Inhaled anesthetics and immobility: mechanisms, mysteries, and minimum alveolar anesthetic concentration

Alcohol solubility in a lipid bilayer: Efficient grand-canonical simulation of an interfacially active molecule

We derive a new density-biased Monte Carlo technique which preserves detailed balance and improves the convergence of grand-canonical simulations of a species with a strong preference for an interfacial region as compared to the bulk. This density-biasing technique is applied to the solubility of "alcohol" molecules in a mesoscopic model of the lipid bilayer, a system which has anesthetic implications but is poorly understood.

Fundamentals of neuronal apoptosis relevant to pediatric anesthesia

The programmed cell death or apoptosis is a complex biochemical process that has risen to prominence in pediatric anesthesia. Preclinical studies report a dose-dependent neuronal apoptosis during synaptogenesis following exposure to intravenous and volatile anesthetic agents. Although emerging clinical data do not universally indicate an increased neurodegenerative risk of general anesthesia in early human life, a great deal of uncertainty was created within the pediatric anesthesia community. This was at least partially caused by the demand of understanding of basic science concepts and knowledge of apoptosis frequently out of reach to the clinician. It is, however, important for the pediatric anesthesiologist to be familiar with the basic science concepts of neuronal apoptosis to be able to critically evaluate current and future preclinical data in this area and future clinical studies. This current review describes the extrinsic and intrinsic pathways involved in the cell death process and discusses techniques commonly employed to determine apoptosis. In addition, potential mechanisms of anesthesia-induced neuronal apoptosis are illustrated in this review.

Reduced immobilizing properties of isoflurane and nitrous oxide in mutant mice lacking the N-methyl-D-aspartate receptor GluR(epsilon)1 subunit are caused by the secondary effects of gene knockout

BACKGROUND

Until recently, the N-methyl-D-aspartate (NMDA) receptor was considered to possibly mediate the immobility produced by inhaled anesthetics such as isoflurane and nitrous oxide. However, new evidence suggests that the role of this receptor in abolition of the movement response may be less important than previously thought. To provide further evidence supporting or challenging this view, we examined the anesthetic potencies of isoflurane and nitrous oxide in genetically modified animals with established NMDA receptor dysfunction caused by GluRepsilon1 subunit knockout.

METHODS

The immobilizing properties of inhaled anesthetics in mice quantitated by the minimum alveolar anesthetic concentration (MAC) were evaluated using the classic tail clamp method.

RESULTS

Compared with wild-type controls, NMDA receptor GluRepsilon1 subunit knockout mice displayed larger isoflurane MAC values indicating a resistance to the immobilizing action of isoflurane. Knockout mice were previously shown to have enhanced monoaminergic tone as a result of genetic manipulation, and this increase in MAC could be abolished in our experiments by pretreatment with the serotonin 5-hydroxytryptamine type 2A receptor antagonist ketanserin or with the dopamine D2 receptor antagonist droperidol at doses that did not affect MAC values in wild-type animals. Mutant mice also displayed resistance to the isoflurane MAC-sparing effect of nitrous oxide, but this resistance was similarly abolished by ketanserin and droperidol. Thus, resistance to the immobilizing action of inhaled anesthetics in knockout mice seems to be secondary to increased monoaminergic activation after knockout rather than a direct result of impaired NMDA receptor function.

CONCLUSIONS

Our results confirm recent findings indicating no critical contribution of NMDA receptors to the immobility induced by isoflurane and nitrous oxide. In addition, they demonstrate the ability of changes secondary to genetic manipulation to affect the results obtained in global knockout studies.

Intranasal application of xenon: describing the pharmacokinetics in experimental animals and the increased pain tolerance within a placebo-controlled experimental human study

BACKGROUND

Pain sensitizes the central nervous system via N-methyl-D-aspartate receptors (NMDARs) leading to an enhancement of pain perception. However, the enhanced responsiveness of pain-processing areas can be suppressed by subanaesthetic doses of the NMDAR antagonist xenon. To analyse the strength of the analgesic effect of low-dose xenon using new economical application methods, we tested xenon applied nasally in an experimental human pain setting.

METHODS

We tested 10 healthy volunteers using a multimodal experimental pain testing in a randomized double-blind placebo-controlled repeated measures study. Xenon was administered using a novel low-pressure intranasal application device. Additionally, we measured xenon concentrations in blood samples obtained from intracranial veins of experimental animals to describe the pharmacokinetics of intranasally applied xenon in the cerebral compartment.

RESULTS

Intranasal application of xenon at a rate of 1.0 litre h(-1) for 30 min significantly increased pain tolerance of volunteers to ischaemic (+128%), cold (+58%), and mechanical (+40%) stimulation (P<0.01). However, 60 min after terminating the application of xenon, there was no significant alteration of pain tolerance compared with placebo. Cranial blood concentrations of xenon in pigs reached a steady state of approximately 450 nl ml(-1) after 5 min.

CONCLUSIONS

In this placebo-controlled experimental human study, we described the increased pain tolerance induced by intranasally applied xenon. On the basis of our results, we conclude that intranasally administered xenon has analgesic properties and suggest that the novel application device presented here offers new possibilities for the administration of NMDAR antagonists within a multimodal analgesia approach.

Bidirectional modulation of isoflurane potency by intrathecal tetrodotoxin and veratridine in rats

BACKGROUND AND PURPOSE

Results from several studies point to voltage-gated Na(+) channels as potential mediators of the immobility produced by inhaled anaesthetics. We hypothesized that the intrathecal administration of tetrodotoxin, a drug that blocks Na(+) channels, should enhance anaesthetic potency, and that concurrent administration of veratridine, a drug that augments Na(+) channel opening, should reverse the increase in potency.

