Gamma-Aminobutyric AcidA Receptors Do Not Mediate the Immobility Produced by Isoflurane

Total Intravenous Anesthesia Protocol for Decreasing Unacceptable Movements during Cerebral Aneurysm Clipping with Motor-Evoked Potential Monitoring: A Historical Control Study and Meta-Analysis

Injury can occur during intraoperative transcranial motor-evoked potential (MEP) monitoring caused by patient movement related to insufficient neuromuscular blocking agent use. Here, we evaluated the incidence of unacceptable movements in patients undergoing intraoperative MEP monitoring following our anesthetic protocol. We reviewed the anesthesia records of 419 patients who underwent unruptured cerebral aneurysm clipping with intraoperative MEP monitoring. The anesthetic protocol included target-controlled infusion with a fixed effect-site propofol concentration of 3 μg/mL and an adjustable effect-site remifentanil concentration of 10-12 ng/mL. We compared our findings of the intraoperative parameters and incidence of spontaneous movement and respiration with those of published meta-analysis studies. Spontaneous movement and respiration occurred in one (0.2%) patient each. The meta-analysis included six studies. The pooled proportions of spontaneous movement and respiration were 6.9% (95% confidence interval [CI], 1.3-16.5%) and 4.1% (95% CI, 0.5-14.1%), respectively. The proportion of spontaneous movement in our study was significantly lower than that in previous studies (p = 0.013), with no significant difference in spontaneous respiration (p = 0.097). Following our center's anesthesia protocol during cerebral aneurysm clipping resulted in a low incidence of spontaneous respiration and movement, indicating its safety for patients undergoing intraoperative MEP monitoring.

Gabra6100Q allele Sprague-Dawley rats have a higher sensitivity to hypnosis induced by isoflurane and ethanol than the wild type rats

BACKGROUND

The neurobiological mechanisms underlying how general anesthetics render a patient's unconsciousness (hypnosis) remains elusive. The role of the cerebellum in hypnosis induced by general anesthetics is unknown. Gabra6^100Q allele Sprague-Dawley (SD) rats have a naturally occurring single nucleotide polymorphism in the GABA_A receptor α6 subunit gene that is expressed exclusively in cerebellum granule cells.

METHODS

We examined the loss of righting reflex (LORR) induced by isoflurane, and ethanol in Gabra6^100Q rats compared with those in wild type (WT) SD rats. We also examined the change of c-Fos expression induced by isoflurane exposure in cerebellum granule cells of both mutant and WT rats.

RESULTS

Gabra6^100Q rats are more sensitive than WT rats to the LORR induced by isoflurane and ethanol. Moreover, isoflurane exposure induced a greater reduction in c-Fos expression in cerebellum granule cells of Gabra6^100Q rats than WT rats.

CONCLUSIONS

Based on these data, we speculate that cerebellum may be involved in the hypnosis induced by some general anesthetics and thus may represent a novel target of general anesthetics.

Effects of sevoflurane and desflurane on the nociceptive responses of substantia gelatinosa neurons in the rat spinal cord dorsal horn: An in vivo patch-clamp analysis

Background

Volatile anesthetics suppress noxiously evoked activity in the spinal dorsal horn, which could contribute in part to analgesia, immobility. Modulation of excitatory and inhibitory synaptic transmission in substantia gelatinosa neurons could lead to the suppression of dorsal horn activity; however, this phenomenon has not yet been investigated fully.

Methods

In urethane-anesthetized rats, extracellular activity of dorsal horn neurons (action potentials) and excitatory/inhibitory postsynaptic currents in substantia gelatinosa neurons were recorded using extracellular and in vivo patch-clamp techniques, respectively, to assess the spontaneous and the noxious-evoked activity. Sevoflurane or desflurane at concentrations ranging from 0.1 to 2 minimum alveolar concentration was administered by inhalation. Hot- and cold-plate tests were performed to assess nociceptive responses during the inhalation of volatile anesthetics at lower anesthetic doses (0.1–0.5 minimum alveolar concentration).

