The effects of general anaesthetics on ligand-gated ion channels

Calcium imaging and analysis of the jugular-nodose ganglia enables identification of distinct vagal sensory neuron subsets

Objective.Sensory nerves of the peripheral nervous system (PNS) transmit afferent signals from the body to the brain. These peripheral nerves are composed of distinct subsets of fibers and associated cell bodies, which reside in peripheral ganglia distributed throughout the viscera and along the spinal cord. The vagus nerve (cranial nerve X) is a complex polymodal nerve that transmits a wide array of sensory information, including signals related to mechanical, chemical, and noxious stimuli. To understand how stimuli applied to the vagus nerve are encoded by vagal sensory neurons in the jugular-nodose ganglia, we developed a framework for micro-endoscopic calcium imaging and analysis.Approach.We developed novel methods forin vivoimaging of the intact jugular-nodose ganglion using a miniature microscope (Miniscope) in transgenic mice with the genetically-encoded calcium indicator GCaMP6f. We adapted the Python-based analysis package Calcium Imaging Analysis (CaImAn) to process the resulting one-photon fluorescence data into calcium transients for subsequent analysis. Random forest classification was then used to identify specific types of neuronal responders.Results.We demonstrate that recordings from the jugular-nodose ganglia can be accomplished through careful surgical dissection and ganglia stabilization. Using a customized acquisition and analysis pipeline, we show that subsets of vagal sensory neurons respond to different chemical stimuli applied to the vagus nerve. Successful classification of the responses with a random forest model indicates that certain calcium transient features, such as amplitude and duration, are important for encoding these stimuli by sensory neurons.Significance.This experimental approach presents a new framework for investigating how individual vagal sensory neurons encode various stimuli on the vagus nerve. Our surgical and analytical approach can be applied to other PNS ganglia in rodents and other small animal species to elucidate previously unexplored roles for peripheral neurons in a diverse set of physiological functions.

Serotonin and Consciousness-A Reappraisal

The serotonergic system of the brain is a major modulator of behaviour. Here we describe a re-appraisal of its function for consciousness based on anatomical, functional and pharmacological data. For a better understanding, the current model of consciousness is expanded. Two parallel streams of conscious flow are distinguished. A flow of conscious content and an affective consciousness flow. While conscious content flow has its functional equivalent in the activity of higher cortico-cortical and cortico-thalamic networks, affective conscious flow originates in segregated deeper brain structures for single emotions. It is hypothesized that single emotional networks converge on serotonergic and other modulatory transmitter neurons in the brainstem where a bound percept of an affective conscious flow is formed. This is then dispersed to cortical and thalamic networks, where it is time locked with conscious content flow at the level of these networks. Serotonin acts in concert with other modulatory systems of the brain stem with some possible specialization on single emotions. Together, these systems signal a bound percept of affective conscious flow. Dysfunctions in the serotonergic system may not only give rise to behavioural and somatic symptoms, but also essentially affect the coupling of conscious affective flow with conscious content flow, leading to the affect-stained subjective side of mental disorders like anxiety, depression, or schizophrenia. The present model is an attempt to integrate the growing insights into serotonergic system function. However, it is acknowledged, that several key claims are still at a heuristic level that need further empirical support.

Sevoflurane modulation of tetrodotoxin-resistant Na+ channels in small-sized dorsal root ganglion neurons of rats

OBJECTIVE

Volatile anesthetics are widely used for general anesthesia during surgical operations. Voltage-gated Na+ channels expressed in central neurons are major targets for volatile anesthetics; but it is unclear whether these drugs modulate native tetrodotoxin-resistant (TTX-R) Na+ channels, which are involved in the development and maintenance of inflammatory pain.

METHODS

In this study, we examined the effects of sevoflurane on TTX-R Na+ currents (INa) in acutely isolated rat dorsal root ganglion neurons, using a whole-cell patch-clamp technique.

RESULTS

Sevoflurane slightly potentiated the peak amplitude of transient TTX-R INa but more potently inhibited slow voltage-ramp-induced persistent INa in a concentration-dependent manner. Sevoflurane (0.86 ± 0.02 mM) (1) slightly shifted the steady-state fast inactivation relationship to hyperpolarizing ranges without affecting the voltage-activation relationship, (2) reduced the extent of use-dependent inhibition of Na+ channels, (3) accelerated the onset of inactivation and (4) delayed the recovery from inactivation of TTX-R Na+ channels. Thus, sevoflurane has diverse effects on TTX-R Na+ channels expressed in nociceptive neurons.

CONCLUSIONS

The present results suggest that the inhibition of persistent INa and the modulation of the voltage dependence and inactivation might be, at least in part, responsible for the analgesic effects elicited by sevoflurane.

Anesthetic care for patients with anti-NMDA receptor encephalitis

Introduction

Anti-N-methyl-D-aspartate (NMDA) receptor encephalitis, an autoimmune disorder resulting from antibodies directed against the NMDA (glutamate) receptor, is the second most frequent cause of immune-mediated encephalitis. To date, the information related to the anesthetic care of children with this disorder is limited to anecdotal reports.

Methods

We reviewed the anesthetic care of six patients with anti-NMDA receptor encephalitis who underwent 21 procedures at our institution from 2014 through 2019.

Results

The study cohort included six patients, ranging in age from 2 to 18 years, who required anesthetic care during 21 procedures. Airway management included a laryngeal mask airway (n = 8), endotracheal intubation (n = 12), and native airway with spontaneous ventilation (n = 1). Intravenous (IV) induction with propofol was used in 17 procedures for five patients, including three that required rapid sequence intubation using rocuronium or succinylcholine. Inhalation induction with sevoflurane in nitrous oxide (N₂O)/oxygen (O₂) was chosen for two procedures in two patients. A combination of both induction techniques was used for two patients in two procedures. Maintenance anesthesia was accomplished with a volatile agent, predominantly sevoflurane, for 18 of the 21 procedures; propofol infusion for one procedure; and single dose of propofol was used for two short procedures. N₂O was not used for maintenance anesthesia in any of the encounters. None of the patients exhibited adverse events, including hemodynamic instability, thermoregulatory problems, or respiratory events perioperatively. Postoperatively, there was no observed deterioration in clinical status attributed to anesthetic care.

Discussion

Multisystem involvement in anti-NMDA receptor encephalitis includes memory loss, behavior irregularity, psychosis, arrhythmias, blood pressure (BP) instability, and hypoventilation. In our study cohort, we noted no intraoperative issues and deterioration in clinical status following the use of volatile anesthetic agents, opioids, dexmedetomidine, and propofol for general anesthesia (GA) or sedation. As ketamine, xenon, and N₂O mediate their anesthetic effects, primarily, through antagonism of NMDA receptors, theoretical concerns suggest that they should be avoided.