EXPERIMENTAL APPROACH

We measured the change in isoflurane potency for reducing movement in response to a painful stimulus as defined by MAC (minimum alveolar concentration of anaesthetic required to abolish movement in 50% of subjects) caused by intrathecal infusion of various concentrations of tetrodotoxin into the lumbothoracic subarachnoid space of rats, and the change in MAC caused by the administration of a fixed dose of tetrodotoxin plus various doses of intrathecal veratridine.

KEY RESULTS

Intrathecal infusion of tetrodotoxin (0.078-0.63 microM) produced a reversible dose-related decrease in MAC, of more than 50% at the highest concentration. Intrathecal co-administration of veratridine (1.6-6.4 microM) reversed this decrease in a dose-related manner, with nearly complete reversal at the highest veratridine dose tested.

CONCLUSIONS AND IMPLICATIONS

Intrathecal administration of tetrodotoxin increases isoflurane potency (decreases isoflurane MAC), and intrathecal administration of veratridine counteracts this effect in vivo. These findings are consistent with a role for voltage-gated Na(+) channel blockade in the immobility produced by inhaled anaesthetics.

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.

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.

Effect of ketamine on the limb withdrawal reflex evoked by transcutaneous electrical stimulation in ponies anaesthetised with isoflurane

The purpose of this study was to evaluate the anti-nociceptive activity of ketamine and isoflurane in horses using a limb withdrawal reflex (WR) model. Single and repeated stimulations were applied to the digital nerve of the left forelimb in ponies anaesthetised with isoflurane before, during and after intravenous administration of racemic ketamine. Surface electromyographic activity was recorded from the deltoid muscle. Higher stimulation intensity was required to evoke a reflex during ketamine administration. Furthermore, the amplitudes of response to stimulations were significantly and dose-dependently depressed and a flattening of the stimulus-response curves was observed. The reflex activity recovered partially once the ketamine infusion finished. The results demonstrated that the limb WR can be used to quantify the temporal effect of ketamine on the sensory-motor processing in ponies anaesthetised with isoflurane.

[Implications of anesthetic techniques on patients' recovery in laparoscopic cholecystectomy]

BACKGROUND/AIM

Different techniques of general anesthesia are used for laparoscopic cholecystectomy (LC). The aim of the study was to establish the best anesthetic technique for achieving better results during awakening affecting not only patient's recovery, but activities of anesthesiological team, as well.

METHODS

The study was conducted as a prospective comparative clinical trial. The patients (n=90) were classified according to the applied anesthetic technique into two groups: Volatile Induction and Maintenance Anaesthesia (VIMA) with sevofluran and Target Controlled Infusion (TCI). The results relating to parameters of recovery after anesthesia and surgery were compared between these two groups. The following parameters were analysed: demographic patients' characteristics, duration of anesthesia, the times to eye opening, to respond to a command, to extubation, and to orientation, from the last anesthetic dose receiving until post anesthesia discharge (PAD), frequency of postoperative nausea, vomiting and agitation (PONVA).

RESULTS

In the examined groups there were no statistically significant differences in the duration of anesthesia (68.29 +/- 6.47 vs 66.29 +/- 11.97 min, p = 0.327). The time to eye opening was significantly shorter in the group VIMA compared to the group TCI (4.49 +/- 1.20 vs 7.42 +/- 1.25 min, p = 0.000), as well as the time to respond to a command (5.93 +/- 1.12 vs 8.47 +/- 1.08 min, p = 0.000). The patients anesthetised with VIMA technique wer estatistically significantly extubated earlier (6.84 +/- 1.19 vs 9.69 +/- 1.31 min, p = 0.000). Considering orientation time, there was also statistically significant difference between the two groups (7.51 +/- 0.97 vs 11.60 +/- 1.75 min, p = 0.000). There was no statistically significant difference in PAD time duration (19.42 +/- 5.99 vs 20.80 +/- 1.59 min, p = 0.142). There were no statistically significant differences in PONVA events between the examined groups.

CONCLUSION

This study showed that VIMA technique with sevofluran in LC provides faster and more qualitative recovery of patients. Thus this technique should be applied in everyday anesthesiological procedures in LC, as well as in other minimally invasive videoendoscopic surgical procedures.

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.

Activation of orexin signal in basal forebrain facilitates the emergence from sevoflurane anesthesia in rat

Orexinergic system may play an important role in the regulation of anesthesia-arousal. However, which region or which pathway mediated the effect of orexins was still unclear. In current study, we investigated whether activation of orexin signals in basal forebrain (BF) may alter electroencephalographic activity, induction and emergence time to sevoflurane anesthesia in rats. Either orexin-A or orexin-B was injected into the BF while measuring electroencephalogram (EEG) under 1.0 minimum alveolar concentration (2.4%) sevoflurane anesthesia. The induction and emergence time of sevoflurane anesthesia were measured respectively after an injection of orexin receptor agonist (orexin-A or orexin-B) or antagonist (SB-334867A) into the BF also. We found that the administration of orexin-A (30, 100 pmol) and orexin-B (100 pmol) changed the burst and suppression patterns to arousal EEG in rat under sevoflurane anesthesia. Comparing with orexin-B, injection of lower dose of orexin-A induced more arousal EEG. Intrabasalis microinjection of orexin-A shorted the emergence time, whereas intrabasalis microinjection of SB-334867A (5 microg, 20 microg) delayed the emergence time to sevoflurane anesthesia, without changing anesthetic induction. These findings indicate that the orexin signals in basal forebrain, a middle region of the cholinergic ventral ascending arousal system, plays a crucial role in the anesthesia-arousal regulation.