Results

At anesthetic doses (1 and 2 minimum alveolar concentration), both sevoflurane and desflurane decreased the frequency of action potentials in the dorsal horn and the activities of excitatory postsynaptic currents in substantia gelatinosa neurons during pinch stimulation and decreased the activities of spontaneous and miniature excitatory postsynaptic currents. Inhibition of the frequencies was more prominent than that of amplitudes in spontaneous and miniature excitatory postsynaptic currents at these anesthetic doses. However, at subanesthetic doses (0.1 and 0.2 minimum alveolar concentration), desflurane facilitated action potentials and excitatory postsynaptic currents. Inhibitory postsynaptic currents were inhibited by both anesthetics at anesthetic doses (1 and 2 minimum alveolar concentration). Hot- or cold-plate tests showed hyperalgesic effects of desflurane at subanesthetic doses (0.1 and 0.2 minimum alveolar concentration) and a dose-dependent analgesic effect of sevoflurane.

Conclusions

Sevoflurane and desflurane at anesthetic doses suppressed dorsal horn activity mainly via inhibition of excitatory postsynaptic currents in substantia gelatinosa neurons, which would contribute to their analgesic properties. Presynaptic mechanisms were likely in excitatory postsynaptic currents inhibition. Desflurane but not sevoflurane may have a hyperalgesic effect at subanesthetic doses.

Effect of anesthesia on motor responses evoked by spinal neural prostheses during intraoperative procedures

OBJECTIVE

The overall goal of this study was to investigate the effects of various anesthetic protocols on the intraoperative responses to intraspinal microstimulation (ISMS). ISMS is a neuroprosthetic approach that targets the motor networks in the ventral horns of the spinal cord to restore function after spinal cord injury. In preclinical studies, ISMS in the lumbosacral enlargement produced standing and walking by activating networks controlling the hindlimb muscles. ISMS implants are placed surgically under anesthesia, and refinements in placement are made based on the evoked responses. Anesthesia can have a significant effect on the responses evoked by spinal neuroprostheses; therefore, in preparation for clinical testing of ISMS, we compared the evoked responses under a common clinical neurosurgical anesthetic protocol with those evoked under protocols commonly used in preclinical studies.

APPROACH

Experiments were conducted in seven pigs. An ISMS microelectrode array was implanted in the lumbar enlargement and responses to ISMS were measured under three anesthetic protocols: (1) isoflurane, an agent used pre-clinically and clinically, (2) total intravenous anesthesia (TIVA) with propofol as the main agent commonly used in clinical neurosurgical procedures, (3) TIVA with sodium pentobarbital, an anesthetic agent used mostly preclinically. Responses to ISMS were evaluated based on stimulation thresholds, movement kinematics, and joint torques. Motor evoked potentials (MEP) and plasma concentrations of propofol were also measured.

MAIN RESULTS

ISMS under propofol anesthesia produced large and functional responses that were not statistically different from those produced under pentobarbital anesthesia. Isoflurane, however, significantly suppressed the ISMS-evoked responses.

SIGNIFICANCE

This study demonstrated that the choice of anesthesia is critical for intraoperative assessments of motor responses evoked by spinal neuroprostheses. Propofol and pentobarbital anesthesia did not overly suppress the effects of ISMS; therefore, propofol is expected to be a suitable anesthetic agent for clinical intraoperative testing of an intraspinal neuroprosthetic system.

Isoflurane Inhibits Dopaminergic Synaptic Vesicle Exocytosis Coupled to CaV2.1 and CaV2.2 in Rat Midbrain Neurons

Volatile anesthetics affect neuronal signaling by poorly understood mechanisms. Activation of central dopaminergic pathways has been implicated in emergence from general anesthesia. The volatile anesthetic isoflurane differentially inhibits glutamatergic and GABAergic synaptic vesicle (SV) exocytosis by reducing presynaptic Ca²⁺ influx without affecting the Ca²⁺-exocytosis relationship, but its effects on dopaminergic exocytosis are unclear. We tested the hypothesis that isoflurane inhibits exocytosis in dopaminergic neurons. We used electrical stimulation or depolarization by elevated extracellular KCl to evoke exocytosis measured by quantitative live-cell fluorescence imaging in cultured rat ventral tegmental area neurons. Using trains of electrically evoked action potentials (APs), isoflurane inhibited exocytosis in dopaminergic neurons to a greater extent (30 ± 4% inhibition; p < 0.0001) than in non-dopaminergic neurons (15 ± 5% inhibition; p = 0.014). Isoflurane also inhibited exocytosis evoked by elevated KCl in dopaminergic neurons (35 ± 6% inhibition; p = 0.0007), but not in non-dopaminergic neurons (2 ± 4% inhibition). Pharmacological isolation of presynaptic Ca²⁺ channel subtypes showed that isoflurane inhibited KCl-evoked exocytosis mediated exclusively by either Ca_V2.1 (P/Q-type Ca²⁺ channels; 30 ± 5% inhibition; p = 0.0002) or by Ca_V2.2 (N-type Ca²⁺ channels; 35 ± 11% inhibition; p = 0.015). Additionally, isoflurane inhibited single AP-evoked Ca²⁺ influx by 41 ± 3% and single AP-evoked exocytosis by 34 ± 6%. Comparable reductions in exocytosis and Ca²⁺ influx were produced by lowering extracellular [Ca²⁺]. Thus, isoflurane inhibits exocytosis from dopaminergic neurons by a mechanism distinct from that in non-dopaminergic neurons involving reduced Ca²⁺ entry through Ca_V2.1 and/or Ca_V2.2.

