THIP conductances by GABAA-receptor modulators in mouse neocortex

THIP is a hypnotic drug, which displays a unique pharmacological profile, because it activates a subset of extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors containing delta-subunits. It is important to study the physiology and pharmacology of these extrasynaptic receptors and to determine how THIP interacts with other hypnotics and anesthetics. Here, we study the modulation of the extrasynaptic response to THIP using three classes of GABA(A)-receptor ligands. In whole cell recordings from mouse neocortical layer 2/3 pyramidal cells, THIP induced an extrasynaptic tonic current of 44 +/- 5 pA. The benzodiazepine site agonist and hypnotic zolpidem (500 nM), which displays selectivity for alpha(1/2/3)- and gamma(2)-containing receptors, did not alter the tonic current induced by THIP. The anesthetic etomidate (1 microM), which shows selectivity for beta(2)- and beta(3)-containing GABA(A) receptors, potentiated the THIP current by 126%. Etomidate also induced a small tonic GABA(A) current per se. The anesthetic propofol (1 microM), which displays broad-spectrum modulatory effects on several GABA(A)-receptor subtypes, enhanced the tonic THIP current by 117%. Finally, all three compounds modulated the function of intrasynaptic receptors activated by synaptically released GABA. Our study shows that the extrasynaptic GABA(A) receptors responsible for the tonic THIP conductance likely do not contain alpha(1)-, alpha(2)-, alpha(3)-, and gamma(2)-subunits. Thus the tonic GABAergic conductance in the neocortex is presumably mediated by alpha(4)beta(2/3)delta receptors, which are likely to play a major role for neocortical excitability. Furthermore, our study has deepened the knowledge about the cellular actions of THIP as well as THIP's interactions with other hypnotics and anesthetics.

Etomidate reduces initiation of backpropagating dendritic action potentials: implications for sensory processing and synaptic plasticity during anesthesia

Anesthetics may induce specific changes that alter the balance of activity within neural networks. Here we describe the effects of the GABA(A) receptor potentiating anesthetic etomidate on sensory processing, studied in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid fish, in vitro. Previous studies have shown that the ELL integrates sensory input and removes predictable features by comparing reafferent sensory signals with a descending electromotor command-driven corollary signal that arrives in part through parallel fiber synapses with the apical dendrites of GABAergic interneurons. These synapses show spike timing-dependent depression when presynaptic activation is associated with postsynaptic backpropagating dendritic action potentials. Under etomidate, almost all neurons become tonically hyperpolarized. The threshold for action potential initiation increased for both synaptic activation and direct intracellular depolarization. Synaptically evoked inhibitory postsynaptic potentials (IPSPs) were also strongly potentiated and prolonged. Current source density analysis showed that backpropagation of action potentials through the apical dendritic arborization in the molecular layer was reduced but could be restored by increasing stimulus strength. These effects of etomidate were blocked by bicuculline or picrotoxin. It is concluded that etomidate affects both tonic and phasic inhibitory conductances at GABA(A) receptors and that increased shunting inhibition at the level of the proximal dendrites also contributes to increasing the threshold for action potential backpropagation. When stimulus strength is sufficient to evoke backpropagation, repetitive association of synaptic excitation with postsynaptic action potential initiation still results in synaptic depression, showing that etomidate does not interfere with the molecular mechanism underlying plastic modulation.

Molecular targets underlying general anaesthesia

The discovery of general anaesthesia, over 150 years ago, revolutionised medicine. The ability to render a patient unconscious and insensible to pain made modern surgery possible and general anaesthetics have become both indispensible as well as one of the most widely used class of drugs. Their extraordinary chemical diversity, ranging from simple chemically inert gases to complex barbiturates, has baffled pharmacologists, and ideas about how they might work have been equally diverse. Until relatively recently, thinking was dominated by the notion that anaesthetics acted 'nonspecifically' by dissolving in the lipid bilayer portions of nerve membranes. While this simple idea could account for the chemical diversity of general anaesthetics, it has proven to be false and it is now generally accepted that anaesthetics act by binding directly to sensitive target proteins. For certain intravenous anaesthetics, such as propofol and etomidate, the target has been identified as the GABA(A) receptor, with particular subunits playing a crucial role. For the less potent inhalational agents, the picture is less clear, although a relatively small number of targets have been identified as being the most likely candidates. In this review, I will describe the work that led up to the identification of the GABA(A) receptor as the key target for etomidate and propofol and contrast this with current progress that has been made in identifying the relevant targets for other anaesthetics, particularly the inhalational agents.

Identification of a Molecular Target Mediating the General Anesthetic Actions of Pentobarbital

Barbiturates were introduced into medical practice in 1934. They are widely used today as general anesthetics. Although in vitro studies revealed that the activity of a variety of ligand-gated channels is modulated by barbiturates, the target(s) mediating the anesthetic actions of barbiturates in vivo are unknown. Studying pentobarbital action in beta3(N265M) mice harboring beta3-containing GABAA receptors insensitive to a variety of general anesthetic agents, we found that the immobilizing action of pentobarbital is mediated fully, and the hypnotic action is mediated in part by this receptor subtype. It was surprising that the respiratory depressant action of pentobarbital is indistinguishable between beta3(N265M) and wild-type mice and thus is mediated by other as-yet-unidentified targets. Whereas the target for the immobilizing and hypnotic actions of pentobarbital seems to be the same as for etomidate and propofol, these latter agents' respiratory depressant actions are mediated by beta3-containing GABAA receptors. Thus, in contrast to etomidate and propofol, pentobarbital can elicit respiratory depression by a beta3-independent pathway. Pentobarbital reduced heart rate and body temperature to a slightly smaller extent in beta3(N265M) mice compared with wild-type mice, indicating that these actions are largely mediated by other targets. Pentobarbital-induced increase of heart rate variability and prolongation of ECG intervals are seen in both beta3(N265M) mice and wild-type mice, suggesting that they are not dependent on beta3-containing GABAA receptors. In summary, we show a clear pharmacological dissociation of the immobilizing/hypnotic and respiratory/cardiovascular actions of pentobarbital.