Isoflurane inhibits growth but does not cause cell death in hippocampal neural precursor cells grown in culture

BACKGROUND

Isoflurane causes long-term hippocampal-dependent learning deficits in rats despite limited isoflurane-induced hippocampal cell death, raising questions about the causality between isoflurane-induced cell death and isoflurane-induced cognitive function. Neurogenesis in the dentate gyrus is required for hippocampal-dependent learning and thus constitutes a potential alternative mechanism by which cognition can be altered after neonatal anesthesia. The authors tested the hypothesis that isoflurane alters proliferation and differentiation of hippocampal neural progenitor cells.

METHODS

Multipotent neural progenitor cells were isolated from pooled rat hippocampi (postnatal day 2) and grown in culture. These cells were exposed to isoflurane and evaluated for cell death using lactate dehydrogenase release, caspase activity, and immunocytochemistry for nuclear localization of cleaved caspase 3. Growth was assessed by cell counting and BrdU incorporation. Expression of markers of stemness (Sox2) and cell division (Ki67) were determined by quantitative polymerase chain reaction. Cell fate selection was assessed using immunocytochemistry to stain for neuronal and glial markers.

RESULTS

Isoflurane did not change lactate dehydrogenase release, activity of caspase 3/7, or the amount of nuclear cleaved caspase 3. Isoflurane decreased caspase 9 activity, inhibited proliferation, and decreased the proportion of cells in s-phase. messenger ribonucleic acid expression of Sox2 (stem cells) and Ki67 (proliferation) were decreased. Differentiating neural progenitor cells more often select a neuronal fate after isoflurane exposure.

CONCLUSIONS

The authors conclude that isoflurane does not cause cell death, but it does act directly on neural progenitor cells independently of effects on the surrounding brain to decrease proliferation and increase neuronal fate selection. These changes could adversely affect cognition after isoflurane anesthesia.

Glycine receptors contribute to hypnosis induced by ethanol

BACKGROUND

Glycine is a major inhibitory neurotransmitter in the adult central nervous system (CNS), and its receptors (GlyRs) are well known for their effects in the spinal cord and the lower brainstem. Accumulating evidence indicates that GlyRs are more widely distributed in the CNS, including many supraspinal regions. Previous in vitro studies have demonstrated that ethanol potentiates the function of these brain GlyRs, yet the behavioral role of the brain GlyRs has not been well explored.

METHODS

Experiments were conducted in rats. The loss of righting reflex (LORR) was used as a marker of the hypnotic state. We compared the LORR induced by systematic administration of ethanol and of ketamine in the absence and presence of the selective glycine receptor antagonist strychnine. Ketamine is a general anesthetic that does not affect GlyRs.

RESULTS

Systemically administered (by intraperitoneal injection) ethanol and ketamine dose-dependently induced LORR in rats. Furthermore, systemically administered (by subcutaneous injection) strychnine dose-dependently reduced the percentage of rats exhibiting LORR induced by ethanol, increased the onset time, and decreased the duration of LORR. Strychnine had no effect, however, on the LORR induced by ketamine.

CONCLUSIONS

Given that hypnosis is caused by neuronal depression in upper brain areas, we therefore conclude that brain GlyRs contribute at least in part to the hypnosis induced by ethanol.

Behavior and cellular evidence for propofol-induced hypnosis involving brain glycine receptors

BACKGROUND

It is well documented that several general anesthetics, including propofol, potentiate glycine receptor function. Furthermore, glycine receptors exist throughout the central nervous system, including areas of the brain thought to be involved in sleep. However, the role of glycine receptors in anesthetic-induced hypnosis has not been determined.

METHODS

Experiments were conducted in rats where the loss of righting reflex (LORR) was used as a marker of the hypnotic state. Propofol-induced LORR was examined in the presence and absence of strychnine (a glycine receptor antagonist), GABAzine (a gamma-aminobutyric acid A receptor antagonist), as well as ketamine (an antagonist of N-methyl-D-aspartic acid subtype of glutamate receptors). Furthermore, the effects of propofol on the currents elicited by glycine and gamma-aminobutyric acid were analyzed in neurons isolated from the posterior hypothalamus of rats. The effects of strychnine and GABAzine on propofol-induced currents were also evaluated.

RESULTS

Strychnine and GABAzine dose-dependently reduced the percentage of rats exhibiting LORR induced by propofol. Furthermore, strychnine significantly increased the onset time and reduced the duration of LORR induced by propofol. In contrast, strychnine did not affect the LORR induced by ketamine. In addition, propofol markedly increased the currents elicited by glycine and GABA of hypothalamic neurons. Conversely, strychnine and GABAzine both profoundly attenuated the current induced by propofol.

CONCLUSION

Strychnine, the glycine receptor antagonist, dose-dependently reduced propofol-induced LORR in rats and propofol-induced current of rat hypothalamic neurons. These results suggest that neuronal glycine receptors partially contribute to propofol-induced hypnosis.

Nicotinic receptor-evoked hippocampal norepinephrine release is highly sensitive to inhibition by isoflurane

BACKGROUND

Inhaled anaesthetics (IAs) produce multiple dose-dependent behavioural effects including amnesia, hypnosis, and immobility in response to painful stimuli that are mediated by distinct anatomical, cellular, and molecular mechanisms. Amnesia is produced at lower anaesthetic concentrations compared with hypnosis or immobility. Nicotinic acetylcholine receptors (nAChRs) modulate hippocampal neural network correlates of memory and are highly sensitive to IAs. Activation of hippocampal nAChRs stimulates the release of norepinephrine (NE), a neurotransmitter implicated in modulating hippocampal synaptic plasticity. We tested the hypothesis that IAs disrupt hippocampal synaptic mechanisms critical to memory by determining the effects of isoflurane on NE release from hippocampal nerve terminals.

