Seeing the Light

Effect of tertiary amine local anesthetics on G protein-coupled receptor lateral diffusion and actin cytoskeletal reorganization

Although widely used clinically, the mechanism underlying the action of local anesthetics remains elusive. Direct interaction of anesthetics with membrane proteins and modulation of membrane physical properties by anesthetics are plausible mechanisms proposed, although a combination of these two mechanisms cannot be ruled out. In this context, the role of G protein-coupled receptors (GPCRs) in local anesthetic action is a relatively new area of research. We show here that representative tertiary amine local anesthetics induce a reduction in two-dimensional diffusion coefficient of the serotonin_1A receptor, an important neurotransmitter GPCR. The corresponding change in mobile fraction is varied, with tetracaine exhibiting the maximum reduction in mobile fraction, whereas the change in mobile fraction for other local anesthetics was not appreciable. These results are supported by quantitation of cellular F-actin, using a confocal microscopic approach previously developed by us, which showed that a pronounced increase in F-actin level was induced by tetracaine. These results provide a novel perspective on the action of local anesthetics in terms of GPCR lateral diffusion and actin cytoskeleton reorganization.

Evaluation of the Inherent Toxicity Concept in Environmental Toxicology and Risk Assessment

Intrinsic/inherent chemical properties are characteristic, irrespective of the number of molecules present. However, toxicity is an extensive/extrinsic biochemical property that depends on the number of molecules. Paracelsus, often considered the father of toxicology, noted that all things are poisonous. Because dose magnitude (i.e., number of molecules) determines the occurrence of poisonous effects, toxicity cannot be an intrinsic/inherent biochemical property. Thus, toxicology's task is to determine case-specific risks resulting in adverse effects produced by the interaction of toxic doses/exposures, toxic mechanisms, and case-specific influencing factors. Experimental testing results are known to vary within and between chemicals, test organisms, and experimental conditions and repetitions; however, hazard-based approaches treat toxicity as a fixed and constant property. A logical alternative is the standard-risk, case-specific risk model. In this approach, testing data are defined as standard risks where the nature, magnitude, and toxicity effect is standardized to the organism, chemical, and test conditions. Interpolation/extrapolation of standard risks to site-specific conditions (i.e., case-specific risks) is challenging, requiring understanding of the influences of the complex interactions within and between differing species, conditions, and toxicity-modifying factors. Therefore, Paracelsus's paradigm is perhaps better abbreviated as "dose-causality-response", because a key interpretive requirement is establishing toxicity causality by separating mode/mechanism of toxic action from modifying factor influences in overall toxicity responses. Unfortunately, the current knowledge base is inadequate. Moving to a standard-risk-specific-risk paradigm would highlight the importance of improving the toxicity causality knowledge base. Thereby, a rationale would be provided for enhancing the design and interpretation of toxicity testing that is necessary for achieving advances in routine translation of standard-risk to specific-risk estimates-the raison d'être of regulatory risk decision making. Environ Toxicol Chem 2020;39:2351-2360. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Carbonic anhydrase enzymes: Likely targets for inhalational anesthetics

