Beta3-containing gamma-aminobutyric acidA receptors are not major targets for the amnesic and immobilizing actions of isoflurane

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

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

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

Isoflurane inhibition of endocytosis is an anesthetic mechanism of action

The mechanisms of volatile anesthetic action remain among the most perplexing mysteries of medicine. Across phylogeny, volatile anesthetics selectively inhibit mitochondrial complex I, and they also depress presynaptic excitatory signaling. To explore how these effects are linked, we studied isoflurane effects on presynaptic vesicle cycling and ATP levels in hippocampal cultured neurons from wild-type and complex I mutant (Ndufs4(KO)) mice. To bypass complex I, we measured isoflurane effects on anesthetic sensitivity in mice expressing NADH dehydrogenase (NDi1). Endocytosis in physiologic concentrations of glucose was delayed by effective behavioral concentrations of isoflurane in both wild-type (τ [unexposed] 44.8 ± 24.2 s; τ [exposed] 116.1 ± 28.1 s; p < 0.01) and Ndufs4(KO) cultures (τ [unexposed] 67.6 ± 16.0 s; τ [exposed] 128.4 ± 42.9 s; p = 0.028). Increasing glucose, to enhance glycolysis and increase ATP production, led to maintenance of both ATP levels and endocytosis (τ [unexposed] 28.0 ± 14.4; τ [exposed] 38.2 ± 5.7; reducing glucose worsened ATP levels and depressed endocytosis (τ [unexposed] 85.4 ± 69.3; τ [exposed] > 1,000; p < 0.001). The block in recycling occurred at the level of reuptake of synaptic vesicles into the presynaptic cell. Expression of NDi1 in wild-type mice caused behavioral resistance to isoflurane for tail clamp response (EC₅₀ Ndi1(-) 1.27% ± 0.14%; Ndi1(+) 1.55% ± 0.13%) and halothane (EC₅₀ Ndi1(-) 1.20% ± 0.11%; Ndi1(+) 1.46% ± 0.10%); expression of NDi1 in neurons improved hippocampal function, alleviated inhibition of presynaptic recycling, and increased ATP levels during isoflurane exposure. The clear alignment of cell culture data to in vivo phenotypes of both isoflurane-sensitive and -resistant mice indicates that inhibition of mitochondrial complex I is a primary mechanism of action of volatile anesthetics.

Isoflurane Potentiation of GABAA Receptors Is Reduced but Not Eliminated by the β3(N265M) Mutation

Background: Mice carrying the GABA_A receptor β3(N265M) point mutation, which renders receptors incorporating β3-subunits insensitive to many general anesthetics, have been used experimentally to link modulation of different receptor subtypes to distinct behavioral endpoints. Remarkably, however, the effect of the mutation on the susceptibility to modulation by isoflurane (a standard reference agent for inhalational vapors) has never been tested directly. Therefore, we compared the modulation by isoflurane of expressed α5β3(N265M)γ2L receptors with their wild type counterparts. Methods: Using whole-cell electrophysiological recording and rapid solution exchange techniques, we tested the effects of isoflurane at concentrations ranging from 80 μM to 320 μM on currents activated by 1 μM GABA. We measured drug modulation of wild-type α5β3γ2L GABA_A receptors and their counterparts harboring the β3(N265M) mutation. Results: Currents elicited by GABA were enhanced two- to four-fold by isoflurane, in a concentration-dependent manner. Under the same conditions, receptors incorporating the β3(N265M) mutation were enhanced by approximately 1.5- to two-fold; i.e., modulation by isoflurane was attenuated by approximately one-half. Direct activation by isoflurane was also present in mutant receptors but also attenuated. Conclusions: In contrast to the complete insensitivity of β3(N265M) mutant receptors to etomidate and propofol, the mutation has only a partial effect on receptor modulation by isoflurane. Therefore, the persistence of isoflurane effects in mutant mice does not exclude a possible contribution of β3-GABA_A receptors.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The Effects of General Anesthetics on Synaptic Transmission

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

Phospholipase C-related inactive protein type-1 deficiency affects anesthetic electroencephalogram activity induced by propofol and etomidate in mice

PURPOSE

The general anesthetics propofol and etomidate mainly exert their anesthetic actions via GABA A receptor (GABA_A-R). The GABA_A-R activity is influenced by phospholipase C-related inactive protein type-1 (PRIP-1), which is related to trafficking and subcellular localization of GABA_A-R. PRIP-1 deficiency attenuates the behavioral reactions to propofol but not etomidate. However, the effect of these anesthetics and of PRIP-1 deficiency on brain activity of CNS are still unclear. In this study, we examined the effects of propofol and etomidate on the electroencephalogram (EEG).

METHODS

The cortical EEG activity was recorded in wild-type (WT) and PRIP-1 knockout (PRIP-1 KO) mice. All recorded EEG data were offline analyzed, and the power spectral density and 95% spectral edge frequency of EEG signals were compared between genotypes before and after injections of anesthetics.