Impact of soman and acetylcholine on the effects of propofol in cultured cortical networks

Patients intoxicated with organophosphorous compounds may need general anaesthesia to enable mechanical ventilation or for control of epileptiform seizures. It is well known that cholinergic overstimulation attenuates the efficacy of general anaesthetics to reduce spontaneous network activity in the cortex. However, it is not clear how propofol, the most frequently used intravenous anaesthetic today, is affected. Here, we investigated the effects of cholinergic overstimulation induced by soman and acetylcholine on the ability of propofol to depress spontaneous action potential activity in organotypic cortical slices measured by extracellular voltage recordings. Cholinergic overstimulation by co-application of soman and acetylcholine (10 μM each) did not reduce the relative inhibition of propofol (1.0 μM; mean normalized action potential firing rate 0.49 ± 0.06 of control condition, p < 0.001, Wilcoxon signed rank test) but clearly reduced its efficacy. Co-application of atropine (10 nM) did not improve the efficacy. Propofol preserved its relative inhibitory potential but did not produce a degree of neuronal depression which can be expected to assure hypnosis in humans. Since a combination with atropine did not improve its efficacy, an increase in dosage will probably be necessary when propofol is used in victims suffering from organophosphorous intoxication.

Opportunities and Challenges for Single-Unit Recordings from Enteric Neurons in Awake Animals

Advanced electrode designs have made single-unit neural recordings commonplace in modern neuroscience research. However, single-unit resolution remains out of reach for the intrinsic neurons of the gastrointestinal system. Single-unit recordings of the enteric (gut) nervous system have been conducted in anesthetized animal models and excised tissue, but there is a large physiological gap between awake and anesthetized animals, particularly for the enteric nervous system. Here, we describe the opportunity for advancing enteric neuroscience offered by single-unit recording capabilities in awake animals. We highlight the primary challenges to microelectrodes in the gastrointestinal system including structural, physiological, and signal quality challenges, and we provide design criteria recommendations for enteric microelectrodes.

Propofol inhibits prokaryotic voltage-gated Na+ channels by promoting activation-coupled inactivation

Propofol is widely used in the clinic for the induction and maintenance of general anesthesia. As with most general anesthetics, however, our understanding of its mechanism of action remains incomplete. Local and general anesthetics largely inhibit voltage-gated Na⁺ channels (Navs) by inducing an apparent stabilization of the inactivated state, associated in some instances with pore block. To determine the biophysical and molecular basis of propofol action in Navs, we investigated NaChBac and NavMs, two prokaryotic Navs with distinct voltage dependencies and gating kinetics, by whole-cell patch clamp electrophysiology in the absence and presence of propofol at clinically relevant concentrations (2-10 µM). In both Navs, propofol induced a hyperpolarizing shift of the pre-pulse inactivation curve without any significant effects on recovery from inactivation at strongly hyperpolarized voltages, demonstrating that propofol does not stabilize the inactivated state. Moreover, there was no evidence of fast or slow pore block by propofol in a non-inactivating NaChBac mutant (T220A). Propofol also induced hyperpolarizing shifts of the conductance-voltage relationships with negligible effects on the time constants of deactivation at hyperpolarized voltages, indicating that propofol does not stabilize the open state. Instead, propofol decreases the time constants of macroscopic activation and inactivation. Adopting a kinetic scheme of Nav gating that assumes preferential closed-state recovery from inactivation, a 1.7-fold acceleration of the rate constant of activation and a 1.4-fold acceleration of the rate constant of inactivation were sufficient to reproduce experimental observations with computer simulations. In addition, molecular dynamics simulations and molecular docking suggest that propofol binding involves interactions with gating machinery in the S4-S5 linker and external pore regions. Our findings show that propofol is primarily a positive gating modulator of prokaryotic Navs, which ultimately inhibits the channels by promoting activation-coupled inactivation.

Assessing anesthetic activity through modulation of the membrane dipole potential

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

Safety and Effectiveness of Endoscopist-Directed Nurse-Administered Sedation during Gastric Endoscopic Submucosal Dissection

BACKGROUND AND AIMS

Endoscopic submucosal dissection (ESD) is routinely performed in treating gastric neoplasia and requires long-term higher levels of sedation. Endoscopist-directed nurse-administered sedation (EDNAS) has not been well studied in ESD. This study aimed to evaluate the safety and effectiveness of EDNAS for ESD.

METHODS

Patients treated with ESD for gastric tumors between 2013 and 2015 were retrospectively collected. Patients were divided into a midazolam-treated group (M group) and a midazolam plus propofol-treated group (MP group). Clinical outcome, safety, effectiveness, adverse events of ESD, and adverse events of sedation were analyzed.

RESULTS

Of 209 collected patients, 83 were in the M group and 126 were in the MP group. Of all patients, 67 patients had the circulatory adverse event during the ESD procedure. Sedation method was the only significant risk factor (M versus MP: 2.17 (1.14-4.15), p = 0.019). In analysis of MP subgroups, 47 patients suffered an adverse event from sedation, and current smoking was the only significant association factor for adverse event (0.15 (0.03-0.68), p = 0.014).

CONCLUSIONS

In performing ESD, the effect of sedation is reduced in smoking patients. EDNAS may be acceptable for ESD under careful monitoring of vital sign and oxygen saturation.

Case report: anaesthetic management of radical gastrectomy for gastric cancer associated with anti-N-methyl-D-aspartate receptor encephalitis

BACKGROUND

Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a rare neurological disorder that is caused by the production of antibodies against NMDARs. As many anaesthetic drugs interact with NMDARs and may worsen the disease and because the disease poses risks, such as cardiovascular events, hyperthermia and respiratory insufficiency, while under anaesthesia, administering anaesthesia to patients with this disorder is clinically challenging.

CASE PRESENTATION

A 55-year-old man with gastric cancer associated with anti-NMDAR encephalitis who was diagnosed 8 months prior was admitted to Peking University Cancer Hospital for tumour resection. Before surgery, the patient's symptoms had been successfully controlled via aggressive immunotherapy. Radical gastrectomy was performed under general anaesthesia induced with remifentanil, propofol, and cisatracurium and maintained with sevoflurane and remifentanil. The patient had a favourable recovery without any adverse symptoms or post-operative complications.