Minimum alveolar concentration: Key concepts and a review of its pharmacological reduction in dogs. Part 1

OBJECTIVE

To outline the major components of the minimum alveolar concentration (MAC) and review the literature in regard to pharmacological manipulation of the MAC of halothane, isoflurane, sevoflurane, enflurane, and desflurane in dogs. The pharmacologic agents included are alpha-2 agonists, benzodiazepines, propofol, maropitant, opioids, lidocaine, acepromazine, non-steroidal anti-inflammatory agents, and NMDA antagonists. Part 1 will focus on summarizing the relevance, measurement, and mechanisms of MAC and review the effects of alpha-2 agonists, benzodiazepines, and propofol on MAC.

DATABASES USED

PubMed, Google Scholar, CAB Abstracts. Search terms used: minimum alveolar concentration, MAC, dog, canine, inhaled anesthetic potency, isoflurane, sevoflurane, desflurane, enflurane, and halothane.

CONCLUSIONS

Many drugs reduce the MAC of inhaled anesthetics in dogs, and allow for a clinically important decrease in inhalant anesthetic use. A decrease in MAC may decrease the adverse cardiovascular and pulmonary effects associated with the use of high concentrations of inhaled anesthetics.

Inflammation Increases Neuronal Sensitivity to General Anesthetics

Background

Critically ill patients with severe inflammation often exhibit heightened sensitivity to general anesthetics; however, the underlying mechanisms remain poorly understood. Inflammation increases the number of γ-aminobutyric acid type A (GABAA) receptors expressed on the surface of neurons, which supports the hypothesis that inflammation increases up-regulation of GABAA receptor activity by anesthetics, thereby enhancing the behavioral sensitivity to these drugs.

Methods

To mimic inflammation in vitro, cultured hippocampal and cortical neurons were pretreated with interleukin (IL)-1β. Whole cell patch clamp methods were used to record currents evoked by γ-aminobutyric acid (GABA) (0.5 μM) in the absence and presence of etomidate or isoflurane. To mimic inflammation in vivo, mice were treated with lipopolysaccharide, and several anesthetic-related behavioral endpoints were examined.

Results

IL-1β increased the amplitude of current evoked by GABA in combination with clinically relevant concentrations of either etomidate (3 μM) or isoflurane (250 μM) (n = 5 to 17, P &lt; 0.05). Concentration–response plots for etomidate and isoflurane showed that IL-1β increased the maximal current 3.3-fold (n = 5 to 9) and 1.5-fold (n = 8 to 11), respectively (P &lt; 0.05 for both), whereas the half-maximal effective concentrations were unchanged. Lipopolysaccharide enhanced the hypnotic properties of both etomidate and isoflurane. The immobilizing properties of etomidate, but not isoflurane, were also increased by lipopolysaccharide. Both lipopolysaccharide and etomidate impaired contextual fear memory.

Conclusions

These results provide proof-of-concept evidence that inflammation increases the sensitivity of neurons to general anesthetics. This increase in anesthetic up-regulation of GABAA receptor activity in vitro correlates with enhanced sensitivity for GABAA receptor–dependent behavioral endpoints in vivo.

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.