METHODS

Isolated nerve terminals prepared from adult male Sprague-Dawley rat hippocampus were radiolabelled with [(3)H]NE and either [(14)C]GABA or [(14)C]glutamate and superfused at 37 degrees C. Release evoked by a 2 min pulse of 100 microM nicotine or 5 microM 4-aminopyridine was evaluated in the presence or absence of isoflurane and/or selective antagonists.

RESULTS

Nicotine-evoked NE release from rat hippocampal nerve terminals was nAChR- and Ca(2+)-dependent, involved both alpha7 and non-alpha7 subunit-containing nAChRs, and was partially dependent on voltage-gated Na(+) channel activation based on sensitivities to various antagonists. Isoflurane inhibited nicotine-evoked NE release (IC(50)=0.18 mM) more potently than depolarization-evoked NE release (IC(50)=0.27 mM, P=0.014), consistent with distinct presynaptic mechanisms of IA action.

CONCLUSIONS

Inhibition of hippocampal nAChR-dependent NE release by subanaesthetic concentrations of isoflurane supports a role in IA-induced amnesia.

Subunit-specific effects of isoflurane on neuronal Ih in HCN1 knockout mice

The ionic mechanisms that contribute to general anesthetic actions have not been elucidated, although increasing evidence has pointed to roles for subthreshold ion channels, such as the HCN channels underlying the neuronal hyperpolarization-activated cationic current (Ih). Here, we used conventional HCN1 knockout mice to test directly the contributions of specific HCN subunits to effects of isoflurane, an inhalational anesthetic, on membrane and integrative properties of motor and cortical pyramidal neurons in vitro. Compared with wild-type mice, residual Ih from knockout animals was smaller in amplitude and presented with HCN2-like properties. Inhibition of Ih by isoflurane previously attributed to HCN1 subunit-containing channels (i.e., a hyperpolarizing shift in half-activation voltage [V1/2]) was absent in neurons from HCN1 knockout animals; the remaining inhibition of current amplitude could be attributed to effects on residual HCN2 channels. We also found that isoflurane increased temporal summation of excitatory postsynaptic potentials (EPSPs) in cortical neurons from wild-type mice; this effect was predicted by simulation of anesthetic-induced dendritic Ih inhibition, which also revealed more prominent summation accompanying shifts in V1/2 (an HCN1-like effect) than decreased current amplitude (an HCN2-like effect). Accordingly, anesthetic-induced EPSP summation was not observed in cortical cells from HCN1 knockout mice. In wild-type mice, the enhanced synaptic summation observed with low concentrations of isoflurane contributed to a net increase in cortical neuron excitability. In summary, HCN channel subunits account for distinct anesthetic effects on neuronal membrane properties and synaptic integration; inhibition of HCN1 in cortical neurons may contribute to the synaptically mediated slow-wave cortical synchronization that accompanies anesthetic-induced hypnosis.

Carbon dioxide negatively modulates N-methyl-D-aspartate receptors

BACKGROUND

Carbon dioxide (CO2) dose-dependently decreases minimum alveolar concentration (MAC) of anaesthetics in rats. CO2 also dose-dependently decreases cerebrospinal fluid pH. N-methyl-D-aspartate (NMDA) channels exhibit pH sensitivity and are putative targets for inhaled anaesthetics. We hypothesized that CO2 dose-dependently decreases rat NMDA channel current via an acidifying effect at concentrations relevant to CO2 MAC.

METHODS

To test this hypothesis, we studied rat NR1/NR2A glutamate receptors expressed in voltage-clamped Xenopus oocytes. To measure pH effects, we used perfusates adjusted between 7.3 and 5.3 with HCl. To measure CO2 effects, we used equimolar sodium perfusates containing either 0 or 24 mM NaHCO3 and CO2 between 0% and 87% atm. Solution compositions were measured using a blood gas analyser with values corrected using a calibrated pH meter and gas chromatograph with solutions at 37 degrees C.

RESULTS

We found that decreasing pH decreased NMDA current. Moreover, pH effects produced by adding CO2 to NaHCO3-containing perfusates were identical to those produced by adding HCl to normal perfusates. The pH inhibiting 50% of NMDA current was 6.52. The CO2 concentration inhibiting 50% of rat NMDA current was 63% for solutions with 24 mM NaHCO3. CO2 exhibited a linear dose-dependent NMDA response analogous to that observed for in vivo CO2 anaesthetic potency in rats.

CONCLUSIONS

CO2 and hydrogen ions act via the same mechanism to inhibit NMDA receptors. Moreover, CO2 inhibits rat NMDA receptors in a manner that is consistent with CO2 MAC-sparing effects in rats.

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.

Anesthetic-like modulation of receptor function by surfactants: a test of the interfacial theory of anesthesia

INTRODUCTION

Inhaled anesthetics are interfacially active, concentrating at interfaces such as the protein/water or bilayer/water interfaces. We tested the hypothesis that interfacial activity was a sufficient condition for anesthetic-like modulation of receptor function by applying surfactants to gamma-aminobutyric acid type A (GABA(A)), glycine, and N-methyl-d-aspartate (NMDA) receptors. We defined anesthetic-like modulation as an increase in currents through native channels that isoflurane and ethanol increased currents through, and a decrease in currents through channels that isoflurane and ethanol decreased currents through. We also tested the null hypothesis that there would be no difference in modulation of channel currents by surfactants in receptors with point mutations that diminished their response to isoflurane and ethanol compared to the native version of these receptors.