CONCLUSIONS

Adequate preparation for surgery is essential for the anaesthetic management of patients with anti-NMDAR encephalitis. These rare patients may benefit from general anaesthesia induced using remifentanil, propofol and cisatracurium and maintained using sevoflurane and remifentanil. Additionally, the use of NMDA antagonists, such as ketamine, nitrous oxide and tramadol, should be avoided.

Anesthetic actions of thiopental remain largely unaffected during cholinergic overstimulation in cultured cortical networks

In case of military or terrorist use of organophosphorus (OP) compounds victims are likely to suffer from not only intoxication but physical trauma as well. Appropriate emergency care may therefore include general anesthesia to allow life-saving surgical intervention. Since there is evidence that drug potency and efficacy of several anesthetics are attenuated by high concentrations of acetylcholine in the CNS, this study was designed to evaluate the anesthetic actions of thiopental during cholinergic overstimulation. Making use of organotypic slice cultures derived from the mouse neocortex, drug effects were assessed by extracellular voltage recordings of network activity at basal cholinergic tone and during simulated cholinergic crisis (high cholinergic tone). The latter was achieved by inhibition of acetylcholinesterases via soman and an ambient acetylcholine concentration of 10μM. The induction of cholinergic crisis in vitro increased the network activity of cortical neurons significantly. Surprisingly, differences in network activity between basal and high cholinergic tone became less pronounced with rising concentrations of thiopental and drug potency and efficacy were almost equivalent. These results clearly distinguish thiopental from previously tested general anesthetics and make it a promising candidate for in vivo studies to identify suitable anesthetics for victims of OP intoxication.

Differential effects of volatile anesthetics on leukocyte integrin macrophage-1 antigen

Macrophage-1 antigen (Mac-1, αMβ2) is a leukocyte adhesion molecule that plays a significant role in leukocyte crawling and phagocytosis, and is homologous to its sister protein leukocyte function-associated antigen-1 (LFA-1, αLβ2). The authors have previously demonstrated that volatile anesthetics isoflurane and sevoflurane bound to and inhibited LFA-1. Here, the hypothesis tested was that isoflurane and sevoflurane would inhibit Mac-1. A binding assay of Mac-1 to its ligand inter-cellular adhesion molecule-1 (ICAM-1) using V-bottom plates was established. The effect of isoflurane and sevoflurane on Mac-1 was examined using this ICAM-1 binding assay and by probing exposure of activation-sensitive epitopes. The docking program Glide was used to predict anesthetic binding site(s) on Mac-1. The functional role of this predicted binding site was then assessed by introducing point mutations in this region. Lastly, the effect of anesthetic on activating mutants was evaluated. The results indicated that isoflurane inhibited binding of Mac-1 to ICAM-1, but sevoflurane did not. Isoflurane also attenuated the exposure of the activation-sensitive epitopes. The docking simulation predicted the isoflurane binding site to be at the cavity underneath the α7 helix of the ligand binding domain (the αM I domain). Point mutants at this predicted binding site contained both activating and deactivating mutants, suggesting its functional significance. The binding of activating mutants αM-Y267A β2-WT and αM-L312A β2-WT to ICAM-1 was not affected by isoflurane, but binding of another activating mutant αM-WT β2-L132A was inhibited supporting the binding of isoflurane to this cavity. The conclusion reached from these findings was that isoflurane inhibited Mac-1 binding to ICAM-1 by binding to the cavity underneath the α7 helix of the αM I domain, but sevoflurane did not. Thus, because these common clinical volatile anesthetics demonstrated different effects on Mac-1, this implied their effects on the immune system might differ.

Potential side effect of propofol and sevoflurane for anesthesia of anti-NMDA-R encephalitis

BACKGROUND

Many anesthetic drugs interact with the NMDA receptor and may therefore alter the clinical presentation of anti-NMDA-R encephalitis.

CASE PRESENTATION

A 24-year-old woman was admitted to hospital for decreased consciousness and hyperthermia. Cerebrospinal fluid analysis revealed lymphocytic pleocytosis, and elevated protein. Cultures were negative. Patient state worsened with agitation, facial dyskinesia, ocular deviation, and limb dystonia. Diagnosis of anti-NMDA-R encephalitis was evidenced by specific antibodies. High doses of methylprednisolone were administered. CT scan disclosed an ovarian teratoma and tumor resection was scheduled under anesthesia with propofol, sufentanil, atracurium and sevoflurane. Sedation after surgery was maintained with propofol. Rapidly after surgery, patient's condition deteriorated with increase of dyskinesias, and two tonic-clonic generalized seizure events.

CONCLUSION

In patients with anti-NMDA-R encephalitis, anesthesia using benzodiazepines, opiates and curares, which fail to interfere with the NMDA pathway, should be preferred.

Reversal of ion-charge selectivity renders the pentameric ligand-gated ion channel GLIC insensitive to anaesthetics

pLGICs (pentameric ligand-gated ion channels) are a family of structurally homologous cation and anion channels involved in neurotransmission. Cation-selective members of the pLGIC family are typically inhibited by general anaesthetics, whereas anion-selective members are potentiated. GLIC is a prokaryotic cation pLGIC and can be inhibited by clinical concentrations of general anaesthetics. The introduction of three mutations, Y221A (Y-3'A), E222P (E-2'P) and N224R (N0'R), at the selectivity filter and one, A237T (A13'T), at the hydrophobic gate, converted GLIC into an anion channel. The mutated GLIC (GLIC4) became insensitive to the anaesthetics propofol and etomidate, as well as the channel blocker picrotoxin. MD (molecular dynamics) simulations revealed changes in the structure and dynamics of GLIC4 in comparison with GLIC, particularly in the tilting angles of the pore-lining helix [TM2 (transmembrane helix 2)] that consequently resulted in different pore radius and hydration profiles. Propofol binding to an intra-subunit site of GLIC shifted the tilting angles of TM2 towards closure at the hydrophobic gate region, consistent with propofol inhibition of GLIC. In contrast, the pore of GLIC4 was much more resilient to perturbation from propofol binding. The present study underscores the importance of pore dynamics and conformation to anaesthetic effects on channel functions.