Opposing actions of sevoflurane on GABAergic and glycinergic synaptic inhibition in the spinal ventral horn

BACKGROUND

The ventral horn is a major substrate in mediating the immobilizing properties of the volatile anesthetic sevoflurane in the spinal cord. In this neuronal network, action potential firing is controlled by GABA(A) and glycine receptors. Both types of ion channels are sensitive to volatile anesthetics, but their role in mediating anesthetic-induced inhibition of spinal locomotor networks is not fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

To compare the effects of sevoflurane on GABAergic and glycinergic inhibitory postsynaptic currents (IPSCs) whole-cell voltage-clamp recordings from ventral horn interneurons were carried out in organotypic spinal cultures. At concentrations close to MAC (minimum alveolar concentration), decay times of both types of IPSCs were significantly prolonged. However, at 1.5 MAC equivalents, GABAergic IPSCs were decreased in amplitude and reduced in frequency. These effects counteracted the prolongation of the decay time, thereby decreasing the time-averaged GABAergic inhibition. In contrast, amplitudes and frequency of glycinergic IPSCs were not significantly altered by sevoflurane. Furthermore, selective GABA(A) and glycine receptor antagonists were tested for their potency to reverse sevoflurane-induced inhibition of spontaneous action potential firing in the ventral horn. These experiments confirmed a weak impact of GABA(A) receptors and a prominent role of glycine receptors at a high sevoflurane concentration.

CONCLUSIONS

At high concentrations, sevoflurane mediates neuronal inhibition in the spinal ventral horn primarily via glycine receptors, and less via GABA(A) receptors. Our results support the hypothesis that the impact of GABA(A) receptors in mediating the immobilizing properties of volatile anesthetics is less essential in comparison to glycine receptors.

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.

GABA(A) receptor antagonism increases NMDA receptor inhibition by isoflurane at a minimum alveolar concentration

OBJECTIVE

At the minimum alveolar concentration (MAC), isoflurane potentiates GABA(A) receptor currents and inhibits NMDA receptor currents, and these actions may be important for producing anesthesia. However, isoflurane modulates GABA(A) receptors more potently than NMDA receptors. The objective of this study was to test whether isoflurane would function as a more potent NMDA receptor antagonist if its efficacy at GABA(A) receptors was decreased.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Fourteen 10-week-old male Sprague-Dawley rats weighing 269 ± 12 g.

METHODS

Indwelling lumbar subarachnoid catheters were surgically placed in isoflurane-anesthetized rats. Two days later, the rats were anesthetized with isoflurane, and artificial CSF containing either 0 or 1 mg kg(-1) picrotoxin, a GABA(A) receptor antagonist, was infused intrathecally at 1 μL minute(-1). The baseline isoflurane MAC was then determined using a standard tail clamp technique. MK801 (dizocilpine), an NMDA receptor antagonist, was then administered intravenously at 0.5 mg kg(-1). Isoflurane MAC was re-measured.

RESULTS

Picrotoxin increased isoflurane MAC by 16% compared to controls. MK801 significantly decreased isoflurane MAC by 0.72% of an atmosphere in controls versus 0.47% of an atmosphere in rats receiving intrathecal picrotoxin.

CONCLUSIONS AND CLINICAL RELEVANCE

A smaller MK801 MAC-sparing effect in the picrotoxin group is consistent with greater NMDA antagonism by isoflurane in these animals, since it suggests that fewer NMDA receptors are available upon which MK801 could act to decrease isoflurane MAC. Decreasing isoflurane GABA(A) potentiation increases isoflurane NMDA antagonism at MAC. Hence, the magnitude of an anesthetic effect on a given channel or receptor at MAC may depend upon effects at other receptors.

General anesthetic actions on GABA(A) receptors

General anesthetic drugs interact with many receptors in the nervous system, but only a handful of these interactions are critical for producing anesthesia. Over the last 20 years, neuropharmacologists have revealed that one of the most important target sites for general anesthetics is the GABA_A receptor. In this review we will discuss what is known about anesthetic – GABA_A receptor interactions.

Gamma-aminobutyric acid type A receptor alpha 4 subunit knockout mice are resistant to the amnestic effect of isoflurane

BACKGROUND

General anesthesia produces multiple end points including immobility, hypnosis, sedation, and amnesia. Tonic inhibition via gamma-aminobutyric acid type A receptors (GABA(A)-Rs) may play a role in mediating behavioral end points that are suppressed by low concentrations of anesthetics (e.g., hypnosis and amnesia). GABA(A)-Rs containing the alpha4 subunit are highly concentrated in the hippocampus and thalamus, and when combined with delta subunits they mediate tonic inhibition, which is sensitive to low concentrations of isoflurane.