RESULTS

PRIP-1 deficiency induced increases in EEG absolute powers, but did not markedly change the relative spectral powers during waking and sleep states in the absence of anesthesia. Propofol administration induced increases in low-frequency relative EEG activity and decreases in SEF95 values in WT but not in PRIP-1 KO mice. Following etomidate injection, low-frequency EEG power was increased in both genotype groups. At high frequency, the relative power in PRIP-1 KO mice was smaller than that in WT mice.

CONCLUSIONS

The lack of PRIP-1 disrupted the EEG power distribution, but did not affect the depth of anesthesia after etomidate administration. Our analyses suggest that PRIP-1 is differentially involved in anesthetic EEG activity with the regulation of GABA_A-R activity.

Isoflurane disrupts excitatory neurotransmitter dynamics via inhibition of mitochondrial complex I

BACKGROUND

The mechanisms of action of volatile anaesthetics are unclear. Volatile anaesthetics selectively inhibit complex I in the mitochondrial respiratory chain. Mice in which the mitochondrial complex I subunit NDUFS4 is knocked out [Ndufs4(KO)] either globally or in glutamatergic neurons are hypersensitive to volatile anaesthetics. The volatile anaesthetic isoflurane selectively decreases the frequency of spontaneous excitatory events in hippocampal slices from Ndufs4(KO) mice.

METHODS

Complex I inhibition by isoflurane was assessed with a Clark electrode. Synaptic function was measured by stimulating Schaffer collateral fibres and recording field potentials in the hippocampus CA1 region.

RESULTS

Isoflurane specifically inhibits complex I dependent respiration at lower concentrations in mitochondria from Ndufs4(KO) than from wild-type mice. In hippocampal slices, after high frequency stimulation to increase energetic demand, short-term synaptic potentiation is less in KO compared with wild-type mice. After high frequency stimulation, both Ndufs4(KO) and wild-type hippocampal slices exhibit striking synaptic depression in isoflurane at twice the 50% effective concentrations (EC₅₀). The pattern of synaptic depression by isoflurane indicates a failure in synaptic vesicle recycling. Application of a selective A₁ adenosine receptor antagonist partially eliminates isoflurane-induced short-term depression in both wild-type and Ndufs4(KO) slices, implicating an additional mitochondria-dependent effect on exocytosis. When mitochondria are the sole energy source, isoflurane completely eliminates synaptic output in both mutant and wild-type mice at twice the (EC₅₀) for anaesthesia.

CONCLUSIONS

Volatile anaesthetics directly inhibit mitochondrial complex I as a primary target, limiting synaptic ATP production, and excitatory vesicle endocytosis and exocytosis.

Isoflurane inhibits synaptic vesicle exocytosis through reduced Ca2+ influx, not Ca2+-exocytosis coupling

Identifying presynaptic mechanisms of general anesthetics is critical to understanding their effects on synaptic transmission. We show that the volatile anesthetic isoflurane inhibits synaptic vesicle (SV) exocytosis at nerve terminals in dissociated rat hippocampal neurons through inhibition of presynaptic Ca(2+) influx without significantly altering the Ca(2+) sensitivity of SV exocytosis. A clinically relevant concentration of isoflurane (0.7 mM) inhibited changes in [Ca(2+)]i driven by single action potentials (APs) by 25 ± 3%, which in turn led to 62 ± 3% inhibition of single AP-triggered exocytosis at 4 mM extracellular Ca(2+) ([Ca(2+)]e). Lowering external Ca(2+) to match the isoflurane-induced reduction in Ca(2+) entry led to an equivalent reduction in exocytosis. These data thus indicate that anesthetic inhibition of neurotransmitter release from small SVs occurs primarily through reduced axon terminal Ca(2+) entry without significant direct effects on Ca(2+)-exocytosis coupling or on the SV fusion machinery. Isoflurane inhibition of exocytosis and Ca(2+) influx was greater in glutamatergic compared with GABAergic nerve terminals, consistent with selective inhibition of excitatory synaptic transmission. Such alteration in the balance of excitatory to inhibitory transmission could mediate reduced neuronal interactions and network-selective effects observed in the anesthetized central nervous system.

Etomidate blocks LTP and impairs learning but does not enhance tonic inhibition in mice carrying the N265M point mutation in the beta3 subunit of the GABA(A) receptor