Urethane suppresses hippocampal CA1 neuron excitability via changes in presynaptic glutamate release and in potassium channel activity

Urethane is a widely used anesthetic for animal experiments. Although urethane is thought to minimally interfere with neurophysiological processes and appears to preserve synaptic signal transmission, it has also been reported to produce depressive effects on neuronal excitability. In the present study, we used electrophysiological recordings to investigate the effects of urethane on rat hippocampal CA1 neurons. Whole-cell recordings were employed in a brain slice preparation to record discharges in current-clamp mode and sEPSCs or mEPSCs in voltage-clamp mode. Urethane was found to significantly increase both the interspike interval and the coefficient of variation of the firing. Moreover, it was found that the inter-event intervals of sEPSC/mEPSCs were increased, but the amplitude and the kinetic properties (rise time and decay time) of the sEPSC/mEPSC were not altered by urethane, which implies that potassium leak currents were involved in such effects. The results suggest that urethane significantly suppresses activity of hippocampal CA1 neurons and alters spontaneous pre-synaptic glutamatergic release possibly by activating potassium leak currents.

Flumazenil expedites recovery from sevoflurane/remifentanil anaesthesia when administered to healthy unpremedicated patients

BACKGROUND AND OBJECTIVE

To investigate the hypothesis that 0.3 mg flumazenil administered to healthy unpremedicated patients at the end of deep surgical sevoflurane/remifentanil anaesthesia would expedite recovery. Flumazenil, an imidazobenzodiazepine derivative, antagonizes the hypnotic/sedative effects of benzodiazepines on γ-aminobutyric acid receptors. However, endogenous benzodiazepine ligands (endozepines) were isolated in mammalian tissues of individuals who had not received benzodiazepines.

METHODS

Twenty-four healthy unpremedicated patients, scheduled to undergo elective surgery requiring general anaesthesia, were randomly allocated to receive either a single dose of 0.3 mg flumazenil (n = 14) or placebo (n = 10) intravenously at the end of the surgical procedure just before the discontinuation of the volatile anaesthetic. After study drug administration, the authors compared various recovery parameters in the flumazenil and control groups.

RESULTS

Median time to spontaneous respiration, eye opening on verbal command, extubation and time to date of birth recollection was significantly shorter in the flumazenil group than in the control group [2.5 min (2.0-3.0) vs. 7.0 min (6.8-8.3), 3.4 min (3.0-4.0) vs. 8.1 min (6.9-10.2), 4.0 min (3.0-5.0) vs. 9.0 min (7.0-10.8) and 4.7 min (4.0-5.0) vs. 10.3 min (8.0-12.0), respectively].

CONCLUSION

Administration of a single dose of 0.3 mg flumazenil to healthy unpremedicated patients at the end of sevoflurane/remifentanil anaesthesia results in earlier emergence from anaesthesia and significantly expedites recovery. This could redefine the role of flumazenil in general anaesthesia, implicating endozepine-dependent mechanisms.

A comparison of visual responses in the lateral geniculate nucleus of alert and anaesthetized macaque monkeys

Despite the increasing use of alert animals for studies aimed at understanding visual processing in the cerebral cortex, relatively little attention has been focused on quantifying the response properties of neurons that provide input to the cortex. Here, we examine the response properties of neurons in the lateral geniculate nucleus (LGN) of the thalamus in the alert macaque monkey and compare these responses to those in the anaesthetized animal. Compared to the anaesthetized animal, we show that magnocellular and parvocellular neurons in the alert animal respond to visual stimuli with significantly higher firing rates. This increase in responsiveness is not accompanied by a change in the shape of neuronal contrast response functions or the strength of centre–surround antagonism; however, it is accompanied by an increased ability of neurons to follow stimuli drifting at higher spatial and temporal frequencies.

Higher susceptibility to halothane modulation in open- than in closed-channel alpha4beta2 nAChR revealed by molecular dynamics simulations

The neuronal alpha4beta2 nicotinic acetylcholine receptor (nAChR) is a potential molecular target for general anesthetics. It is unclear, however, whether anesthetic action produces the same effect on the open and closed channels. Computations parallel to our previous open channel study (J. Phys. Chem. B 2009, 113, 12581) were performed on the closed-channel alpha4beta2 nAChR to investigate the conformation-dependent anesthetic effects on channel structures and dynamics. Flexible ligand docking and over 20 ns molecular dynamics simulations revealed similar halothane-binding sites in the closed and open channels. The sites with relatively high binding affinities (approximately -6.0 kcal/mol) were identified at the interface of extracellular (EC) and transmembrane (TM) domains or at the interface between alpha4 and beta2 subunits. Despite similar sites for halothane binding, the closed-channel conformation showed much less sensitivity than the open channel to the structural and dynamical perturbations from halothane. Compared to the systems without anesthetics, the amount of water inside the pore decreased by 22% in the presence of halothane in the open channel but only by 6% in the closed channel. Comparison of the nonbonded interactions at the EC/TM interfaces suggested that the beta2 subunits were more prone than the alpha4 subunits to halothane binding. In addition, our data support the notion that halothane exerts its effect by disturbing the quaternary structure and dynamics of the channel. The study concludes that sensitivity and global dynamics responsiveness of alpha4beta2 nAChR to halothane are conformation dependent. The effect of halothane on the global dynamics of the open-channel conformation might also account for the action of other inhaled general anesthetics.

Inhalational anaesthetics and n-alcohols share a site of action in the neuronal Shaw2 Kv channel

BACKGROUND AND PURPOSE

Neuronal ion channels are key targets of general anaesthetics and alcohol, and binding of these drugs to pre-existing and relatively specific sites is thought to alter channel gating. However, the underlying molecular mechanisms of this action are still poorly understood. Here, we investigated the neuronal Shaw2 voltage-gated K(+) (K(v)) channel to ask whether the inhalational anaesthetic halothane and n-alcohols share a binding site near the activation gate of the channel.

EXPERIMENTAL APPROACH

Focusing on activation gate mutations that affect channel modulation by n-alcohols, we investigated n-alcohol-sensitive and n-alcohol-resistant K(v) channels heterologously expressed in Xenopus oocytes to probe the functional modulation by externally applied halothane using two-electrode voltage clamping and a gas-tight perfusion system.

KEY RESULTS

Shaw2 K(v) channels are reversibly inhibited by halothane in a dose-dependent and saturable manner (K(0.5)= 400 microM; n(H)= 1.2). Also, discrete mutations in the channel's S4S5 linker are sufficient to reduce or confer inhibition by halothane (Shaw2-T330L and K(v)3.4-G371I/T378A respectively). Furthermore, a point mutation in the S6 segment of Shaw2 (P410A) converted the halothane-induced inhibition into halothane-induced potentiation. Lastly, the inhibition resulting from the co-application of n-butanol and halothane is consistent with the presence of overlapping binding sites for these drugs and weak binding cooperativity.

CONCLUSIONS AND IMPLICATIONS

These observations strongly support a molecular model of a general anaesthetic binding site in the Shaw2 K(v) channel. This site may involve the amphiphilic interface between the S4S5 linker and the S6 segment, which plays a pivotal role in K(v) channel activation.