METHODS

The effect of seven surfactants with different head group charges (anionic, cationic, zwitterionic, and uncharged) and tail lengths (8 carbons and 12 carbons) on homomeric wild type alpha1 and mutant alpha(1) (S267I) glycine receptors, wild type alpha(1)beta(2)gamma(2s) and mutant alpha(1)(S270I)beta(2)gamma(2s) GABA(A) receptors, and wild type NR1/NR2A and mutant NR1(F639A)/NR2A NMDA receptors was studied. Receptors were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamping.

RESULTS

All seven surfactants, isoflurane, and ethanol enhanced GABA(A) receptor function. Six of seven surfactants, isoflurane, and ethanol enhanced glycine receptor function. Six of seven surfactants, isoflurane, and ethanol inhibited NMDA receptor function. For the mutant receptors, five of seven surfactants increased currents through GABA(A) receptors, whereas six of seven surfactants increased currents through glycine receptors. Six of seven surfactants decreased currents through the NMDA receptor. In contrast to isoflurane and ethanol, surfactants as a group did not diminish modulation of mutant compared to wild type receptors.

CONCLUSION

These findings identify another large class of compounds (surfactants) that modulate the function of GABA(A), glycine, and NMDA receptors in a manner that is qualitatively similar to inhaled anesthetics. We cannot reject the hypothesis that interfacial activity is a sufficient condition for anesthetic-like modulation of these receptors. Mutations that diminish the modulatory effect of isoflurane and ethanol did not diminish the modulatory effect of the surfactants.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors participate in the analgesic but not hypnotic effects of emulsified halogenated anaesthetics

The present study was designed to investigate the role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in hypnosis and analgesia induced by emulsified inhalation anaesthetics. After having established the mice model of hypnosis and analgesia by intraperitoneally injecting appropriate doses of emulsified enflurane, isoflurane or sevoflurane, we intracerebroventricularly or intrathecally injected different doses of AMPA and then observed the effects on the sleep time using hypnosis test and the tail-withdrawal latency using the tail-withdrawal test. In hypnosis test, AMPA (50, 75 and 100 ng, intracerebroventricularly) had no distinctive effects on the sleep time of the mice treated with emulsified inhalation anaesthetics (P > 0.05). In tail-withdrawal test, AMPA (0.25, 0.5 and 1.0 ng, intrathecally) significantly and dose-dependently decreased the tail-withdrawal latency (P < 0.05 or P < 0.01) in the mice treated with emulsified anaesthetics. These results suggest that AMPA receptors may participate in the analgesic but not in the hypnotic effects induced by emulsified enflurane, isoflurane or sevoflurane.

Inhaled anesthetics do not combine to produce synergistic effects regarding minimum alveolar anesthetic concentration in rats

BACKGROUND

We hypothesized that pairs of inhaled anesthetics having divergent potencies [one acting weakly at minimum alveolar anesthetic concentration (MAC); one acting strongly at MAC] on specific receptors/channels might act synergistically, and that such deviations from additivity would support the notion that anesthetics act on multiple sites to produce anesthesia.

METHODS

Accordingly, we studied the additivity of MAC for 11 anesthetic pairs divergently (one weakly, one strongly) affecting a specific receptor/channel at MAC. By "divergently," we usually meant that at MAC the more strongly acting anesthetic enhanced or blocked the in vitro receptor or channel at least twice (and usually more) as much as did the weakly acting anesthetic. The receptors/channels included: TREK-1 and TASK-3 potassium channels; and gamma-aminobutyric acid type A, glycine, N-methyl-D-aspartic acid, and acetylcholine receptors. We also studied the additivity of cyclopropane-benzene because the N-methyl-D-aspartic acid blocker MK-801 had divergent effects on the MACs of these anesthetics. We also studied four pairs that included nitrous oxide because nitrous oxide had been reported to produce infraadditivity (antagonism) when combined with isoflurane.

RESULTS

All combinations produced a result within 10% of that which would be predicted by additivity except for the combination of isoflurane with nitrous oxide where infraadditivity was found.

CONCLUSIONS

Such results are consistent with the notion that inhaled anesthetics act on a single site to produce immobility in the face of noxious stimulation.

Isoflurane preconditioning reduces mouse microglial activation and injury induced by lipopolysaccharide and interferon-gamma

Activation and injury of microglial cells are involved in a broad range of brain diseases including stroke, brain infection and neurodegenerative diseases. However, there is very little information regarding how to reduce microglial reaction and preserve these cells to provide neuroprotection. Here, we showed that the incubation of C8-B4 mouse microglial cells with lipopolysaccharide (LPS) plus interferon-gamma (IFNgamma) for 24 h decreased the viability of these cells. Pretreatment of these cells with 1%, 2% or 3% isoflurane, a commonly used volatile anesthetic, for 1 h at 30 min before the exposure to LPS plus IFNgamma attenuated the reduction of cell viability (preconditioning effect). LPS plus IFNgamma also activated these microglial cells to express inducible nitric oxide synthase (iNOS) and to induce accumulation of nitrite, a stable oxidation product of nitric oxide, in the incubation medium. Isoflurane preconditioning attenuated these LPS plus IFNgamma effects on the iNOS expression and nitrite accumulation. Aminoguanidine, an iNOS inhibitor, attenuated the LPS plus IFNgamma-induced glutamate release and decrease of microglial viability. Isoflurane preconditioning also reduced LPS plus IFNgamma-induced glutamate release. Exogenous glutamate decreased microglial viability. Finally, the isoflurane preconditioning-induced protection was abolished by chelerythrine, a protein kinase C inhibitor. These results suggest that LPS plus IFNgamma activates the iNOS-nitric oxide-glutamate pathway to induce microglial injury and that this activation is attenuated by isoflurane preconditioning. Protein kinase C may be involved in the isoflurane preconditioning effects.