Enhancement of tonic inhibition mediated by extrasynaptic α5-subunit containing GABAA receptors (GABAARs) has been proposed as the mechanism by which a variety of anesthetics, including the general anesthetic etomidate, impair learning and memory. Since α5 subunits preferentially partner with β3 subunits, we tested the hypothesis that etomidate acts through β3-subunit containing GABAARs to enhance tonic inhibition, block LTP, and impair memory. We measured the effects of etomidate in wild type mice and in mice carrying a point mutation in the GABAAR β3-subunit (β3-N265M) that renders these receptors insensitive to etomidate. Etomidate enhanced tonic inhibition in CA1 pyramidal cells of the hippocampus in wild type but not in mutant mice, demonstrating that tonic inhibition is mediated by β3-subunit containing GABAARs. However, despite its inability to enhance tonic inhibition, etomidate did block LTP in brain slices from mutant mice as well as in those from wild type mice. Etomidate also impaired fear conditioning to context, with no differences between genotypes. In studies of recombinant receptors expressed in HEK293 cells, α5β1γ2L GABAARs were insensitive to amnestic concentrations of etomidate (1 μM and below), whereas α5β2γ2L and α5β3γ2L GABAARs were enhanced. We conclude that etomidate enhances tonic inhibition in pyramidal cells through its action on α5β3-containing GABAA receptors, but blocks LTP and impairs learning by other means - most likely by modulating α5β2-containing GABAA receptors. The critical anesthetic targets underlying amnesia might include other forms of inhibition imposed on pyramidal neurons (e.g. slow phasic inhibition), or inhibitory processes on non-pyramidal cells (e.g. interneurons).

Volatile anesthetics inhibit sodium channels without altering bulk lipid bilayer properties

Although general anesthetics are clinically important and widely used, their molecular mechanisms of action remain poorly understood. Volatile anesthetics such as isoflurane (ISO) are thought to alter neuronal function by depressing excitatory and facilitating inhibitory neurotransmission through direct interactions with specific protein targets, including voltage-gated sodium channels (Na(v)). Many anesthetics alter lipid bilayer properties, suggesting that ion channel function might also be altered indirectly through effects on the lipid bilayer. We compared the effects of ISO and of a series of fluorobenzene (FB) model volatile anesthetics on Na(v) function and lipid bilayer properties. We examined the effects of these agents on Na(v) in neuronal cells using whole-cell electrophysiology, and on lipid bilayer properties using a gramicidin-based fluorescence assay, which is a functional assay for detecting changes in lipid bilayer properties sensed by a bilayer-spanning ion channel. At clinically relevant concentrations (defined by the minimum alveolar concentration), both the FBs and ISO produced prepulse-dependent inhibition of Na(v) and shifted the voltage dependence of inactivation toward more hyperpolarized potentials without affecting lipid bilayer properties, as sensed by gramicidin channels. Only at supra-anesthetic (toxic) concentrations did ISO alter lipid bilayer properties. These results suggest that clinically relevant concentrations of volatile anesthetics alter Na(v) function through direct interactions with the channel protein with little, if any, contribution from changes in bulk lipid bilayer properties. Our findings further suggest that changes in lipid bilayer properties are not involved in clinical anesthesia.

Mutations M287L and Q266I in the glycine receptor α1 subunit change sensitivity to volatile anesthetics in oocytes and neurons, but not the minimal alveolar concentration in knockin mice

BACKGROUND

Volatile anesthetics (VAs) alter the function of key central nervous system proteins but it is not clear which, if any, of these targets mediates the immobility produced by VAs in the face of noxious stimulation. A leading candidate is the glycine receptor, a ligand-gated ion channel important for spinal physiology. VAs variously enhance such function, and blockade of spinal glycine receptors with strychnine affects the minimal alveolar concentration (an anesthetic EC50) in proportion to the degree of enhancement.

METHODS

We produced single amino acid mutations into the glycine receptor α1 subunit that increased (M287L, third transmembrane region) or decreased (Q266I, second transmembrane region) sensitivity to isoflurane in recombinant receptors, and introduced such receptors into mice. The resulting knockin mice presented impaired glycinergic transmission, but heterozygous animals survived to adulthood, and we determined the effect of isoflurane on glycine-evoked responses of brainstem neurons from the knockin mice, and the minimal alveolar concentration for isoflurane and other VAs in the immature and mature knockin mice.

RESULTS

Studies of glycine-evoked currents in brainstem neurons from knockin mice confirmed the changes seen with recombinant receptors. No increases in the minimal alveolar concentration were found in knockin mice, but the minimal alveolar concentration for isoflurane and enflurane (but not halothane) decreased in 2-week-old Q266I mice. This change is opposite to the one expected for a mutation that decreases the sensitivity to volatile anesthetics.

CONCLUSION

Taken together, these results indicate that glycine receptors containing the α1 subunit are not likely to be crucial for the action of isoflurane and other VAs.

Induced changes in protein receptors conferring resistance to anesthetics

PURPOSE OF REVIEW

Although general anesthetics have been provided effectively for many years, their exact molecular underpinnings remain relatively unknown. In this article, we discuss the recent findings associated with resistance to anesthetic effects as a way of shedding light on these mechanisms.

RECENT FINDINGS

The original theories of anesthetic action based upon their effects on cellular membranes have given way to specific theories concerning direct effects on ion channel proteins. These molecular targets are intimately involved in the conduct of neuronal signaling within the central nervous system and are thought to be essential in the modulation of conscious states. It is the lack of a thorough understanding of unperturbed consciousness that fosters great difficulty in understanding how anesthetics alter this conscious state. However, one very fruitful line of analysis in the quest for such answers lies in the examination of both in-vitro and in-vivo ion channel systems that seem to maintain variable levels of resistance to anesthetics.