Computational studies on the interactions of inhalational anesthetics with proteins

Despite the widespread clinical use of anesthetics since the 19th century, a clear understanding of the mechanism of anesthetic action has yet to emerge. On the basis of early experiments by Meyer, Overton, and subsequent researchers, the cell's lipid membrane was generally concluded to be the primary site of action of anesthetics. However, later experiments with lipid-free globular proteins, such as luciferase and apoferritin, shifted the focus of anesthetic action to proteins. Recent experimental studies, such as photoaffinity labeling and mutagenesis on membrane proteins, have suggested specific binding sites for anesthetic molecules, further strengthening the proteocentric view of anesthetic mechanism. With the increased availability of high-resolution crystal structures of ion channels and other integral membrane proteins, as well as the availability of powerful computers, the structure-function relationship of anesthetic-protein interactions can now be investigated in atomic detail. In this Account, we review recent experiments and related computer simulation studies involving interactions of inhalational anesthetics and proteins, with a particular focus on membrane proteins. Globular proteins have long been used as models for understanding the role of protein-anesthetic interactions and are accordingly examined in this Account. Using selected examples of membrane proteins, such as nicotinic acetyl choline receptor (nAChR) and potassium channels, we address the issues of anesthetic binding pockets in proteins, the role of conformation in anesthetic effects, and the modulation of local as well as global dynamics of proteins by inhaled anesthetics. In the case of nicotinic receptors, inhalational anesthetic halothane binds to the hydrophobic cavity close to the M2-M3 loop. This binding modulates the dynamics of the M2-M3 loop, which is implicated in allosterically transmitting the effects to the channel gate, thus altering the function of the protein. In potassium channels, anesthetic molecules preferentially potentiate the open conformation by quenching the motion of the aromatic residues implicated in the gating of the channel. These simulations suggest that low-affinity drugs (such as inhalational anesthetics) modulate the protein function by influencing local as well as global dynamics of proteins. Because of intrinsic experimental limitations, computational approaches represent an important avenue for exploring the mode of action of anesthetics. Molecular dynamics simulations-a computational technique frequently used in the general study of proteins-offer particular insight in the study of the interaction of inhalational anesthetics with membrane proteins.

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.

Simultaneous electroencephalography and functional magnetic resonance imaging of general anesthesia

It has been long appreciated that anesthetic drugs induce stereotyped changes in electroencephalogram (EEG), but the relationships between the EEG and underlying brain function remain poorly understood. Functional imaging methods including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have become important tools for studying how anesthetic drugs act in the human brain to induce the state of general anesthesia. To date, no investigation has combined functional MRI with EEG to study general anesthesia. We report here a paradigm for conducting combined fMRI and EEG studies of human subjects under general anesthesia. We discuss the several technical and safety problems that must be solved to undertake this type of multimodal functional imaging and show combined recordings from a human subject. Combined fMRI and EEG exploits simultaneously the high spatial resolution of fMRI and the high temporal resolution of EEG. In addition, combined fMRI and EEG offers a direct way to relate established EEG patterns induced by general anesthesia to changes in neural activity in specific brain regions as measured by changes in fMRI blood oxygen level dependent (BOLD) signals.

Acute alcohol action and desensitization of ligand-gated ion channels

Ethanol exerts its biological actions through multiple receptors, including ion channels. Ion channels that are sensitive to pharmacologically relevant ethanol concentrations constitute a heterogeneous set, including structurally unrelated proteins solely sharing the property that their gating is regulated by a ligand(s). Receptor desensitization is almost universal among these channels, and its modulation by ethanol may be a crucial aspect of alcohol pharmacology and effects in the body. We review the evidence documenting interactions between ethanol and ionotropic receptor desensitization, and the contribution of this interaction to overall ethanol action on channel function. In some cases, such as type 3 serotonin, nicotinic acetylcholine, GABA-A, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, ethanol actions on apparent desensitization play a significant role in acute drug action on receptor function. In a few cases, mutagenesis helped to identify different areas within a receptor protein that differentially sense n-alcohols, resulting in differential modulation of receptor desensitization. However, desensitization of a receptor is linked to a variety of biochemical processes that may alter protein conformation, such as the lipid microenvironment, post-translational channel modification, and channel subunit composition, the relative contribution of these processes to ethanol interactions with channel desensitization remains unclear. Understanding interactions between ethanol and ionotropic receptor desensitization may help to explain different ethanol actions 1) when ethanol is evaluated in vitro on cloned channel proteins, 2) under physiological or pathological conditions or in distinct cell domains with modified ligand concentration and/or receptor conformation. Finally, receptor desensitization is likely to participate in molecular and, possibly, behavioral tolerance to ethanol, which is thought to contribute to the risk of alcoholism.

A hypothesis on the origin and evolution of the response to inhaled anesthetics

In this article, I present an evolutionary explanation for why organisms respond to inhaled anesthetics. It is conjectured that organisms today respond to inhaled anesthetics owing to the sensitivity of ion channels to inhaled anesthetics, which in turn has arisen by common descent from ancestral, anesthetic-sensitive ion channels in one-celled organisms (i.e., that the response to anesthetics did not arise as an adaptation of the nervous system, but rather of ion channels that preceded the origin of multicellularity). This sensitivity may have been refined by continuing selection at synapses in multicellular organisms. In particular, it is hypothesized that 1) the beneficial trait that was selected for in one-celled organisms was the coordinated response of ion channels to compounds that were present in the environment, which influenced the conformational equilibrium of ion channels; 2) this coordinated response prevented the deleterious consequences of entry of positive charges into the cell, thereby increasing the fitness of the organism; and 3) these compounds (which may have included organic anions, cations, and zwitterions as well as uncharged compounds) mimicked inhaled anesthetics in that they were interfacially active, and modulated ion channel function by altering bilayer properties coupled to channel function. The proposed hypothesis is consistent with known properties of inhaled anesthetics. In addition, it leads to testable experimental predictions of nonvolatile compounds having anesthetic-like modulatory effects on ion channels and in animals, including endogenous compounds that may modulate ion channel function in health and disease. The latter included metabolites that are increased in some types of end-stage organ failure, and genetic metabolic diseases. Several of these predictions have been tested and proved to be correct.

