Nonanesthetics can suppress learning

Assessing anesthetic activity through modulation of the membrane dipole potential

There is great individual variation in response to general anesthetics (GAs) leading to difficulties in optimal dosing and sometimes even accidental awareness during general anesthesia (AAGA). AAGA is a rare, but potentially devastating, complication affecting between 0.1% and 2% of patients undergoing surgery. The development of novel personalized screening techniques to accurately predict a patient’s response to GAs and the risk of AAGA remains an unmet clinical need. In the present study, we demonstrate the principle of using a fluorescent reporter of the membrane dipole potential, di-8-ANEPPs, as a novel method to monitor anesthetic activity using a well-described inducer/noninducer pair. The membrane dipole potential has previously been suggested to contribute a novel mechanism of anesthetic action. We show that the fluorescence ratio of di-8-ANEPPs changed in response to physiological concentrations of the anesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F₃), but not the structurally similar noninducer, 1,2-dichlorohexafluorocyclobutane (F₆), to artificial membranes and in vitro retinal cell systems. Modulation of the membrane dipole provides an explanation to overcome the limitations associated with the alternative membrane-mediated mechanisms of GA action. Furthermore, by combining this technique with noninvasive retinal imaging technologies, we propose that this technique could provide a novel and noninvasive technique to monitor GA susceptibility and identify patients at risk of AAGA.

Emulsified isoflurane combined with therapeutic hypothermia improves survival and neurological outcomes in a rat model of cardiac arrest

Emulsified isoflurane (EIso), when introduced following cardiopulmonary resuscitation (CPR), significantly improves survival and neurological outcomes in a rat model of cardiac arrest (CA). The present study aimed to examine whether EIso combined with therapeutic hypothermia (TH) confers an additive neuroprotective effect. Adult male Sprague-Dawley rats that were subjected to return of spontaneous circulation (ROSC) after a 6-min asphyxia-induced CA were randomized to five groups: Sham group, control group under normothermic conditions, EIso group (4 ml/kg for 30 min under normothermic conditions), TH group (33°C for 2 h), and EIso plus TH group. Survival conditions and neurological outcomes were evaluated at 1 day and 7 days after ROSC. Animal survival rate at 7 days after ROSC was 30.7% in the CPR group, 60% in the EIso group, 63.6% in the TH group and 72.7% in the EIso plus TH group. EIso, TH and EIso plus TH yielded significant improvements in survival rates, neural deficit score and cognitive function, and ameliorated hippocampal CA1 region cell injury and apoptosis at 1 day and 7 days after ROSC compared with the CPR group. Combined therapy of EIso and TH was superior to EIso or TH alone, suggesting that combined EIso and TH treatment results in significant improvements in survival and neurological outcomes, and was more effective than independent EIso or TH treatment.

HCN1 Channels Contribute to the Effects of Amnesia and Hypnosis but not Immobility of Volatile Anesthetics

BACKGROUND

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) subtype 1 (HCN1) channels have been identified as targets of ketamine to produce hypnosis. Volatile anesthetics also inhibit HCN1 channels. However, the effects of HCN1 channels on volatile anesthetics in vivo are still elusive. This study uses global and conditional HCN1 knockout mice to evaluate how HCN1 channels affect the actions of volatile anesthetics.

METHODS

Minimum alveolar concentrations (MACs) of isoflurane and sevoflurane that induced immobility (MAC of immobility) and/or hypnosis (MAC of hypnosis) were determined in wild-type mice, global HCN1 knockout (HCN1) mice, HCN1 channel gene with 2 lox-P sites flanking a region of the fourth exon of HCN1 (HCN1) mice, and forebrain-selective HCN1 knockout (HCN1: cre) mice. Immobility of mice was defined as no purposeful reactions to tail-clamping stimulus, and hypnosis was defined as loss of righting reflex. The amnestic effects of isoflurane and sevoflurane were evaluated by fear-potentiated startle in these 4 strains of mice.

RESULTS

All MAC values were expressed as mean ± SEM. For MAC of immobility of isoflurane, no significant difference was found among wild-type, HCN1, HCN1, and HCN1: cre mice (all ~1.24%-1.29% isoflurane). For both HCN1 and HCN1: cre mice, the MAC of hypnosis for isoflurane (each ~1.05% isoflurane) was significantly increased over their nonknockout controls: HCN1 versus wild-type (0.86% ± 0.03%, P < 0.001) and HCN1: cre versus HCN1 mice (0.84% ± 0.03%, P < 0.001); no significant difference was found between HCN1 and HCN1: cre mice. For MAC of immobility of sevoflurane, no significant difference was found among wild-type, HCN1, HCN1, and HCN1: cre mice (all ~2.6%-2.7% sevoflurane). For both HCN1 and HCN1: cre mice, the MAC of hypnosis for sevoflurane (each ~1.90% sevoflurane) was significantly increased over their nonknockout controls: HCN1 versus wild-type (1.58% ± 0.05%, P < 0.001) and HCN1: cre versus HCN1 mice (1.56% ± 0.05%, P < 0.001). No significant difference was found between HCN1 and HCN1: cre mice. By fear-potentiated startle experiments, amnestic effects of isoflurane and sevoflurane were significantly attenuated in HCN1 and HCN1: cre mice (both P < 0.002 versus wild-type or HCN1 mice). No significant difference was found between HCN1 and HCN1: cre mice.