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.

Awareness during pediatric anesthesia: what is the position of European pediatric anesthesiologists?

BACKGROUND

The incidence of awareness in the pediatric population is reported as high as 1 : 125. An online survey was conducted about the current perception and practice of members of the British and French pediatric anesthesia societies regarding awareness during general anesthesia.

METHODS

Following the approval of the executive committees of the British and French pediatric anesthesia societies, members with a valid email contact address were invited to participate in a web-based survey. Perceived risk factors, use of awareness monitors, pre- and postoperative discussions of awareness as well as personal experience were enquired.

RESULTS

A total of 302 (51%) responded to the email survey. More than 60% indicated that awareness is a problem in pediatric anesthesia with the majority estimating an incidence of 1 : 1000. Almost half (49%) the respondents believe that awareness is age-dependent and 50% are not concerned below 1 month of age. More than 86% of respondents do not discuss the risk of awareness with the parents or actively look for awareness despite 27% reporting at least one episode in their practice. Intra-operative monitoring almost exclusively consists of clinical signs and endtidal anesthetic concentrations. Bispectral index (BIS) monitoring is routinely used by approximately 10% of the surveyed members.

CONCLUSIONS

This survey demonstrates that European pediatric anesthesiologists perceive awareness as a major problem. However, none seems to address the issue openly or looks for its presence routinely. The vast majority of pediatric anesthesiologists rely almost exclusively on clinical monitoring and endtidal anesthetic concentrations for its detection.

The effect of varying continuous propofol infusions on plasma cyclic guanosine 3',5'-monophosphate concentrations in anesthetized children

BACKGROUND

The glutamate-nitric oxide-cyclic guanosine 3',5'-monophosphate (cGMP) pathway is potentially an effective target for general anesthetics. Plasma cGMP concentrations are reduced after an increase in predicted plasma propofol concentrations during sedation in healthy adult volunteers. We hypothesized that an increase in measured plasma propofol concentration leads to a reduction in plasma cGMP in anesthetized children.

METHODS

Eighteen healthy children aged 46.8 (+/-19.6) mo, requiring general anesthesia for lower body surgical procedures were enrolled. After inhaled induction, tracheal intubation and initiation of intermittent positive pressure ventilation, caudal epidural analgesia was performed. Anesthesia was maintained using a continuous propofol infusion adapted from a previously published regimen to achieve predicted propofol plasma concentration of 6, 3, and 1.5 microg/mL after 30, 50, and 70 min, respectively. Samples for propofol and cGMP plasma concentrations were collected and analyzed using high-performance liquid chromatography and an enzyme immunoassay system.

RESULTS

The plasma cGMP concentrations varied significantly (median [range]) 19.2 [11.8-23.5], 21.3 [14.6-30.8], and 24.9 [15.7-37.8] nmol/L among each predicted plasma propofol concentration, P < 0.0001. The correlation coefficient (r) was -0.62.

CONCLUSIONS

This study demonstrates that an increase in plasma propofol concentration leads to a decrease in plasma cGMP in healthy children, and could serve as a biochemical marker for depth of propofol anesthesia in children.

Sevoflurane 0.25 MAC preferentially affects higher order association areas: a functional magnetic resonance imaging study in volunteers

BACKGROUND

Functional magnetic resonance imaging (fMRI) can objectively measure the subjective effects of anesthesia. Memory-related regions (association areas) are affected by subanesthetic doses of volatile anesthetics. In this study we measured the regional neuronal effects of 0.25 MAC sevoflurane in healthy volunteers and differentiated the effect between primary cortical regions and association areas.

METHODS

The effect of 0.25 MAC sevoflurane on visual, auditory, and motor activation was studied in 16 ASA I volunteers. With fMRI (3 Tesla Siemens magnetom), regional cerebral blood flow (rCBF) was measured by the pulsed arterial spin labeling technique. Subjects inhaled a mixture of O2 and 0.25 MAC sevoflurane and standard ASA monitoring was performed. Visual, auditory, and motor activation tasks were used. rCBF was measured in the awake state and during inhalation of 0.25 MAC sevoflurane, without and with activation. The change in rCBF (deltaCBF) with 0.25 MAC Sevoflurane during baseline state and with activation was calculated in 11 regions of interest related to visual, auditory, and motor activation tasks.

RESULTS

The change from baseline rCBF with 0.25 MAC sevoflurane was not statistically significant in the 11 regions of interest. With activation there was a significant increase in CBF in several regions. However, only in the primary and secondary visual cortices (V1, V2), thalamus, hippocampus, and supplementary motor area was the decrease in activation with 0.25 MAC sevoflurane statistically significant (P < 0.05).

CONCLUSION

Memory-related regions (association areas) are affected by subanesthetic concentrations of volatile anesthetics. Using fMRI, this study showed that 0.25 MAC sevoflurane predominantly affects the primary visual cortex, the related association cortex, and certain other higher order association cortices.