GABAA receptors, anesthetics and anticonvulsants in brain development

GABA, acting via GABA(A) receptors, is well-accepted as the main inhibitory neurotransmitter of the mature brain, where it dampens neuronal excitability. The receptor's properties have been studied extensively, yielding important information about its structure, pharmacology, and regulation that are summarized in this review. Several GABAergic drugs have been commonly used as anesthetics, sedatives, and anticonvulsants for decades. However, findings that GABA has critical functions in brain development, in particular during the late embryonic and neonatal period, raise worthwhile questions regarding the side effects of GABAergic drugs that may lead to long-term cognitive deficits. Here, we will review some of these drugs in parallel with the control of CNS development that GABA exerts via activation of GABA(A) receptors. This review aims to provide a basic science and clinical perspective on the function of GABA and related pharmaceuticals acting at GABA(A) receptors.

Interaction of anesthetics with open and closed conformations of a potassium channel studied via molecular dynamics and normal mode analysis

A variety of experiments suggest that membrane proteins are important targets of anesthetic molecules, and that ion channels interact differently with anesthetics in their open and closed conformations. The availability of an open and a closed structural model for the KirBac1.1 potassium channel has made it possible to perform a comparative analysis of the interactions of anesthetics with the same channel in its open and closed states. To this end, all-atom molecular dynamics simulations supplemented by normal mode analysis have been employed to probe the interactions of the inhalational anesthetic halothane with both an open and closed conformer of KirBac1.1 embedded in a lipid bilayer. Normal mode analysis on the closed and open channel, in the presence and absence of halothane, reveals that the anesthetic modulates the global as well as the local dynamics of both conformations differently. In the case of the open channel, the observed reduction of flexibility of residues in the inner helices suggests a functional modification action of anesthetics on ion channels. In this context, preferential quenching of the aromatic residue motion and modulation of global dynamics by halothane may be seen as steps toward potentiating or favoring open state conformations. These molecular dynamics simulations provide the first insights into possible specific interactions between anesthetic molecules and ion channels in different conformations.

Effect of common anesthetics on dendritic properties in layer 5 neocortical pyramidal neurons

Understanding the impact of active dendritic properties on network activity in vivo has so far been restricted to studies in anesthetized animals. However, to date no study has been made to determine the direct effect of the anesthetics themselves on dendritic properties. Here, we investigated the effects of three types of anesthetics commonly used for animal experiments (urethane, pentobarbital and ketamine/xylazine). We investigated the generation of calcium spikes, the propagation of action potentials (APs) along the apical dendrite and the somatic firing properties in the presence of anesthetics in vitro using dual somatodendritic whole cell recordings. Calcium spikes were evoked with dendritic current injection and high-frequency trains of APs at the soma. Surprisingly, we found that the direct actions of anesthetics on calcium spikes were very different. Two anesthetics (urethane and pentobarbital) suppressed dendritic calcium spikes in vitro, whereas a mixture of ketamine and xylazine enhanced them. Propagation of spikes along the dendrite was not significantly affected by any of the anesthetics but there were various changes in somatic firing properties that were highly dependent on the anesthetic. Last, we examined the effects of anesthetics on calcium spike initiation and duration in vivo using high-frequency trains of APs generated at the cell body. We found the same anesthetic-dependent direct effects in addition to an overall reduction in dendritic excitability in anesthetized rats with all three anesthetics compared with the slice preparation.

Volatile aromatic anesthetics variably impact human gamma-aminobutyric acid type A receptor function

BACKGROUND

The gamma-aminobutyric acid type A (GABA(A)) and N-methyl-d-aspartate (NMDA) receptors are important inhibitory and excitatory neurotransmitter receptors, respectively, in the central nervous system. At the concentrations required to produce immobility in the face of a noxious stimulus, volatile aromatic anesthetics inhibit NMDA receptors to varying degrees, strongly suggesting that they also act at other targets to produce immobilization. In this study, we sought to assess the potential role that GABA(A) receptors play in mediating the behavioral actions of volatile aromatic anesthetics.

METHODS

Electrophysiological techniques were used to quantify the effects of eight volatile aromatic anesthetics and three clinical anesthetics on currents mediated by alpha1beta2gamma2L GABA(A) receptors expressed in Xenopus oocytes.

RESULTS

At equivalent minimal alveolar anesthetic concentration multiples, volatile aromatic anesthetics vary widely in the degrees to which they enhance GABA(A) receptor-mediated currents elicited by low concentrations of GABA. In general, anesthetics that inhibit NMDA receptors most, enhanced GABA(A) receptors least. This reciprocal relationship between anesthetic potency on GABA(A) versus NMDA receptors was also observed for the clinical anesthetics isoflurane, halothane, and cyclopropane. Studies using a range of GABA concentrations indicated that volatile aromatic anesthetics enhance GABA(A) receptor activity by shifting the open-close (gating) equilibrium towards the open channel state.

CONCLUSIONS

These findings suggest that GABA(A) receptors contribute variably to the behavioral actions of volatile anesthetics and imply that the molecular determinants of anesthetic action on NMDA and GABA(A) receptors are distinctly different.

Functional changes in the vanilloid receptor subtype 1 channel during and after acute desensitization

Agonist-induced desensitization of the transient receptor potential vanilloid receptor-1 (TRPV1) is one of the key strategies that offer a way to alleviate neuropathic and inflammatory pain. This process is initiated by TRPV1 receptor activation and the subsequent entry of extracellular Ca(2+) through the channel into sensory neurones. One of the prominent mechanisms responsible for TRPV1 desensitization is dephosphorylation of the TRPV1 protein by the Ca(2+)/calmodulin-dependent enzyme, phosphatase 2B (calcineurin). Of several consensus phosphorylation sites identified so far, the most notable are two sites for Ca(2+)/calmodulin dependent kinase II (CaMKII) at which the dynamic equilibrium between the phosphorylated and dephosphorylated states presumably regulates agonist binding. We examined the mechanisms of acute Ca(2+)-dependent desensitization using whole-cell patch-clamp techniques in human embryonic kidney (HEK) 293T cells expressing the wild type or CaMKII phosphorylation site mutants of rat TRPV1. The nonphosphorylatable mutant S502A/T704I was capsaicin-insensitive but the S502A/T704A construct was fully functional, indicating a requirement for a specific residue at position 704. A point mutation at the nearby conserved residue R701 strongly affected the heat, capsaicin and pH-evoked currents. As this residue constitutes a stringent CaMKII consensus site but is also predicted to be involved in the interaction with membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)), these data suggest that in addition to dephosphorylation, or as its consequence, a short C-terminal juxtamembrane segment adjacent to the transient receptor potential box composed of R701 and T704 might be involved in the decelerated gating kinetics of the desensitized TRPV1 channel.

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.