METHODS

In this study, we used a GABA(A) alpha4 receptor knockout mouse line to evaluate the contribution of alpha4-containing GABA(A)-Rs to the effects of immobility, hypnosis, and amnesia produced by isoflurane. Knockout mice and their wild-type counterparts were assessed on 3 behavioral tests: conditional fear (to assess amnesia), loss of righting reflex (to assess hypnosis), and the minimum alveolar concentration of inhaled anesthetic necessary to produce immobility in response to noxious stimulation in 50% of subjects (to assess immobility).

RESULTS

Genetic inactivation of the alpha4 subunit reduced the amnestic effect of isoflurane, minimally affected loss of righting reflex, and had no effect on immobility.

CONCLUSIONS

These results lend support to the hypothesis that different sites of action mediate different anesthetic end points and suggest that alpha4-containing GABA(A)-Rs are important mediators of the amnestic effect of isoflurane on hippocampal-dependent declarative memory.

Gamma-aminobutyric acid type A receptor alpha 4 subunit knockout mice are resistant to the amnestic effect of isoflurane

BACKGROUND

General anesthesia produces multiple end points including immobility, hypnosis, sedation, and amnesia. Tonic inhibition via gamma-aminobutyric acid type A receptors (GABA(A)-Rs) may play a role in mediating behavioral end points that are suppressed by low concentrations of anesthetics (e.g., hypnosis and amnesia). GABA(A)-Rs containing the alpha4 subunit are highly concentrated in the hippocampus and thalamus, and when combined with delta subunits they mediate tonic inhibition, which is sensitive to low concentrations of isoflurane.

METHODS

In this study, we used a GABA(A) alpha4 receptor knockout mouse line to evaluate the contribution of alpha4-containing GABA(A)-Rs to the effects of immobility, hypnosis, and amnesia produced by isoflurane. Knockout mice and their wild-type counterparts were assessed on 3 behavioral tests: conditional fear (to assess amnesia), loss of righting reflex (to assess hypnosis), and the minimum alveolar concentration of inhaled anesthetic necessary to produce immobility in response to noxious stimulation in 50% of subjects (to assess immobility).

RESULTS

Genetic inactivation of the alpha4 subunit reduced the amnestic effect of isoflurane, minimally affected loss of righting reflex, and had no effect on immobility.

CONCLUSIONS

These results lend support to the hypothesis that different sites of action mediate different anesthetic end points and suggest that alpha4-containing GABA(A)-Rs are important mediators of the amnestic effect of isoflurane on hippocampal-dependent declarative memory.

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.

Intrathecal glycine significantly decreases the minimum alveolar concentration of isoflurane in rats

OBJECTIVE

To evaluate the effect of intrathecal administration of glycine on the minimum alveolar concentration (MAC) of isoflurane in rats.

METHODS

Intrathecal catheters were implanted in 40 adult male rats anesthetized with isoflurane. Baseline MAC of isoflurane was measured during the infusion of artificial cerebrospinal fluid (CSF) alone. Subsequently, 10, 40, 80, 160, and 300 mmol/L of glycine dissolved in artificial CSF were infused for two hours at the same rate as under control conditions, and MAC for isoflurane was re-determined.

RESULTS

Intrathecal administration of glycine produced a significant, dose-dependent decrease in MAC for isoflurane (up to -65.2% +/- 16.2%).

CONCLUSIONS

Intrathecal administration of glycine decreases anesthetic requirement This result supports the idea that glycine receptors may be important to the immobilizing effect of anesthetics that enhance glycine receptor function such as isoflurane.

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.

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.

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.

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.

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.

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.

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.

Chirality in anesthesia II: stereoselective modulation of ion channel function by secondary alcohol enantiomers

Chirality has been proposed as a means for distinguishing relevant from irrelevant molecular targets of action, but the sensitivity and specificity of this test is unknown for volatile anesthetics. We applied enantiomers of two chiral anesthetic alcohols (2-butanol and 2-pentanol) that are enantioselective for the minimum alveolar concentration (MAC) preventing movement in 50% of animals and one (2-hexanol) that was not to frog oocytes. Each oocyte expressed one of three anesthetic-sensitive ion channels: a Twik-related-spinal cord K+ (TRESK) channel, a gamma-amino butyric acid type A (GABA(A)) receptor and an N-methyl-d-aspartate (NMDA) receptor. Using voltage-clamp techniques, we found that 2-butanol was not enantioselective for any channel (e.g., 16 mM 2-butanol R(-) and S(-) enantiomers decreased current through an NMDA receptors by 44% +/- 3% [mean +/- se] and 37% +/- 4%, respectively); 2-pentanol was enantioselective for one channel (the GABA(A) receptor, the enantiomers increasing current by 277% +/- 20% and 141% +/- 30%); 2-hexanol was enantioselective for both GABA(A) and NMDA receptors (e.g., decreasing current through the NMDA receptor by 19% +/- 3% and 43% +/- 5%). We calculated the sensitivity and specificity of chirality as a test of anesthetic relevance under two scenarios: 1) all three channels were relevant mediators of MAC and 2) no channel was a mediator of MAC. These sensitivities and specificities were poor because there is no consistent correspondence between receptor and whole animal results. We recommend that enantioselectivity not be used as a test of relevance for inhaled anesthetic targets.