SUMMARY

Information about the possible targets and molecular nature of anesthetic action is being derived from studies of anesthetic resistance in γ aminobutyric acid receptors, tandem pore potassium channels, and an apparently wide variety of protein systems within the nematode, Caenorhabditis elegans.

Altered anesthetic sensitivity of mice lacking Ndufs4, a subunit of mitochondrial complex I

Anesthetics are in routine use, yet the mechanisms underlying their function are incompletely understood. Studies in vitro demonstrate that both GABA(A) and NMDA receptors are modulated by anesthetics, but whole animal models have not supported the role of these receptors as sole effectors of general anesthesia. Findings in C. elegans and in children reveal that defects in mitochondrial complex I can cause hypersensitivity to volatile anesthetics. Here, we tested a knockout (KO) mouse with reduced complex I function due to inactivation of the Ndufs4 gene, which encodes one of the subunits of complex I. We tested these KO mice with two volatile and two non-volatile anesthetics. KO and wild-type (WT) mice were anesthetized with isoflurane, halothane, propofol or ketamine at post-natal (PN) days 23 to 27, and tested for loss of response to tail clamp (isoflurane and halothane) or loss of righting reflex (propofol and ketamine). KO mice were 2.5 - to 3-fold more sensitive to isoflurane and halothane than WT mice. KO mice were 2-fold more sensitive to propofol but resistant to ketamine. These changes in anesthetic sensitivity are the largest recorded in a mammal.

Active emergence from propofol general anesthesia is induced by methylphenidate

BACKGROUND

A recent study showed that methylphenidate induces emergence from isoflurane general anesthesia. Isoflurane and propofol are general anesthetics that may have distinct molecular mechanisms of action. The objective of this study was to test the hypothesis that methylphenidate actively induces emergence from propofol general anesthesia.

METHODS

Using adult rats, the effect of methylphenidate on time to emergence after a single bolus of propofol was determined. The ability of methylphenidate to restore righting during a continuous target-controlled infusion (TCI) of propofol was also tested. In a separate group of rats, a TCI of propofol was established and spectral analysis was performed on electroencephalogram recordings taken before and after methylphenidate administration.

RESULTS

Methylphenidate decreased median time to emergence after a single dose of propofol from 735 s (95% CI: 598-897 s, n = 6) to 448 s (95% CI: 371-495 s, n = 6). The difference was statistically significant (P = 0.0051). During continuous propofol anesthesia with a median final target plasma concentration of 4.0 μg/ml (95% CI: 3.2-4.6, n = 6), none of the rats exhibited purposeful movements after injection of normal saline. After methylphenidate, however, all six rats promptly exhibited arousal and had restoration of righting with a median time of 82 s (95% CI: 30-166 s). Spectral analysis of electroencephalogram data demonstrated a shift in peak power from δ (less than 4 Hz) to θ (4-8 Hz) and β (12-30 Hz) after administration of methylphenidate, indicating arousal in 4/4 rats.

CONCLUSIONS

Methylphenidate decreases time to emergence after a single dose of propofol, and induces emergence during continuous propofol anesthesia in rats. Further study is warranted to test the hypothesis that methylphenidate induces emergence from propofol general anesthesia in humans.

Gamma-aminobutyric acid type A receptor β3 subunit forebrain-specific knockout mice are resistant to the amnestic effect of isoflurane

BACKGROUND

β3 containing γ-aminobutyric acid type A receptors (GABA(A)-Rs) mediate behavioral end points of IV anesthetics such as immobility and hypnosis. A knockout mouse with targeted forebrain deletion of the β3 subunit of the GABA(A)-R shows reduced sensitivity to the hypnotic effect of etomidate, as measured by the loss of righting reflex. The end points of amnesia and immobility produced by an inhaled anesthetic have yet to be evaluated in this conditional knockout.

METHODS

We assessed forebrain selective β3 conditional knockout mice and their littermate controls for conditional fear to evaluate amnesia and MAC, the minimum alveolar concentration of inhaled anesthetic necessary to produce immobility in response to noxious stimulation, to assess immobility. Suppression of conditional fear was assessed for etomidate and isoflurane, and MAC was assessed for isoflurane.

RESULTS

Etomidate equally suppressed conditional fear for both genotypes. The knockout showed resistance to the suppression of conditional fear produced by isoflurane in comparison with control littermates. Controls and knockouts did not differ in isoflurane MAC values.

CONCLUSIONS

These results suggest that β3 containing GABA(A)-Rs in the forebrain contribute to hippocampal-dependent memory suppressed by isoflurane, but not etomidate.