Anesthetic-like modulation of a gamma-aminobutyric acid type A, strychnine-sensitive glycine, and N-methyl-d-aspartate receptors by coreleased neurotransmitters

INTRODUCTION

A mechanism of anesthesia has recently been proposed which predicts that coreleased neurotransmitters may modulate neurotransmitter receptors for which they are not the native agonist in a manner similar to anesthetics.

METHODS

We tested this prediction by applying acetylcholine to a NR1/NR2A N-methyl-d-aspartate receptor, glycine to a wild-type alpha(1)beta(2) and anesthetic-resistant alpha(1)(S270I)beta(2) gamma-amino-butyric acid (GABA) type A receptor, and GABA to a homomeric alpha(1) wild type and anesthetic-resistant alpha(1) S267I glycine receptor. Receptors were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamping.

RESULTS

We found inhibition of N-methyl-d-aspartate receptor function by acetylcholine, enhancement of glycine receptor function by GABA, and enhancement of GABA type A receptor function by glycine. As expected of compounds with anesthetic activity, GABA showed far less potentiation (enhancement) of the function of the anesthetic-resistant S267I glycine receptor than that of the wild-type receptor. Glycine potentiated the function of wild-type GABA type A receptors but inhibited the function of the anesthetic-resistant S270I GABA type A receptor.

CONCLUSIONS

These results show that neurotransmitters that are coreleased onto anesthetic-sensitive receptors may modulate the function of receptors for which they are not the native agonist via an anesthetic-like mechanism. These findings lend support to a recent theory of anesthetic action.

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.

Anaesthesia in patients suffering from organophosphorus intoxication—interactions between general anaesthetics and acetylcholine in cortical networks in vitro

In scenarios of mass destruction it is likely that victims are intoxicated by organophosphates and, at the same time, physically injured. Organophosphate compounds produce excessive cholinergic overstimulation in the CNS via blocking acetylcholinesterase activity. The specifics of acute care and anaesthesia in physically traumatized and intoxicated patients are largely unknown. Recent studies in animals and human subjects demonstrated that acetylcholinesterase inhibitors reverse anaesthesia. Two distinct mechanisms are potentially involved. First, acetylcholine produces an excitatory drive onto neurons, thereby counterbalancing the inhibitory actions of anaesthetics. Anaesthesia is reversed because it critically depends on a distinctive depression of several central nervous functions. Second, cholinergic stimulation may affect the mechanisms by which anaesthetics mediate their depressant actions on central neurons. In this case acetylcholine reverses anaesthesia by decreasing the potency of anaesthetic agents. In order to identify potential mechanisms involved in cholinergic reversal of anaesthesia we have investigated interactions between acetylcholine and the volatile anaesthetic sevoflurane in isolated cortical brain slices. Our results provide evidence that cholinergic stimulation counterbalances the effects of general anaesthetics by increasing neuronal excitability, and, in addition, by decreasing anaesthetic potency. These findings imply that in patients suffering from organophosphorus intoxication dose requirements for general anaesthetics are considerably increased.

Propofol modulates inner retina function in Beagles

PURPOSE

To further understand a common veterinary anesthetic, propofol (2,6- di-isopropylphenol) and effects of infusion rates on the retinal neurons in Beagle dogs.

METHODS

Standard full-field blue xenon-flash stimulation elicited responses of dark-adapted eyes, which were recorded from dogs before and after a propofol infusion rate increase.

RESULTS

Electroretinogram b-waves increased significantly after the infusion rate increase and decreased with decline (P < 0.0001). Also, a weak significance (P = 0.041) for a-wave peak amplitude increase was found after infusion rate increase. The initial part (first 18 ms) of the leading edge of the a-wave remained unchanged. No significant differences in times to a- and b-wave peaks were found.

CONCLUSION

Enhanced b-wave response and decline is due to sensitivity of postreceptoral cells, possibly interplexiform and amacrine cells, to propofol concentration.

Use of Anesthetic Agents in Neonates and Young Children

BACKGROUND

Some drugs used for sedation and anesthesia produce histopathologic central nervous system changes in juvenile animal models. These observations have raised concerns regarding the use of these drugs in pediatric patients. We summarized the findings in developing animals and describe the steps that the Food and Drug Administration (FDA) and others are taking to assess potential risks in pediatric patients. The FDA views this communication as opening a dialog with the anesthesia community to address this issue.

METHODS

We reviewed the available animal studies literature examining the potential neurotoxic effects of commonly used anesthetic drugs on the developing brain. The search strategy involved crossing the keywords neurotoxic and neuroapoptosis with the following general and specific terms: anesthetic, N-methyl-d-aspartate (NMDA), ketamine, midazolam, lorazepam, fentanyl, methadone, morphine, meperidine, isoflurane, nitrous oxide, sevoflurane, halothane, enflurane, desflurane, propofol, etomidate, barbiturate, methoxyflurane, and chloral hydrate. We summarized several studies sponsored by the FDA in rats and monkeys, initially examining the potential for ketamine, as a prototypical agent, to induce neurodegeneration in the developing brain.

RESULTS

Numerous animal studies in rodents indicate that NMDA receptor antagonists, including ketamine, induce neurodegeneration in the developing brain. The effects of ketamine are dose dependent. The data suggest that limiting exposure limits the potential for neurodegeneration. There is also evidence that other general anesthetics, such as isoflurane, can induce neurodegeneration in rodent models, which may be exacerbated by concurrent administration of midazolam or nitrous oxide. There are very few studies that have examined the potential functional consequences of the neurodegeneration noted in the animal models. However, the studies that have been reported suggest subtle, but prolonged, behavioral changes in rodents. Although the doses and durations of ketamine exposure that resulted in neurodegeneration were slightly larger than those used in the clinical setting, those associated with isoflurane were not. There are insufficient human data to either support or refute the clinical applicability of these findings.

CONCLUSIONS

Animal studies suggest that neurodegeneration, with possible cognitive sequelae, is a potential long-term risk of anesthetics in neonatal and young pediatric patients. The existing nonclinical data implicate not only NMDA-receptor antagonists, but also drugs that potentiate gamma-aminobutyric acid signal transduction, as potentially neurotoxic to the developing brain. The potential for the combination of drugs that have activity at both receptor systems or that can induce more or less neurotoxicity is not clear; however, recent nonclinical data suggest that some combinations may be more neurotoxic than the individual components. The lack of information to date precludes the ability to designate any one anesthetic agent or regimen as safer than any other. Ongoing studies in juvenile animals should provide additional information regarding the risks. The FDA anticipates working with the anesthesia community and pharmaceutical industry to develop strategies for further assessing the safety of anesthetics in neonates and young children, and for providing data to guide clinicians in making the most informed decisions possible when choosing anesthetic regimens for their pediatric patients.