CONCLUSIONS

Forebrain HCN1 channels contribute to hypnotic and amnestic effects of volatile anesthetics, but HCN1 channels are not involved in the immobilizing actions of volatile anesthetics.

Distinctive recruitment of endogenous sleep-promoting neurons by volatile anesthetics and a nonimmobilizer

BACKGROUND

Numerous studies demonstrate that anesthetic-induced unconsciousness is accompanied by activation of hypothalamic sleep-promoting neurons, which occurs through both pre- and postsynaptic mechanisms. However, the correlation between drug exposure, neuronal activation, and onset of hypnosis remains incompletely understood. Moreover, the degree to which anesthetics activate both endogenous populations of γ-aminobutyric acid (GABA)ergic sleep-promoting neurons within the ventrolateral preoptic (VLPO) and median preoptic nuclei remains unknown.

METHODS

Mice were exposed to oxygen, hypnotic doses of isoflurane or halothane, or 1,2-dichlorohexafluorocyclobutane (F6), a nonimmobilizer. Hypothalamic brain slices prepared from anesthetic-naive mice were also exposed to oxygen, volatile anesthetics, or F6 ex vivo, both in the presence and absence of tetrodotoxin. Double-label immunohistochemistry was performed to quantify the number of c-Fos-immunoreactive nuclei in the GABAergic subpopulation of neurons in the VLPO and the median preoptic areas to test the hypothesis that volatile anesthetics, but not nonimmobilizers, activate sleep-promoting neurons in both nuclei.

RESULTS

In vivo exposure to isoflurane and halothane doubled the fraction of active, c-Fos-expressing GABAergic neurons in the VLPO, whereas F6 failed to affect VLPO c-Fos expression. Both in the presence and absence of tetrodotoxin, isoflurane dose-dependently increased c-Fos expression in GABAergic neurons ex vivo, whereas F6 failed to alter expression. In GABAergic neurons of the median preoptic area, c-Fos expression increased with isoflurane and F6, but not with halothane exposure.

CONCLUSIONS

Anesthetic unconsciousness is not accompanied by global activation of all putative sleep-promoting neurons. However, within the VLPO hypnotic doses of volatile anesthetics, but not nonimmobilizers, activate putative sleep-promoting neurons, correlating with the appearance of the hypnotic state.

Inhaled anesthetics in horses

Inhaled agents represent an important and useful class of drugs for equine anesthesia. This article reviews the ether-type anesthetics in contemporary use, their uptake and elimination, their mechanisms of action, and their desirable and undesirable effects in horses.

The effect of sevoflurane anesthesia on cognitive function and the expression of Insulin-like Growth Factor-1 in CA1 region of hippocampus in old rats

To investigate the effects of sevoflurane on cognitive function in old Sprague-Dawley (SD) rats and the expression of insulin-like growth factor-1 (IGF-1) in CA1 region of hippocampus. Forty Sprague-Dawley rats of 12 months old were randomly divided into five groups: the normal control group; 1.5% sevoflurane I group (be tested after received 1.5% sevoflurane for 1 day); 1.5% sevoflurane II group (be tested after received 1.5% sevoflurane for 7 day); 3.0% sevoflurane I group (be tested after received 3.0% sevoflurane for 1 day) and 3.0% sevoflurane II group (be tested after received 3.0% sevoflurane for 7 day). All SD rats were received 1.5 or 3.0% sevoflurane in a special glass anesthesia box for 2 h respectively, except for the normal control group. Y-maze was used to test the ability of learning and memory after being received sevoflurane for 1 or 7 days at the same moment portion. The altered expression level of IGF-1 in the hippocampus was tested to compare its transcripts by RT-PCR analysis. The results showed that 3% sevoflurane induced the decline of cognitive function and significantly deceased the IGF-1 expression at mRNA levels at 1 day in the 3.0% sevoflurane I group when compared with the normal control group. However, there were no significant difference among the other groups when compared with normal control group. Therefore, administration of sevoflurane might temporally affect the ability of cognitive function of rats through suppressing the IGF-1 expression at mRNA levels in hippocampus.