Beyond classical benzodiazepines: novel therapeutic potential of GABAA receptor subtypes

GABA(A) (γ-aminobutyric acid, type A) receptors are a family of ligand-gated ion channels that are essential for the regulation of central nervous system function. Benzodiazepines - which non-selectively target GABA(A) receptors containing the α1, α2, α3 or α5 subunits - have been in clinical use for decades and are still among the most widely prescribed drugs for the treatment of insomnia and anxiety disorders. However, their use is limited by side effects and the risk of drug dependence. In the past decade, the identification of separable key functions of GABA(A) receptor subtypes suggests that receptor subtype-selective compounds could overcome the limitations of classical benzodiazepines; furthermore, they might be valuable for novel indications such as chronic pain, depression, schizophrenia, cognitive enhancement and stroke.

Identification and characterization of anesthetic targets by mouse molecular genetics approaches

PURPOSE

It is now generally accepted that proteins are the primary targets of general anesthetics. However, the demonstration that the activity of a protein is altered by general anesthetics at clinically relevant concentrations in vitro does not provide direct evidence that this target mediates pharmacological actions of general anesthetics. Here we report on advances that have been made in identifying the contribution of individual ligand-gated ion channels to defined anesthetic endpoints using molecular mouse genetics.

PRINCIPAL FINDINGS

Gamma-aminobutyric acid (GABA)(A) receptor subtypes defined by the presence of the α1, α4, α5, β2, and β3 subunits and two-pore domain potassium channels (TASK-1, TASK-3, and TREK) have been discovered to mediate, at least in part, the hypnotic, immobilizing or amnestic actions of intravenous and volatile general anesthetics. Moreover, using tissues from genetically modified mice, specific functions of GABA(A) receptor subtypes in cortical and spinal neuronal networks were identified.

CONCLUSION

Genetically modified mice have been very useful for research on mechanisms of anesthesia and have contributed to the functional identification of general anesthetic targets and of the role of these targets in neuronal networks.

A transmembrane amino acid in the GABAA receptor β2 subunit critical for the actions of alcohols and anesthetics

Alcohols and inhaled anesthetics enhance the function of GABA(A) receptors containing α, β, and γ subunits. Molecular analysis has focused on the role of the α subunits; however, there is evidence that the β subunits may also be important. The goal of our study was to determine whether Asn265, which is homologous to the site implicated in the α subunit (Ser270), contributes to an alcohol and volatile anesthetic binding site in the GABA(A) receptor β(2) subunit. We substituted cysteine for Asn265 and exposed the mutant to the sulfhydryl-specific reagent octyl methanethiosulfonate (OMTS). We used two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes and found that, after OMTS application, GABA-induced currents were irreversibly potentiated in mutant α(1)β(2)(N265C)γ(2S) receptors [but not α(1)β(2)(I264C)γ(2S)], presumably because of the covalent linking of octanethiol to the thiol group in the substituted cysteine. It is noteworthy that this effect was blocked when OMTS was applied in the presence of octanol. We found that potentiation by butanol, octanol, or isoflurane in the N265C mutant was nearly abolished after the application of OMTS, suggesting that an alcohol and volatile anesthetic binding site at position 265 of the β(2) subunit was irreversibly occupied by octanethiol and consequently prevented butanol or isoflurane from binding and producing their effects. OMTS did not affect modulation or direct activation by pentobarbital, but there was a partial reduction of allosteric modulation by flunitrazepam and alphaxalone in mutant α(1)β(2)(N265C)γ(2S) receptors after OMTS was applied. Our findings provide evidence that Asn265 may contribute to an alcohol and anesthetic binding site.

Genetic reduction of GABA(A) receptor gamma2 subunit expression potentiates the immobilizing action of isoflurane

Potentiation of inhibitory gamma-aminobutyric acid subtype A (GABA(A)) receptor function is involved in the mechanisms of anesthetic action. The present study examined the immobilizing action of the volatile anesthetic isoflurane in mice with double knockout (DKO) of phospholipase C-related inactive protein (PRIP)-1 and -2. Both of these proteins play important roles in the expression of GABA(A) receptors containing the gamma2 subunit on the neuronal cell surface. Immunohistochemistry for GABA(A) receptor subunits demonstrated reduced expression of gamma2 subunits in the spinal cord of the DKO mice. Immunohistochemistry also revealed up-regulation of the alpha1 and beta3 subunits even though there were no apparent differences in the immunoreactivities for the beta2 subunits between wild-type and DKO mice. The tail-clamp method was used to evaluate the anesthetic/immobilizing effect of isoflurane and the minimum alveolar concentration (MAC) was significantly lower in DKO mice compared with wild-type controls (1.07+/-0.01% versus 1.36+/-0.04% atm), indicating an increased sensitivity to isoflurane in DKO mice. These immunohistochemical and pharmacological findings suggest that reduced expression of the GABA(A) receptor gamma2 subunit affects the composition and function of spinal GABA(A) receptors and potentiates the immobilizing action of isoflurane.