The effect of cigarette smoking on the hypnotic efficacy of propofol

The bispectral index (BIS) was used to examine the hypnotic efficacy of propofol in 25 smokers (20 cigarettes/day for 2 years) and 24 matched non-smokers (same gender, age, height, weight). BIS was recorded at baseline, at four incremental effect-site concentrations of propofol and at loss of consciousness. Compared with non-smokers, smokers were found to have higher BIS values at baseline (mean (SD)) (97 (1) vs 98 (1)), at 0.7 microg x ml(-1) (95 (3) vs 97 (1)) and at 1.1 microg x ml(-1) (89 (6) vs 94 (4)), p = 0.0099, and they lost consciousness at higher propofol concentrations (2.0 (0.4) vs 2.4 (0.8) microg x ml(-1)), p = 0.03, and at lower BIS values (66 (10) vs 60 (10)), p = 0.04. The hypnotic efficacy of propofol is reduced when used at low effect-site concentrations in smokers. This phenomenon may have some impact on the management of smokers undergoing sedation using target controlled infusion systems.

Urethane inhibits the GABAergic neurotransmission in the nucleus of the solitary tract of rat brain stem slices

Because urethane is a widely used anesthetic in animal experimentation, in the present study, we evaluated its effects on neurons of the nucleus of the solitary tract (NTS) in brain stem slices from young rats (25-30 days old). Using the whole cell configuration of the patch-clamp technique, spontaneous postsynaptic currents (sPSCs) and evoked excitatory postsynaptic currents (eEPSCs) were recorded. Urethane (20 mM) decreased by approximately 60% the frequency of GABAergic sPSCs (1.0 +/- 0.2 vs. 0.4 +/- 0.1 Hz) but did not change the frequency, amplitude, or half-width of glutamatergic events or TTX-resistant inhibitory sPSCs [miniature inhibitory postsynaptic currents (IPSCs)]. Miniature IPSCs were measured in the presence of urethane plus 1 mM diazepam (1 mM), and no changes were seen in their amplitude. This suggests that the GABA concentration in the NTS synapses is set at saturating level. We also evaluated the effect of urethane on eEPSCs, and no significant change was observed in the amplitude of N-methyl-d-aspartate [NMDA; 44.2 +/- 11.5 vs. 37.6 +/- 10.6 pA (holding potential = 40 mV)] and non-NMDA currents [204.4 +/- 35.5 vs. 196.6 +/- 31.2 pA (holding potential = -70 mV)]. Current-clamp experiments showed that urethane did not alter the action potential characteristics and passive membrane properties. These data suggest that urethane has an inhibitory effect on GABAergic neurons in the NTS but does not change the spontaneous or evoked excitatory responses.

Halothane affects focal adhesion proteins in the A 549 cells

Halothane is a volatile anaesthetic, which is known to induce alterations in cellular plasma membranes, modulating the physical state of the membrane lipids and/or interacting directly with membrane-bound proteins, such as integrin receptors. Integrin-mediated cell adhesion is a general property of eukaryotic cells, which is closely related to cell viability. Our previous investigations showed that halothane is toxic for A 549 lung carcinoma cells when applied at physiologically relevant concentrations and causes inhibition of adhesion to collagen IV. The present study is focused on the mechanisms underlying halothane toxicity. Our results imply that physiologically relevant concentrations of halothane disrupt focal adhesion contacts in A 549 cells, which is accompanied with suppression of focal adhesion kinase activity and paxillin phosphorylation, and not with proteolytic changes or inhibition of vinculin and paxillin expression.We suggest that at least one of the toxic effects of halothane is due to a decreased phosphorylation of the focal contact proteins.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.

Allosteric modulation of ligand-gated ion channels

Ligand-gated ion channels (LGICs) are cell surface proteins that play an important role in fast synaptic transmission and in the modulation of cellular activity. Due to their intrinsic properties, LGICs respond to neurotransmitters and other effectors (e.g. pH) and transduce the binding of a ligand into an electrical current on a microsecond timescale. Following activation, LGICs open allowing an ion flux across the cell membrane. Depending upon the charge and concentration of ions, the flux can cause a depolarization or hyperpolarization, thus modulating excitability of the cell. While our understanding of LGICs has significantly progressed during the past decade, many properties of these proteins are still poorly understood, in particular their modulation by allosteric effectors. LGICs are often thought as a simple on-off switches. However, a closer look at these receptors reveals a complex behavior and a wide repertoire of subtle modulation by intrinsic and extrinsic factors. From a physiological point of view, this modulation can be seen as an additional level of complexity in the cell signaling process. Here we review the allosteric modulation of LGICs in light of the latest findings and discuss the suitability of this approach to the design of new therapeutic molecules.

Perturbation of Voltage-Sensitive Ca2+ Channel Function by Volatile Organic Solvents

The mechanisms underlying the acute neurophysiological and behavioral effects of volatile organic compounds (VOCs) remain to be elucidated. However, the function of neuronal ion channels is perturbed by VOCs. The present study examined effects of toluene (TOL), trichloroethylene (TCE), and perchloroethylene (PERC) on whole-cell calcium current (ICa) in nerve growth factor-differentiated pheochromocytoma (PC12) cells. All three VOCs affected ICa in a reversible, concentration-dependent manner. At +10-mV test potentials, VOCs inhibited ICa, whereas at test potentials of -20 and -10 mV, they potentiated it. The order of potency for inhibition (IC50) was PERC (270 microM) > TOL (720 microM) > TCE (1525 microM). VOCs also changed ICa inactivation kinetics from a single- to double-exponential function. Voltage-ramp experiments suggested that VOCs shifted ICa activation in a hyperpolarizing direction; this was confirmed by calculating the half-maximal voltage of activation (V1/2, act) in the absence and presence of VOCs using the Boltzman equation. V(1/2, act) was shifted from approximately -2 mV in control to -11, -12, and -16 mV by TOL, TCE, and PERC, respectively. Similarly, VOCs shifted the half-maximal voltage of steady-state inactivation (V1/2, inact) from approximately -16 mV in control to -32, -35, and -20 mV in the presence of TOL, TCE, and PERC, respectively. Inhibition of ICa by TOL was confirmed in primary cultures of cortical neurons, where 827 microM TOL inhibited current by 61%. These data demonstrate that VOCs perturb voltage-sensitive Ca2+ channel function in neurons, an effect that could contribute to the acute neurotoxicity of these compounds.