Drug development in anaesthesia: industrial perspective

PURPOSE OF REVIEW

In this review, we summarize the new drugs in development in the anaesthesia field.

RECENT FINDINGS

There are some interesting approaches, including pro-drugs of propofol such as Aquavan (MGI Pharma, Bloomington, Minnesota, USA) and novel 'soft-drug' sedatives and hypnotics (e.g. CNS-7259X and TD-4756) as well as a novel approach to terminate the action of steroidal neuromuscular blockers (sugammadex). There is also significant activity in the field of novel analgesics. Particularly addressing the fields of sedatives, hypnotics and neuromuscular blockers, however, there is relatively little drug discovery activity currently. Part of the reason for this may be that the mechanisms of action of anaesthetics are not fully understood. This cannot be the whole story, however, since attractive new targets have recently been identified. For example, an agent with selective actions at the beta3-containing subunit of the gamma-amino butyric acid-A receptor is likely to have the hypnotic effects of propofol without the cardiac depressant side-effects.

SUMMARY

We consider the main reason for low activity is the perception in industry that there is little need for new drugs in anaesthesia because the needs are well addressed by existing agents. If this is not the case then anaesthesiologists need to be more effective in communicating their requirements.

CaMK-II modulation of GABAA receptors expressed in HEK293, NG108-15 and rat cerebellar granule neurons

The gamma-aminobutyric acid type A (GABA(A)) receptor is a pentameric ligand-gated ion channel responsible for fast synaptic inhibition in the brain. Phosphorylation of the GABA(A) receptor by serine/threonine protein kinases, at residues located in the intracellular loop between the third and fourth transmembrane domains of each subunit, can dynamically modulate receptor trafficking and function. In this study, we have assessed the effect that Ca(2+)-calmodulin-dependent protein kinase-II (CaMK-II) has on GABA(A) receptors. The intracellular application of preactivated CaMK-II failed to modulate the function of alphabeta and alphabetagamma subunit GABA(A) receptors heterologously expressed in human embryonic kidney (HEK)293 cells. However, application of similarly preactivated alpha-CaMK-II significantly potentiated the amplitudes of whole-cell GABA currents recorded from rat cultured cerebellar granule neurons and from recombinant GABA(A) receptors expressed in neuroblastoma, NG108-15, cells. The modulation by alpha-CaMK-II of current amplitude depended upon the subunit composition of GABA(A) receptors. alpha-CaMK-II potentiated GABA currents recorded from alpha1beta3 and alpha1beta3gamma2 GABA(A) receptors, but was unable to functionally modulate beta2 subunit-containing receptors. Similar results were obtained from beta2 -/- mouse cerebellar granule cell cultures and from rat granule cell cultures overexpressing recombinant alpha1beta2 or alpha1beta3 GABA(A) receptors. alpha-CaMK-II had a greater effect on the modulation of GABA responses mediated by alpha1beta3gamma2 compared with alpha1beta3 receptors, indicating a possible role for the gamma2 subunit in CaMK-II-mediated phosphorylation. In conclusion, CaMK-II can upregulate the function of GABA(A) receptors expressed in neurons or a neuronal cell line that is dependent on the beta subunit co-assembled into the receptor complex.

Effects of General Anesthesia on High-Frequency Oscillations in Somatosensory Evoked Potentials

To determine the characteristics of high-frequency oscillations (HFOs) of cortical somatosensory evoked potentials (SEPs), the effect of general anesthesia on HFOs and low-frequency primary cortical responses was studied. The authors recorded SEPs elicited by median nerve stimulation directly from human brains of seven patients who underwent implantation of subdural electrodes before surgical treatment of intractable epilepsy. Recordings were made before and during general anesthesia. Changes in the number of HFOs and amplitude ratios of HFOs/primary cortical responses were analyzed. Under general anesthesia, the number of HFO peaks and the amplitude ratios were significantly decreased. General anesthesia induced remarkably decreased HFO activities when compared to low-frequency SEPs, suggesting that each of those originated from different generators. Possible relations between gamma-amino-butyric acid (GABA)ergic inhibitory interneurons and HFOs are discussed.

Synthesis of trifluoromethylaryl diazirine and benzophenone derivatives of etomidate that are potent general anesthetics and effective photolabels for probing sites on ligand-gated ion channels

To locate the binding sites of general anesthetics on ligand-gated ion channels, two derivatives of the intravenous general anesthetic etomidate (2-ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate), in which the 2-ethyl group has been replaced by photoactivable groups based on either aryl diazirine or benzophenone chemistry, have been synthesized and characterized pharmacologically. TDBzl-etomidate (4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate) and BzBzl-etomidate (4-benzoylbenzyl-1-(1-phenylethyl)-1H-imidazole-5-carboxylate are both potent general anesthetics with half-effective anesthetic concentrations of 700 and 220 nM, respectively. Both agents resembled etomidate in enhancing currents elicited by low concentrations of GABA on heterologously expressed GABAA receptors and in shifting the GABA concentration-response curve to lower concentrations. They also allosterically enhanced the binding of flunitrazepam to mammalian brain GABAA receptors. Both agents were also effective and selective photolabels, photoincorporating into some, but not all, subunits of the Torpedo nicotinic acetylcholine receptor to a degree that was allosterically regulated by an agonist or a noncompetitive inhibitor. Thus, they have the necessary pharmacological and photochemical properties to be useful in identifying the site of etomidate-induced anesthesia.