Gamma-amino butyric acid type A receptor mutations at beta2N265 alter etomidate efficacy while preserving basal and agonist-dependent activity

BACKGROUND

Etomidate acts at gamma-Aminobutyric acid type A (GABAA) receptors containing beta2 or beta3, but not beta1 subunits. Mutations at beta residue 265 (Ser in beta1; Asn in beta2 or beta3) profoundly affect etomidate sensitivity. Whether these mutations alter etomidate binding remains uncertain.

METHODS

Heterologously expressed alpha1beta2gamma2L GABAA receptors and receptors with beta2(N265S) or beta2(N265M) mutations were studied electrophysiologically in both Xenopus oocytes and HEK293 cells. Experiments quantified the impact of beta2N265 mutations or substituting beta1 for beta2 on basal channel activation, GABA EC50, maximal GABA efficacy, etomidate-induced leftward shift in GABA responses, etomidate direct activation, and rapid macrocurrent kinetics. Results were analyzed in the context of an established allosteric co-agonist mechanism.

RESULTS

Mutations produced only small changes in basal channel activity, GABA EC50, maximal GABA efficacy, and macrocurrent kinetics. Relative to wild-type, beta2(N265S) reduced etomidate enhancement of apparent GABA affinity six-fold, and it reduced etomidate direct activation efficacy 14-fold. beta2(N265M) totally eliminated both etomidate modulation of GABA responses and direct channel activation. Mechanism-based analysis showed that the function of both mutants remains consistent with the allosteric co-agonist model and that beta2(N265S) reduced etomidate allosteric efficacy five-fold, whereas etomidate-binding affinity dropped threefold. Experiments swapping beta2 subunits for beta1 indicated that etomidate efficacy is reduced 34-fold, whereas binding affinity drops less than two-fold.

CONCLUSIONS

Mutations at beta2N265 profoundly alter etomidate sensitivity with only small changes in basal and GABA-dependent channel activity. Mutations at the beta2N265 residue or replacement of beta2 with beta1 influence etomidate efficacy much more than binding to inactive receptors.

Sodium channels and the synaptic mechanisms of inhaled anaesthetics

General anaesthetics act in an agent-specific manner on synaptic transmission in the central nervous system by enhancing inhibitory transmission and reducing excitatory transmission. The synaptic mechanisms of general anaesthetics involve both presynaptic effects on transmitter release and postsynaptic effects on receptor function. The halogenated volatile anaesthetics inhibit neuronal voltage-gated Na(+) channels at clinical concentrations. Reductions in neurotransmitter release by volatile anaesthetics involve inhibition of presynaptic action potentials as a result of Na(+) channel blockade. Although voltage-gated ion channels have been assumed to be insensitive to general anaesthetics, it is now evident that clinical concentrations of volatile anaesthetics inhibit Na(+) channels in isolated rat nerve terminals and neurons, as well as heterologously expressed mammalian Na(+) channel alpha subunits. Voltage-gated Na(+) channels have emerged as promising targets for some of the effects of the inhaled anaesthetics. Knowledge of the synaptic mechanisms of general anaesthetics is essential for optimization of anaesthetic techniques for advanced surgical procedures and for the development of improved anaesthetics.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.

The effects of aromatic anesthetics on dorsal horn neuronal responses to noxious stimulation

BACKGROUND

Gamma-aminobutyric acid type A receptor potentiation and/or N-methyl-d-aspartate (NMDA) receptor inhibition might explain the anesthetic properties of fluorinated aromatic compounds. We hypothesized that depression of dorsal horn neuronal responses to noxious stimulation would correlate with the magnitude of effect of benzene (BNZ), o-difluorobenzene, and hexafluorobenzene (HFB) on NMDA receptors.

METHODS

Rats were anesthetized with desflurane. A T13-L1 laminectomy allowed extracellular recording of neuronal activity from the lumbar spinal cord. After discontinuing desflurane administration, MAC for each aromatic anesthetic was determined. A 5-s noxious mechanical stimulus was then applied to the hindpaw receptive field of nociceptive dorsal horn neurons, and single-neuron responses were recorded at 0.8 and 1.2 MAC. These responses were also recorded in decerebrate rats receiving BNZ and HFB at 0-1.2 MAC.

RESULTS

In intact rats, depression of responses of dorsal horn neurons to noxious stimulation by peri-MAC increases in BZN, o-difluorobenzene, and HFB correlated directly with their in vitro capacity to block NMDA receptors. In decerebrate rats, 1.2 MAC BNZ depressed nociceptive responses by 60%, with a further percentage decrease continuing from 0.8 to 1.2 MAC approximately equal to that found in intact rats. In decerebrate rats, HFB caused a progressive dose-related decrease in MAC (maximum 25%), but in intact rats, an increase from 0.8 to 1.2 neuronal response caused an (insignificant) increase in neuronal response.