Inhalational anesthetics such as isoflurane, desflurane and halothane are the mainstay medications for surgical procedures; upon inhalation, they produce anesthesia described as reversible unconsciousness with the features of amnesia, sleep, immobility and analgesia. To date, how they produce anesthesia is unknown. This study proposes that carbonic anhydrase enzymes are likely targets mediating the actions of inhalational anesthetics. Carbonic anhydrase enzymes, commonly expressed in living organisms, utilize carbon dioxide (CO₂) as a substrate and can generate H⁺ and HCO₃^- from CO₂ with a great efficiency. There are remarkable lines of evidence for their likely roles in mediating anesthetic actions. Firstly, carbonic anhydrase enzymes are extensively expressed in the brain and spinal cord, and their importance in the brain activity, especially for the GABA and NMDA receptor signaling pathways, has been demonstrated in numerous studies. According to these studies, they provide HCO₃^- for GABA-A receptor activities and also buffer HCO₃^- excess resulting from NMDA receptor activation. Activation of GABA-A and inhibition of NMDA receptors are associated with the induction of anesthesia by the intravenous general anesthetics propofol and ketamine, respectively. Secondly, the carbonic anhydrase inhibitors topiramate and zonisamide are effectively used in the treatment of epilepsy for decades; their chronic use results in the requirement of increased levels of amobarbital in order to produce anesthesia in the epileptic patients during WADA test. In addition, given that CO₂ is a substrate for these enzymes, their tertiary structure is likely has a hydrophobic pocket suitable for the anesthetic molecules to bind. Inhalational anesthetic molecules, which are lipophilic and inert in nature, have an ability to cross the membranes and inhibit carbonic anhydrases, which might not be accessible by topiramate and zonisamide. Unlike carbonic anhydrase inhibitors, they could bind to the hydrophobic pocket for CO₂ molecules and produce a profound effect called anesthesia. Finally, there is a great deal of similarities between the physiological actions of inhalational anesthetics and carbonic anhydrase inhibitors; moreover well-known side effects of inhalational anesthetics could be associated with the inhibition of carbonic anhydrases. Therefore, this article presents a hypothesis that the anesthetic actions of inhalational anesthetics could be due to their inhibitory effects on the carbonic anhydrases. Investigating this hypothesis might lead to the development of new safer anesthetics, and more importantly it might reveal an endogenous anesthetic pathway, in which the carbonic anhydrase system is a component along with the GABA-A and NMDA receptor systems.

Sevoflurane inhibits presynaptic calcium influx without affecting presynaptic action potentials in hippocampal CA1 region

Although diverse effects of volatile anesthetics have been investigated in various studies, the mechanisms of action of such anesthetics, especially sevoflurane, remain elusive. In contrast to their potent modulation of inhibitory synaptic transmission there is little information about their effects on excitatory transmission in the brain. In this study, we examined the effect of sevoflurane on the excitatory synaptic transmission at CA1 synapses in hippocampal slices of mice. Sevoflurane at 5% was mixed with 95% O₂ and 5% CO₂ and bubbled in artificial cerebral spinal fluid (0.69 mM). Extracellular recordings of the field excitatory postsynaptic potential (fEPSP) and presynaptic fiber volley (FV) were made at physiological temperature. In addition, fluorescent measurements of presynaptic Ca²⁺ transients were performed while simultaneously recording fEPSP. Application of sevoflurane reduced the amplitude of fEPSP (45 ± 8%, n = 5). This effect was accompanied by concurrent enhancement of the paired-pulse facilitation of fEPSP (127 ± 5%, n = 12), suggesting a possible presynaptic site of action of sevoflurane. The amplitude of FV was not significantly affected (102 ± 5%, n = 5). In contrast, fluorescent measurements revealed that presynaptic Ca²⁺ influx was suppressed by sevoflurane (69 ± 5%, n = 7), as was simultaneously recorded fEPSP (44 ± 5%, n = 7). Our results suggest that sevoflurane potently suppresses excitatory synaptic transmission via inhibition of presynaptic Ca²⁺ influx without affecting presynaptic action potentials.

Organic solvent-induced changes in membrane geometry in human SH-SY5Y neuroblastoma cells - a common narcotic effect?

Exposure to organic solvents may cause narcotic effects. At the cellular level, these narcotic effects have been associated with a reduction in neuronal excitability caused by changes in membrane structure and function. In order to critically test whether changes in membrane geometry contribute to these narcotic effects, cultured human SH-SY5Y neuroblastoma cells have been exposed to selected organic solvents. The solvent-induced changes in cell membrane capacitance were investigated using the whole-cell patch clamp technique for real-time capacitance measurements. Exposure of SH-SY5Y cells to the cyclic hydrocarbons m-xylene, toluene, and cyclohexane caused a rapid and reversible increase of membrane capacitance. The aliphatic, nonpolar n-hexane did not cause a detectable change of whole-cell membrane capacitance, whereas the amphiphiles n-hexanol and n-hexylamine caused an increase of membrane capacitance and a concomitant reduction in membrane resistance. Despite a large difference in dielectric properties, the chlorinated hydrocarbons 1,1,2,2-tetrachoroethane and tetrachloroethylene caused a similar magnitude increase in membrane capacitance. The theory on membrane capacitance has been applied to deduce changes in membrane geometry caused by solvent partitioning. Although classical observations have shown that solvents increase the membrane capacitance per unit area of membrane, i.e., increase membrane thickness, the present results demonstrate that solvent partitioning predominantly leads to an increase in membrane surface area and to a lesser degree to an increase in membrane thickness. Moreover, the present results indicate that the physicochemical properties of each solvent are important determinants for its specific effects on membrane geometry. This implies that the hypothesis that solvent partitioning is associated with a common perturbation of membrane structure needs to be revisited and cannot account for the commonly observed narcotic effects of different organic solvents.