Anaesthetic mechanisms: update on the challenge of unravelling the mystery of anaesthesia

General anaesthesia is administered each day to thousands of patients worldwide. Although more than 160 years have passed since the first successful public demonstration of anaesthesia, a detailed understanding of the anaesthetic mechanism of action of these drugs is still lacking. An important early observation was the Meyer-Overton correlation, which associated the potency of an anaesthetic with its lipid solubility. This work focuses attention on the lipid membrane as a likely location for anaesthetic action. With the advent of cellular electrophysiology and molecular biology techniques, tools to dissect the components of the lipid membrane have led, in recent years, to the widespread acceptance of proteins, namely receptors and ion channels, as more likely targets for the anaesthetic effect. Yet these accumulated data have not produced a comprehensive explanation for how these drugs produce central nervous system depression. In this review, we follow the story of anaesthesia mechanisms research from its historical roots to the intensely neurophysiological research regarding it today. We will also describe recent findings that identify specific neuroanatomical locations mediating the actions of some anaesthetic agents.

Protein crystallography under xenon and nitrous oxide pressure: comparison with in vivo pharmacology studies and implications for the mechanism of inhaled anesthetic action

In contrast with most inhalational anesthetics, the anesthetic gases xenon (Xe) and nitrous oxide (N(2)O) act by blocking the N-methyl-d-aspartate (NMDA) receptor. Using x-ray crystallography, we examined the binding characteristics of these two gases on two soluble proteins as structural models: urate oxidase, which is a prototype of a variety of intracellular globular proteins, and annexin V, which has structural and functional characteristics that allow it to be considered as a prototype for the NMDA receptor. The structure of these proteins complexed with Xe and N(2)O were determined. One N(2)O molecule or one Xe atom binds to the same main site in both proteins. A second subsite is observed for N(2)O in each case. The gas-binding sites are always hydrophobic flexible cavities buried within the monomer. Comparison of the effects of Xe and N(2)O on urate oxidase and annexin V reveals an interesting relationship with the in vivo pharmacological effects of these gases, the ratio of the gas-binding sites' volume expansion and the ratio of the narcotic potency being similar. Given these data, we propose that alterations of cytosolic globular protein functions by general anesthetics would be responsible for the early stages of anesthesia such as amnesia and hypnosis and that additional alterations of ion-channel membrane receptor functions are required for deeper effects that progress to "surgical" anesthesia.

Partitioning of anesthetics into a lipid bilayer and their interaction with membrane-bound peptide bundles

Molecular dynamics simulations have been performed to investigate the partitioning of the volatile anesthetic halothane from an aqueous phase into a coexisting hydrated bilayer, composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids, with embedded alpha-helical peptide bundles based on the membrane-bound portions of the alpha- and delta-subunits, respectively, of nicotinic acetylcholine receptor. In the molecular dynamics simulations halothane molecules spontaneously partitioned into the DOPC bilayer and then preferentially occupied regions close to lipid headgroups. A single halothane molecule was observed to bind to tyrosine (Tyr-277) residue in the alpha-subunit, an experimentally identified specific binding site. The binding of halothane attenuated the local loop dynamics of alpha-subunit and significantly influenced global concerted motions suggesting anesthetic action in modulating protein function. Steered molecular dynamics calculations on a single halothane molecule partitioned into a DOPC lipid bilayer were performed to probe the free energy profile of halothane across the lipid-water interface and rationalize the observed spontaneous partitioning. Partitioned halothane molecules affect the hydrocarbon chains of the DOPC lipid, by lowering of the hydrocarbon tilt angles. The anesthetic molecules also caused a decrease in the number of peptide-lipid contacts. The observed local and global effects of anesthetic binding on protein motions demonstrated in this study may underlie the mechanism of action of anesthetics at a molecular level.

Selective enhancement of tonic GABAergic inhibition in murine hippocampal neurons by low concentrations of the volatile anesthetic isoflurane

Volatile (inhaled) anesthetics cause amnesia at concentrations well below those that cause loss of consciousness and immobility; however, the underlying neuronal mechanisms are unknown. Although many anesthetics increase inhibitory GABAergic synaptic transmission, this effect occurs only at high concentrations (>100 microm). Molecular targets for low concentrations of inhaled anesthetics have not been identified. Here, we report that a tonic inhibitory conductance in hippocampal pyramidal neurons generated by alpha5 subunit-containing GABA(A) receptors is highly sensitive to low concentrations of the volatile anesthetic isoflurane (ISO) (25 and 83.3 microm). The alpha5 subunit is necessary for enhancement of the tonic current by these low concentrations of isoflurane because potentiation is absent in neurons from alpha5-/- mice. Furthermore, ISO (25 microm) potentiated recombinant human alpha5beta3gamma2L GABA(A) receptors, whereas this effect was not seen with alpha1beta3gamma2L GABA(A) receptors. These studies suggest that an increased tonic inhibition in the hippocampus may contribute to amnestic properties of volatile anesthetics.