CONCLUSIONS

The findings in intact rats suggest that NMDA blockade contributes to the depression of dorsal horn neurons to nociceptive stimulation by fluorinated aromatic anesthetics. These results, combined with the additional findings in decerebrate rats, suggest that supraspinal effects (perhaps on gamma-aminobutyric acid type A receptors) may have a supraspinal facilitatory effect on nociception for HFB.

Positron emission tomography and target-controlled infusion for precise modulation of brain drug concentration

INTRODUCTION

There are several instances when it is desirable to control brain concentration of pharmaceuticals, e.g., to modulate the concentration of anesthetic agents to different desired levels fitting to different needs during the course of surgery. This has so far only been possible using indirect estimates of drug concentration such as assuming constant relation between tissue and blood including extrapolations from animals.

METHODS

A system for controlling target tissue concentration (UIPump) was used to regulate whole-brain concentrations of a central benzodiazepine receptor antagonist at therapeutic levels with input from brain kinetics as determined with PET. The system was tested by using pharmacological doses of flumazenil mixed with tracer amounts of [11C]flumazenil. Flumazenil was used as a model compound for anesthesia. An infusion scheme to produce three different steady-state levels in sequence was designed based on kinetic curves obtained after bolus injection. The subjects (Sprague-Dawley rats, n=6) were monitored in a microPET scanner during the whole experiment to verify resulting brain kinetic curves.

RESULTS

A steady-state brain concentration was rapidly achieved corresponding to a whole-brain concentration of 118+/-6 ng/ml. As the infusion rate decreased to lower the exposure by a factor of 2, the brain concentration decreased to 56+/-4 ng/ml. A third increased steady-state level of anesthesia corresponding to a whole-brain concentration of 107+/-7 ng/ml was rapidly achieved.

CONCLUSION

The experimental setup with computerized pump infusion and PET supervision enables accurate setting of target tissue drug concentration.

MK-801 enhances gabaculine-induced loss of the righting reflex in mice, but not immobility

PURPOSE

gamma-Aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptors are important targets for anesthetic action at the in vitro cellular level. Gabaculine is a GABA-trans-aminase inhibitor that increases endogenous GABA in the brain, and enhances GABA activity. We have recently shown that unconsciousness is associated with the enhanced GABA activity due to gabaculine, but that immobility is not. MK-801 is a selective NMDA channel blocker. In this study, we examined behaviourally whether gabaculine in combination with MK-801 could produce these components of the general anesthetic state. We further compared the effect of MK-801 with ketamine, another NMDA channel blocker.

METHODS

All drugs were administered intraperitoneally to adult male ddY mice. To assess the general anesthetic components, two endpoints were used. One was loss of the righting reflex (LORR; as a measure of unconsciousness) and the other was loss of movement in response to tail-clamp stimulation (as a measure of immobility).

RESULTS

Large doses of MK-801 alone (10-50 mg.kg(-1)) induced neither LORR nor immobility in response to noxious stimulation. However, even a small dose (0.2 mgxkg(-1)) significantly enhanced gabaculine-induced LORR (P < 0.05), although gabaculine in combination with MK-801 (0.2-10 mgxkg(-1)) produced no immobility. However, gabaculine plus a subanesthetic dose of ketamine (30 mgxkg(-1)), which acts on NMDA, opioid and nicotinic acetylcholine receptors and neuronal Na(+) channels, suppressed the pain response, but did not achieve a full effect. Ketamine alone dose-dependently produced both LORR and immobility.

CONCLUSION

These findings suggest that gabaculine-induced LORR is modulated by blocking NMDA receptors, but that immobility is not mediated through GABA or NMDA receptors.

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.

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.

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.

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.

Do dopamine receptors mediate part of MAC?

BACKGROUND

Depletion of central nervous system catecholamines, including dopamine, can decrease MAC (the minimum alveolar concentration of an inhaled anesthetic required to suppress movement in response to a noxious stimulus in 50% of test subjects); release of central nervous system catecholamines, including dopamine, can increase MAC; and increased free dopamine concentrations in the striatum can decrease MAC. Such findings suggest that dopamine receptors might mediate part of the capacity of inhaled anesthetics to provide immobility in the face of noxious stimulation.

METHODS

We measured the effect of blockade of D2 dopamine-mediated transmission with 0.3 mg/kg or 3.0 mg/kg droperidol on the MAC of cyclopropane, desflurane, halothane, isoflurane, or sevoflurane in rats, and the effect of 3.0 mg/kg droperidol on the dose or concentration of etomidate (an anesthetic known to act principally by enhancing the response of gamma-aminobutyric acid(A) receptors to gamma-aminobutyric acid) required to suppress movement in response to noxious stimulation.

RESULTS

Blockade of D2 dopamine-mediated transmission with droperidol does not decrease the MAC of cyclopropane, desflurane, halothane, isoflurane, or sevoflurane or its equivalent for etomidate in rats.

CONCLUSIONS

These data, plus data from studies by others about D1 dopamine receptors, indicate that dopamine receptors do not mediate the immobility produced by inhaled anesthetics.