Changes in plasma orexin-A levels in sevoflurane-remifentanil anesthesia in young and elderly patients undergoing elective lumbar surgery

BACKGROUND

Delayed emergence from general anesthesia frequently occurs in elderly patients, but the reason is not clear. Orexin has been shown to be involved in arousal from general anesthesia. In this study, we examined plasma orexin-A levels in both elderly and young patients during the anesthesia arousal cycle.

METHODS

We recruited 41 patients scheduled for elective lumbar surgery and eventually evaluated 34 patients. Patients were divided into a young group (age 30-55, N = 16) and an elderly group (age 65-77, N = 18). Anesthesia with sevoflurane-remifentanil was titrated to maintain the Bispectral Index between 45 and 65. The times from stopping anesthesia to eyes opening and extubation were recorded. Arterial blood was collected, and plasma orexin-A was determined by radioimmunoassay at the following 4 time points: preanesthesia (T0), 1 hour after anesthesia induction (T1), emergence (5 minutes after tracheal extubation) (T2), and 30 minutes after tracheal extubation (T3).

RESULTS

The times from stopping anesthesia to eyes opening and tracheal extubation were both significantly longer in the elderly group than in the young group (P = 0.004, P = 0.01, respectively). Basal (T0) orexin-A levels were higher in the elderly group than in the young group (T0, 26.13 ± 1.25 vs 17.9 ± 1.30 pg/mL, P < 0.0001). Plasma orexin-A levels did not change during induction of anesthesia in either group but significantly increased at T2 (vs T0, P <0.0001) in both elderly (35.0 ± 1.7 pg/mL) and young (29.2 ± 1.9 pg/mL) groups. Orexin-A levels were significantly higher in the elderly than in the young group at T1, T2, and T3.

CONCLUSION

Plasma orexin-A levels are not responsible for the delayed emergence from general anesthesia in elderly patients.

Cerebral mechanisms of general anesthesia

How does general anesthesia (GA) work? Anesthetics are pharmacological agents that target specific central nervous system receptors. Once they bind to their brain receptors, anesthetics modulate remote brain areas and end up interfering with global neuronal networks, leading to a controlled and reversible loss of consciousness. This remarkable manipulation of consciousness allows millions of people every year to undergo surgery safely most of the time. However, despite all the progress that has been made, we still lack a clear and comprehensive insight into the specific neurophysiological mechanisms of GA, from the molecular level to the global brain propagation. During the last decade, the exponential progress in neuroscience and neuro-imaging led to a significant step in the understanding of the neural correlates of consciousness, with direct consequences for clinical anesthesia. Far from shutting down all brain activity, anesthetics lead to a shift in the brain state to a distinct, highly specific and complex state, which is being increasingly characterized by modern neuro-imaging techniques. There are several clinical consequences and challenges that are arising from the current efforts to dissect GA mechanisms: the improvement of anesthetic depth monitoring, the characterization and avoidance of intra-operative awareness and post-anesthesia cognitive disorders, and the development of future generations of anesthetics.