Membrane structural perturbations caused by anesthetics and nonimmobilizers: a molecular dynamics investigation

The structural perturbations of the fully hydrated dimyristoyl-phosphatidylcholine bilayer induced by the presence of hexafluoroethane C(2F6), a "nonimmobilizer," have been examined by molecular dynamics simulations and compared with the effects produced by halothane CF3CHBrCl, an "anesthetic," on a similar bilayer (DPPC) (Koubi et al., Biophys. J. 2000. 78:800). We find that the overall structure of the lipid bilayer and the zwitterionic head-group dipole orientation undergo only a slight modification compared with the pure lipid bilayer, with virtually no change in the potential across the interface. This is in contrast to the anesthetic case in which the presence of the molecule led to a large perturbation of the electrostatic potential across to the membrane interface. Similarly, the analysis of the structural and dynamical properties of the lipid core are unchanged in the presence of the nonimmobilizer although there is a substantial increase in the microscopic viscosity for the system containing the anesthetic. These contrasting perturbations of the lipid membrane caused by those quite similarly sized molecules may explain the difference in their physiological effects as anesthetics and nonimmobilizers, respectively.

Halothane binding to a G protein coupled receptor in retinal membranes by photoaffinity labeling

General anesthetics have been reported to alter the functions of G protein coupled receptor (GPCR) signaling systems. To determine whether these effects might be mediated by direct binding interactions with the GPCR or its associated G protein, we studied the binding character of halothane on mammalian rhodopsin, structurally the best understood GPCR, by using direct photoaffinity labeling with [(14)C]halothane. In the bleached bovine rod disk membranes (RDM), opsin and membrane lipids were dominantly photolabeled with [(14)C]halothane, but none of the three G protein subunits were labeled. In opsin itself, halothane labeling was inhibited by unlabeled halothane with an IC(50) of 0.9 mM and a Hill coefficient of -0.8. The stoichiometry was 1.1:1.0 (halothane:opsin molar ratio). The IC(50) values of isoflurane and 1-chloro-1,2, 2-trifluorocyclobutane were 5.0 and 15 mM, respectively. Ethanol had no effect on opsin labeling by halothane. A nonimmobilizer, 1, 2-dichlorohexafluorocyclobutane, inhibited halothane labeling by 50% at 0.05 mM. The present results demonstrate that halothane binds specifically and selectively to GPCRs in the RDM. The absence of halothane binding to any of the G protein subunits strongly suggests that the functional effects of halothane on GPCR signaling systems are mediated by direct interactions with receptor proteins.

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Effects of ethanol and anesthetics on type 1 and 5 metabotropic glutamate receptors expressed in Xenopus laevis oocytes

Previous studies have demonstrated that ethanol and volatile anesthetics inhibit the function of some metabotropic (G protein-coupled) receptors, including the 5-hydroxytryptamine2 and muscarinic cholinergic receptors. The metabotropic glutamate receptors (mGluRs) show little sequence homology with most other metabotropic receptors and are important modulators of synaptic transmission in the mammalian central nervous system. It was of interest to determine drug actions on these receptors, and we investigated the effects of ethanol, halothane, the anesthetic compound F3 (1-chloro-1,2,2-trifluorocyclobutane), and the nonanesthetics F6 (1,2-dichlorohexafluorocyclobutane) and F8 (2,3-chlorooctafluorobutane) on the function of mGluR1 and mGluR5 expressed in Xenopus laevis oocytes. Halothane, F3, and ethanol inhibited mGluR5-induced Ca(2+)-dependent Cl- currents, yet pharmacologically relevant concentrations of these compounds had little effect on the glutamate-induced currents in the oocytes expressing mGluR1. F6 had inhibitory effects on both receptors, and F8 did not affect either mGluR1 or mGluR5 function. The protein kinase C (PKC) inhibitor GF109203X enhanced the glutamate-induced current, and the PKC activator phorbol-12-myristate-13-acetate inhibited this current in the oocytes expressing mGluR5, but these compounds had little effect on mGluR1 function. GF109203X abolished the inhibitory effects of halothane, F3, and ethanol on mGluR5s. Conversely, the phosphatase inhibitor calyculin A prolonged the action of halothane and ethanol. Furthermore, mutation of a PKC consensus site (Ser890) of mGluR5 abolished the inhibitory effects of halothane, F3, and ethanol. These results suggest that ethanol and volatile anesthetics inhibit mGluR5 because they promote PKC-mediated phosphorylation.