Effects of isoflurane and enflurane on GABA A and glycine receptors contribute equally to depressant actions on spinal ventral horn neurones in rats

BACKGROUND

Volatile anaesthetics are widely used agents in clinical anaesthesia, although their mechanism of action is poorly understood. In particular, the dominant molecular mechanisms by which volatile anaesthetics depress spinal neurones and thereby mediate spinal effects such as immobility have recently become a matter of dispute. As GABAA and glycine receptors are potential candidates we investigated the impact of both receptor systems in mediating the depressant effects of isoflurane and enflurane on spinal neurones in rats.

METHODS

The effects of isoflurane and enflurane on spontaneous action potential firing were investigated by extracellular voltage recordings from ventral horn interneurones in cultured spinal cord tissue slices obtained from embryonic rats (E 14-15).

RESULTS

Isoflurane and enflurane reduced spontaneous action potential firing. Concentrations causing half-maximal effects (isoflurane: 0.17 mM; enflurane: 0.50 mM) were less than EC50-immobility (isoflurane: 0.32 mM; enflurane: 0.62 mM). Effects of isoflurane were mediated by 39% by glycine receptors and 36% by GABAA receptors. The effects of enflurane were mediated 26% by GABAA receptors and 29% by glycine receptors.

CONCLUSION

These results demonstrate that the effects of isoflurane and enflurane on GABAA and glycine receptors contribute almost equally to their depressant actions on spinal ventral horn neurones in rats. The fraction of inhibition mediated by both receptor systems differs between specific volatile anaesthetics. Our data argue against the theory that a dominant molecular mechanism accounts for spinal effects of volatile anaesthetics.

Excitatory and inhibitory actions of isoflurane in bovine chromaffin cells

Isoflurane, a halogenated volatile anesthetic, is thought to produce anesthesia by depressing CNS function. Many anesthetics, including isoflurane, are thought to modulate and/or directly activate GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors. We previously showed that activation of the GABA(A) receptors, with specific agonists, leads to cellular excitation resulting from the depolarized anion equilibrium potential. In this study, our goal was to determine whether isoflurane mimicked this response and to explore the functional consequences of this activation. Furthermore, we sought to study the actions of isoflurane on nicotinic acetylcholine receptors (nAChRs) as they mediate the "sympathetic drive" in these cells. For these studies the Ca(2+)-indicator dye fura-2 was used to assay [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release. We show that bovine adrenal chromaffin cells were excited by isoflurane at clinically relevant concentrations. Isoflurane directly activated GABA(A) receptors found in chromaffin cells, which depolarized the cells and elevated [Ca(2+)](i). Application of isoflurane directly to the chromaffin cells elicited catecholamine secretion from these cells. At the same time, isoflurane suppressed activation of nAChRs, which presumably blocks "sympathetic drive" to the chromaffin cells. These latter results may help explain why isoflurane produces the hypotension observed clinically.

GABA(A) receptor diversity and pharmacology

Because of its control of spike-timing and oscillatory network activity, gamma-aminobutyric acid (GABA)-ergic inhibition is a key element in the central regulation of somatic and mental functions. The recognition of GABA(A) receptor diversity has provided molecular tags for the analysis of distinct neuronal networks in the control of specific pharmacological and physiological brain functions. Neurons expressing alpha(1)GABA(A) receptors have been found to mediate sedation, whereas those expressing alpha(2)GABA(A) receptors mediate anxiolysis. Furthermore, associative temporal and spatial memory can be regulated by modulating the activity of hippocampal pyramidal cells via extrasynaptic alpha(5)GABA(A) receptors. In addition, neurons expressing alpha(3)GABA(A) receptors are instrumental in the processing of sensory motor information related to a schizophrenia endophenotype. Finally, during the postnatal development of the brain, the maturation of GABAergic interneurons seems to provide the trigger for the experience-dependent plasticity of neurons in the visual cortex, with alpha(1)GABA(A) receptors setting the time of onset of a critical period of plasticity. Thus, particular neuronal networks defined by respective GABA(A) receptor subtypes can now be linked to the regulation of various clearly defined behavioural patterns. These achievements are of obvious relevance for the pharmacotherapy of certain brain disorders, in particular sleep dysfunctions, anxiety disorders, schizophrenia and diseases associated with memory deficits.

Isoflurane-induced surgical tolerance mediated only in part by beta3-containing GABA(A) receptors

The targets which mediate the actions of the volatile general anaesthetic isoflurane are unknown. Based on pharmacological studies using GABA(A) receptor antagonists it has recently been suggested that GABA(A) receptors would not mediate the immobilizing action of isoflurane. Using the beta3(N265M) knock-in mouse model we found that the mutant mice were less sensitive to the immobilizing action of isoflurane, indicating a role of beta3-containing GABA(A) receptors in mediating immobility. At high concentrations isoflurane also immobilizes beta3(N265M) mice, indicating that other targets also mediate immobility. Thus, our findings support a multisite model for the immobilizing action of isoflurane.