The neurobiology of alcohol consumption and alcoholism: an integrative history

Studies of the neurobiological predisposition to consume alcohol (ethanol) and to transition to uncontrolled drinking behavior (alcoholism), as well as studies of the effects of alcohol on brain function, started a logarithmic growth phase after the repeal of the 18th Amendment to the United States Constitution. Although the early studies were primitive by current technological standards, they clearly demonstrated the effects of alcohol on brain structure and function, and by the end of the 20th century left little doubt that alcoholism is a "disease" of the brain. This review traces the history of developments in the understanding of ethanol's effects on the most prominent inhibitory and excitatory systems of brain (GABA and glutamate neurotransmission). This neurobiological information is integrated with knowledge of ethanol's actions on other neurotransmitter systems to produce an anatomical and functional map of ethanol's properties. Our intent is limited in scope, but is meant to provide context and integration of the actions of ethanol on the major neurobiologic systems which produce reinforcement for alcohol consumption and changes in brain chemistry that lead to addiction. The developmental history of neurobehavioral theories of the transition from alcohol drinking to alcohol addiction is presented and juxtaposed to the neurobiological findings. Depending on one's point of view, we may, at this point in history, know more, or less, than we think we know about the neurobiology of alcoholism.

Evaluation of critical body residue data for acute narcosis in aquatic organisms

The Environmental Residue Effects Database was evaluated to identify critical body residues of organic chemicals causing acute baseline neutral narcosis in aquatic organisms. Over 15 000 records for >400 chemicals were evaluated. Mean molar critical body residues in the final data set of 161 records for 29 chemicals were within published ranges but varied within and among chemicals and species (~3 orders of magnitude), and lipid normalization did not consistently decrease variability. All 29 chemicals can act as baseline neutral narcotics, but chemicals and/or their metabolites may also act by nonnarcotic modes of action. Specifically, nonnarcotic toxicity of polycyclic aromatic hydrocarbons and/or their biotransformation derivatives may be a significant source of variability. Complete testing of the narcosis-critical body residue hypothesis was confounded by data gaps for key toxicity modifying factors such as metabolite formation/toxicity, lipid content/composition, other modes of toxic action, and lack of steady-state status. Such problems impede determination of the precise, accurate toxicity estimates necessary for sound toxicological comparisons. Thus, neither the data nor the chemicals in the final data set should be considered definitive. Changes to testing designs and methods are necessary to improve data collection and critical body residue interpretation for hazard and risk assessment. Each of the toxicity metrics discussed-wet weight and lipid weight critical body residues, volume fraction in organism lipid, and chemical activity-has advantages, but all are subject to the same toxicity modifying factors.

The size of the unbranched aliphatic chain determines the immunomodulatory potency of short and long chain n-alkanols

Aliphatic n-alkanols are a family of ubiquitous substances that display general anesthetic properties in accordance to their degree of hydrophobicity. In addition, the immunomodulatory activity of one of its members, ethanol, has long been recognized. We reasoned that because unbranched aliphatic n-alkanols are structurally very similar they might have an immunological impact that mirrors their anesthetic potency. We report the impact of the homologous C1-C12 alcohol series on the ability of activated primary human lymphocytes to produce IFN-γ. Methanol enhanced IFN-γ production whereas C2-C10 alcohols reduced the release of this cytokine. The activity of the n-alkanol series was observed within a wide concentration window ranging from millimolar levels for short chain alcohols to micromolar amounts for C7-C10 alcohols. There was a clear correlation between immunomodulatory activity and hydrophobicity of the compounds, but a cutoff effect was evident at C11. n-Alkanols were shown to act downstream of the cell membrane because T cell receptor early signaling was preserved. The activation of the nuclear factor of activated T cells (NFAT) was down-regulated progressively in accordance to the size of the n-alkanol aliphatic chains with a clear downward trend that was interrupted at C11. The nuclear factor-κB (NF-κB) signaling was also compromised, but the cutoff appeared earlier at C10. The pattern of immunomodulation and transcriptional dysregulation induced by the n-alkanol series suggested the existence of interaction pockets of defined dimensions within intracellular targets that compromise the activation of NFAT and NF-κB transcription factors and ultimately modulate the effector function of the T lymphocyte.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Binding site and affinity prediction of general anesthetics to protein targets using docking

BACKGROUND

The protein targets for general anesthetics remain unclear. A tool to predict anesthetic binding for potential binding targets is needed. In this study, we explored whether a computational method, AutoDock, could serve as such a tool.