TASK-3 knockout mice exhibit exaggerated nocturnal activity, impairments in cognitive functions, and reduced sensitivity to inhalation anesthetics

The TASK-3 channel is an acid-sensitive two-pore-domain K+ channel, widely expressed in the brain and probably involved in regulating numerous neuronal populations. Here, we characterized the behavioral and pharmacological phenotypes of TASK-3 knockout (KO) mice. Circadian locomotor activity measurements revealed that the nocturnal activity of the TASK-3 KO mice was increased by 38% (P < 0.01) compared with wild-type littermate controls, light phase activity being similar. Although TASK-3 channels are abundant in cerebellar granule cells, the KO mice performed as well as the wild-type mice in walking on a rotating rod or along a 1.2-cm-diameter beam. However, they fell more frequently from a narrower 0.8-cm beam. The KO mice showed impaired working memory in the spontaneous alternation task, with the alternation percentage being 62 +/- 3% for the wild-type mice and 48 +/- 4% (P < 0.05) for the KO mice. Likewise, during training for the Morris water-maze spatial memory task, the KO mice were slower to find the hidden platform, and in the probe trial, the female KO mice visited fewer times the platform quadrant than the male KO and wild-type mice. In pharmacological tests, the TASK-3 KO mice showed reduced sensitivity to the inhalation anesthetic halothane and the cannabinoid receptor agonist WIN55212-2 mesylate [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] but unaltered responses to the alpha2 adrenoceptor agonist dexmedetomidine, the i.v. anesthetic propofol, the opioid receptor agonist morphine, and the local anesthetic lidocaine. Overall, our results suggest important contributions of TASK-3 channels in the neuronal circuits regulating circadian rhythms, cognitive functions, and mediating specific pharmacological effects.

Mouse chromosome 7 harbors a quantitative trait locus for isoflurane minimum alveolar concentration

BACKGROUND

The minimum alveolar concentration (MAC) of isoflurane is a quantitative trait because it varies continuously in a population. The location on the genome of genes or other genetic elements controlling quantiative traits is called quantitative trait loci (QTLs). In this study we sought to detect a quantitative trait locus underlying isoflurane MAC in mice.

METHODS

To accomplish this, two inbred mouse strains differing in isoflurane MAC, the C57BL/6J and LP/J mouse strains, were bred through two generations to produce genetic recombination. These animals were genotyped for microsatellite markers. We also applied an independent, computational method for identifying QTL-regulating differences in isoflurane MAC. In this approach, the isoflurane MAC was measured in a panel of 19 inbred strains, and computationally searched for genomic intervals where the pattern of genetic variation, based on single nucleotide polymorphisms, correlated with the differences in isoflurane MAC among inbred strains.

RESULTS AND CONCLUSIONS

Both methods of genetic analysis identified a QTL for isoflurane MAC that was located on the proximal part of mouse chromosome 7.

SPINAL ?-AMINO-3-HYDROXY-5-METHYL-4-ISOXAZOLEPROPIONIC ACID RECEPTORS MAY MEDIATE THE ANALGESIC EFFECTS OF EMULSIFIED HALOGENATED ANAESTHETICS

1. The present study was designed to investigate the relationship between spinal cord alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the analgesic effects of emulsified halogenated anaesthetics. 2. After having established the mouse model of analgesia by intraperitoneal or subcutaneous injections of appropriate doses of emulsified enflurane, isoflurane or sevoflurane, we injected different doses of AMPA intrathecally and observed effects on the pain threshold using the hot-plate and acetic acid-induced writhing tests. 3. The results showed that intrathecal injection of AMPA (0.25, 0.5 and 1.0 ng) did not affect the pain threshold on the hot-plate test or the writhing times in conscious mice. In contrast, AMPA (0.25, 0.5 and 1.0 ng intrathecally) significantly and dose-dependently decreased the pain threshold on the hot-plate test and increased the writhing times in mice treated with emulsified anaesthetics. 4. These results suggest that spinal AMPA receptors may be important targets for the analgesic effects of emulsified enflurane, isoflurane and sevoflurane.

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.

Pretreatment with volatile anesthetics, but not with the nonimmobilizer 1,2-dichlorohexafluorocyclobutane, reduced cell injury in rat cerebellar slices after an in vitro simulated ischemia

A prior exposure to the volatile anesthetic isoflurane has been shown to induce neuroprotection in rats. This phenomenon is called preconditioning. We designed this study to determine whether the potency of volatile anesthetics in inducing neuropreconditioning is related to their potency to induce anesthesia. Cerebellar slices of adult male Sprague-Dawley rats were exposed to various concentrations of isoflurane, halothane, sevoflurane, desflurane or the nonimmobilizer 1,2-dichlorohexafluorocyclobutane for 15 min, followed by a 15-min drug-free period, and then were subjected to oxygen-glucose deprivation for 10 min at 37 degrees C. After a 5-h recovery at 37 degrees C, brain slices were used for quantification of cell injury by spectrophotometric measurement of formazan produced from 2,3,5-triphenyltetrazolium chloride. All four volatile anesthetics induced a concentration-dependent preconditioning effect. The EC50 for this effect induced by isoflurane, halothane, sevoflurane or desflurane was 221, 173, 184 and 929 microM, respectively. This EC50 was linearly correlated with the aqueous concentration of one minimum alveolar concentration. The volatile anesthetic preconditioning-induced neuroprotection was abolished by DL-threo-beta-hydroxyaspartic acid, DL-threo-beta-benzyloxyaspartate or dihydrokainate, glutamate transporter inhibitors. The volatile nonimmobilizer 1,2-dichlorohexafluorocyclobutane at any concentrations tested in the study did not induce a significant preconditioning effect. Isoflurane preconditioning did not change the oxygen-glucose deprivation-induced glutamate accumulation. These results suggest that the preconditioning-induced neuroprotection by volatile anesthetics is not agent-specific. Mechanisms that are involved in inducing anesthesia may contribute to the induction of preconditioning effect by volatile anesthetics. Modification of glutamate transporter activity may be one of such mechanisms to induce these protective effects.