Volatile anesthetic effects on glutamate versus GABA release from isolated rat cortical nerve terminals: 4-aminopyridine-evoked release

Inhibition of glutamatergic excitatory neurotransmission and potentiation of GABA-mediated inhibitory transmission are possible mechanisms involved in general anesthesia. We compared the effects of three volatile anesthetics (isoflurane, enflurane, or halothane) on 4-aminopyridine (4AP)-evoked release of glutamate and GABA from isolated rat cerebrocortical nerve terminals (synaptosomes). Synaptosomes were prelabeled with l-[(3)H]glutamate and [(14)C]GABA, and release was evoked by superfusion with pulses of 1 mM 4AP in the absence or presence of 1.9 mM free Ca(2+). All three volatile anesthetics inhibited Ca(2+)-dependent glutamate and GABA release; IC(50) values for glutamate were comparable to clinical concentrations (1-1.6x MAC), whereas IC(50) values for GABA release exceeded clinical concentrations (>2.2x MAC). All three volatile anesthetics inhibited both Ca(2+)-independent and Ca(2+)-dependent 4AP-evoked glutamate release equipotently, whereas inhibition of Ca(2+)-dependent 4AP-evoked GABA release was less potent than inhibition of Ca(2+)-independent GABA release. Inhibition of Ca(2+)-independent 4AP-evoked glutamate release was more potent than that of GABA release for isoflurane and enflurane but equipotent for halothane. Tetrodotoxin inhibited both Ca(2+)-independent and Ca(2+)-dependent 4AP-evoked glutamate and GABA release equipotently, consistent with Na(+) channel involvement. In contrast to tetrodotoxin, volatile anesthetics exhibited selective effects on 4AP-evoked glutamate versus GABA release, consistent with distinct mechanisms of action. Preferential inhibition of Ca(2+)-dependent 4AP-evoked glutamate release versus GABA release supports the hypothesis that reduced excitatory neurotransmission relative to inhibitory neurotransmission contributes to volatile anesthetic actions.

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.

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

Mice bearing an N265M point mutation in the gamma-aminobutyric acid (GABA)(A) receptor beta3 subunit resist various anesthetic effects of propofol and etomidate. They also require a 16% larger concentration of enflurane and a 21% larger concentration of halothane to abolish the withdrawal reflex than do wild-type mice. Using a Pavlovian test, we measured whether this mutation increased the concentration of isoflurane required to impair learning and memory relative to wild-type mice. We found that the concentration was not significantly increased. We also measured MAC (the minimum alveolar concentration required to eliminate movement in response to noxious stimulation in 50% of subjects). Isoflurane MAC for mutant mice (1.93% +/- 0.0.03%; mean +/- se; n = 14) was 17.0% larger than MAC for wild-type mice (1.65 +/- 0.04; n = 14; P < 0.001). Similarly, the cyclopropane MAC for mutant mice (27.6% +/- 0.55%; n = 16) was 13.6% larger than MAC for wild-type mice (24.3 +/- 0.46; n = 8; P < 0.01). The increase in MAC for cyclopropane was unexpected, because published reports find only minimal actions at alpha1beta2gamma2 GABA(A) receptors whereas isoflurane provides a large enhancement. Consistent with previous work on alpha1beta2gamma2 GABA(A) receptors, we found in Xenopus oocytes that 5 MAC cyclopropane enhanced the effect of GABA on alpha1beta2gamma2 GABA(A) receptors by only 76%, and by a nearly identical enhancement in alpha1beta3gamma2, and alpha6beta3gamma2 receptors. In contrast, a much smaller concentration of isoflurane (1 MAC) produced a 160% to 310% enhancement in these receptors. If, relative to isoflurane, cyclopropane minimally increases GABA-induced chloride currents at any GABA(A) receptor subtype, the present data for MAC are consistent with the notion that GABA(A) receptors do not mediate the immobility produced by inhaled anesthetics.

IMPLICATIONS

The results of the present study indicate that beta3-containing gamma-aminobutyric acidA receptors do not mediate the amnesia produced by isoflurane and do not mediate, or only partially mediate, the immobility produced by inhaled anesthetics.

Thiopental produces immobility primarily by supraspinal actions in rats

The spinal cord mediates most of the immobilizing action of inhaled anesthetics. In the present study we investigated whether spinal or supraspinal sites mediate the immobilizing action of thiopental in rats. Thiopental was administered IV, intrathecally (IT), intracerebroventricularly (ICV), or simultaneously IT and ICV. Only the IV infusion produced anesthesia, defined as immobility in response to application of a tail clamp (i.e., the equivalent of minimum alveolar concentration, MAC). Consequently, the MAC-sparing effect (for isoflurane) of thiopental was used to assess the immobilizing contribution of IT and ICV infusions of thiopental. Thiopental concentrations were determined in whole brain, spinal cord, and a slice of cerebral cortex distant from the infusion sites. These concentrations were correlated with the MAC-sparing effect of the thiopental infusions in a multiple regression model. To assess the rate at which thiopental penetrates the cord, rat spinal cords were equilibrated in a bath of thiopental ex vivo and the concentration of thiopental in the cord was measured as a function of equilibration time. This was repeated in vivo with IT infusions of thiopental spanning the time of the behavioral studies. We found that IT or ICV infusion of thiopental 25 microg/min decreased isoflurane MAC <25%. The associated thiopental concentrations in the spinal cord after IT infusion, and in the whole brain after ICV infusion of 25 microg/min thiopental, exceeded by 500% and 680%, respectively, the concentrations found in the spinal cord and in the whole brain after IV infusion of thiopental in a dose that produced anesthesia in the absence of isoflurane. The percentage decrease in the MAC of isoflurane correlated primarily with the concentration of thiopental found in cerebral tissue not in contact with the cerebral ventricles. The spinal cord infusion produced an approximately 20% decrease in MAC. Ex vivo IT thiopental readily diffused into the spinal cord, with a time constant of approximately 1 h. We conclude that, unlike inhaled anesthetics, the immobilizing action of thiopental is largely supraspinal. Centers in the brain other than those near the third and fourth ventricles produce the greatest effect.

Multiple synaptic and membrane sites of anesthetic action in the CA1 region of rat hippocampal slices

BACKGROUND

Anesthesia is produced by a depression of central nervous system function, however, the sites and mechanisms of action underlying this depression remain poorly defined. The present study compared and contrasted effects produced by five general anesthetics on synaptic circuitry in the CA1 region of hippocampal slices.

RESULTS

At clinically relevant and equi-effective concentrations, presynaptic and postsynaptic anesthetic actions were evident at glutamate-mediated excitatory synapses and at GABA-mediated inhibitory synapses. In addition, depressant effects on membrane excitability were observed for CA1 neuron discharge in response to direct current depolarization. Combined actions at several of these sites contributed to CA1 circuit depression, but the relative degree of effect at each site was different for each anesthetic studied. For example, most of propofol's depressant effect (> 70 %) was reversed with a GABA antagonist, but only a minor portion of isoflurane's depression was reversed (< 20 %). Differences were also apparent on glutamate synapses-pentobarbital depressed transmission by > 50 %, but thiopental by only < 25 %.

CONCLUSIONS

These results, in as much as they may be relevant to anesthesia, indicate that general anesthetics act at several discrete sites, supporting a multi-site, agent specific theory for anesthetic actions. No single effect site (e.g. GABA synapses) or mechanism of action (e.g. depressed membrane excitability) could account for all of the effects produced for any anesthetic studied.

Molecular and neuronal substrates for general anaesthetics

Although general anaesthesia has been of tremendous importance for the development of surgery, the underlying mechanisms by which this state is achieved are only just beginning to be understood in detail. In this review, we describe the neuronal systems that are thought to be involved in mediating clinically relevant actions of general anaesthetics, and we go on to discuss how the function of individual drug targets, in particular GABA(A)-receptor subtypes, can be revealed by genetic studies in vivo.