Riluzole, a Glutamate Release Inhibitor, Induces Loss of Righting Reflex, Antinociception, and Immobility in Response to Noxious Stimulation in Mice

BACKGROUND

The general anesthetic state comprises behavioral and perceptual components, including amnesia, unconsciousness, analgesia, and immobility. In vitro, glutamatergic excitatory neurons are important targets for anesthetic action at the cellular and microcircuits levels. Riluzole (2-amino-6-[trifluoromethoxy]benzothiazole) is a neuroprotective drug that inhibits glutamate release from nerve terminals in the central nervous system. Here, we examined in vivo the ability of riluzole to produce components of the general anesthetic state through a selective blockade of glutamatergic neurotransmission.

METHODS

Riluzole was administered intraperitoneally in adult male ddY mice. To assess the general anesthetic components, three end-points were used: 1) loss of righting reflex (LORR; as a measure of unconsciousness), 2) loss of movement in response to noxious stimulation (as a measure of immobility), and 3) loss of nociceptive response (as a measure of analgesia).

RESULTS

The intraperitoneal administration of riluzole induced LORR in a dose-dependent fashion with a 50% effective dose value of 27.4 (23.3-32.2; 95% confidence limits) mg/kg. The behavioral and microdialysis studies revealed that time-course changes in impairment and LORR induced by riluzole corresponded with decreased glutamate levels in the mouse brain. This suggests that riluzole-induced LORR (unconsciousness) could result, at least in part, from its ability to decrease brain glutamate concentrations. Riluzole dose-dependently produced not only LORR, but also loss of movement in response to painful stimulation (immobility), and loss of nociceptive response (analgesia) with 50% effective dose values of 43.0 (37.1-49.9), and 10.0 (7.4-13.5) mg/kg, respectively. These three dose-response curves were parallel, suggesting that the behavioral effects of riluzole may be mediated through a common site of action.

CONCLUSIONS

These findings suggest that riluzole-induced LORR, immobility, and antinociception appear to be associated with its ability to inhibit glutamatergic neurotransmission in the central nervous system.

Increased γ-Aminobutyric Acid Levels in Mouse Brain Induce Loss of Righting Reflex, but Not Immobility, in Response to Noxious Stimulation

BACKGROUND

The general anesthetic state comprises behavioral and perceptual components, including amnesia, unconsciousness, and immobility. gamma-Aminobutyric acidergic (GABAergic) inhibitory neurotransmission is an important target for anesthetic action at the in vitro cellular level. In vivo, however, the functional relevance of enhancing GABAergic neurotransmission in mediating essential components of the general anesthetic state is unknown. Gabaculine is a GABA-transaminase inhibitor that inhibits degradation of released GABA, and consequently increases endogenous GABA in the central nervous system. Here, we examined, behaviorally, the ability of increased GABA levels to produce components of the general anesthetic state.

METHODS

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

RESULTS

Gabaculine induced LORR in a dose-dependent fashion with a 50% effective dose of 100 (75-134; 95% confidence limits) mg/kg. The behavioral and microdialysis studies revealed that the endogenous GABA-induced LORR occurred in a brain concentration-dependent manner. However, even larger doses of gabaculine (285-400 mg/kg) produced no loss of tail-clamp response. In contrast, all the tested volatile anesthetics concentration-dependently abolished both righting and tail-clamp response, supporting the evidence that volatile anesthetics act on a variety of molecular targets.

CONCLUSIONS

These findings indicate that LORR is associated with enhanced GABAergic neurotransmission, but that immobility in response to noxious stimulation is not, suggesting that LORR and immobility are mediated through different neuronal pathways and/or regions in the central nervous system.

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.

Bupivacaine caudal epidural anesthesia: assessing the effect of general anesthetic technique on block onset

BACKGROUND

The primary objective of this prospective, randomized trial was to compare the effect of propofol and sevoflurane on effectiveness of regional anesthesia. As a secondary objective, we aimed at evaluating the influence of age on neuraxial block profile.

METHODS

One hundred and thirteen healthy children aged <10 years, scheduled for general or urological surgical procedures were randomly allocated to receive either propofol or sevoflurane induction and maintenance. Children received caudal or lumbar epidural block depending on their weight and expected surgery. Time to onset of surgical anesthesia, intraoperative analgesic effectiveness, residual motor blockade, postoperative pain, and adverse effects were evaluated. To assess the influence of age on these measures children were further divided into three age groups: 0-24, 25-60 and >60 months.

RESULTS

The mean time to onset of surgical anesthesia was significantly shorter in the sevoflurane group than the propofol group (3.1 vs 4.7 min, P < 0.01), independent of the type of regional technique employed. Distress on arousal in recovery was increased in children receiving sevoflurane (P = 0.03). There was no significant difference in residual motor blockade between the groups but children between 0 and 24 months receiving sevoflurane had a higher incidence of residual motor blockade 3 h after local anesthetic injection (P = 0.01).

CONCLUSIONS

This study has demonstrated that general anesthesia with sevoflurane decreases the time to onset of surgical anesthesia relative to propofol anesthesia. This effect was most marked when sevoflurane anesthesia preceded caudal epidural blockade. The basis for this effect is most likely to be related to differential binding of the two anesthetic agents to receptors in the spinal cord that mediate immobility in response to surgical stimuli.