METHODS

High-resolution crystal data of water-soluble proteins (cytochrome C, apoferritin, and human serum albumin), and a membrane protein (a pentameric ligand-gated ion channel from Gloeobacter violaceus [GLIC]) were used. Isothermal titration calorimetry (ITC) experiments were performed to determine anesthetic affinity in solution conditions for apoferritin. Docking calculations were performed using DockingServer with the Lamarckian genetic algorithm and the Solis and Wets local search method (http://www.dockingserver.com/web). Twenty general anesthetics were docked into apoferritin. The predicted binding constants were compared with those obtained from ITC experiments for potential correlations. In the case of apoferritin, details of the binding site and their interactions were compared with recent cocrystallization data. Docking calculations for 6 general anesthetics currently used in clinical settings (isoflurane, sevoflurane, desflurane, halothane, propofol, and etomidate) with known 50% effective concentration (EC(50)) values were also performed in all tested proteins. The binding constants derived from docking experiments were compared with known EC(50) values and octanol/water partition coefficients for the 6 general anesthetics.

RESULTS

All 20 general anesthetics docked unambiguously into the anesthetic binding site identified in the crystal structure of apoferritin. The binding constants for 20 anesthetics obtained from the docking calculations correlate significantly with those obtained from ITC experiments (P = 0.04). In the case of GLIC, the identified anesthetic binding sites in the crystal structure are among the docking predicted binding sites, but not the top ranked site. Docking calculations suggest a most probable binding site located in the extracellular domain of GLIC. The predicted affinities correlated significantly with the known EC(50) values for the 6 frequently used anesthetics in GLIC for the site identified in the experimental crystal data (P = 0.006). However, predicted affinities in apoferritin, human serum albumin, and cytochrome C did not correlate with these 6 anesthetics' known experimental EC(50) values. A weak correlation between the predicted affinities and the octanol/water partition coefficients was observed for the sites in GLIC.

CONCLUSION

We demonstrated that anesthetic binding sites and relative affinities can be predicted using docking calculations in an automatic docking server (AutoDock) for both water-soluble and membrane proteins. Correlation of predicted affinity and EC(50) for 6 frequently used general anesthetics was only observed in GLIC, a member of a protein family relevant to anesthetic mechanism.

Short-term immunological effects of non-ethanolic short-chain alcohols

Short-chain alcohols are embedded into several aspects of modern life. The societal costs emanating from the long history of use and abuse of the prototypical example of these molecules, ethanol, have stimulated considerable interest in its general toxicology. A much more modest picture exists for other short-chain alcohols, notably as regards their immunotoxicity. A large segment of the general population is potentially exposed to two of these alcohols, methanol and isopropanol. Their ubiquitous nature and their eventual use as ethanol surrogates are predictably associated to accidental or deliberate poisoning. This review addresses the immunological consequences of acute exposure to methanol and isopropanol. It first examines the general mechanisms of short-chain alcohol-induced biological dysregulation and then provides a tentative model to explain the molecular events that underlie the immunological dysfunction produced by methanol and isopropanol. The time-related context of serum alcohol concentrations in acute poisoning, as well as the clinical implications of their short-term immunotoxicity, is also discussed.

The in vivo genotoxicity of cisplatin, isoflurane and halothane evaluated by alkaline comet assay in Swiss albino mice

The aim of this study was to evaluate the genotoxicity of repeated exposure to isoflurane or halothane and compare it with the genotoxicity of repeated exposure to cisplatin. We also determined the genotoxicity of combined treatment with inhalation anaesthetics and cisplatin on peripheral blood leucocytes (PBL), brain, liver and kidney cells of mice. The mice were divided into six groups as follows: control, cisplatin, isoflurane, cisplatin-isoflurane, halothane and cisplatin-halothane, and were exposed respectively for three consecutive days. The mice were treated with cisplatin or exposed to inhalation anaesthetic; the combined groups were exposed to inhalation anaesthetic after treatment with cisplatin. The alkaline comet assay was performed. All drugs had a strong genotoxicity (P<0.05 vs. control group) in all of the observed cells. Isoflurane caused stronger DNA damage on the PBL and kidney cells, in contrast to halothane, which had stronger genotoxicity on brain and liver cells. The combination of cisplatin and isoflurane induced lower genotoxicity on PBL than isoflurane alone (P<0.05). Halothane had the strongest effect on brain cells, but in the combined treatment with cisplatin, the effect decreased to the level of cisplatin alone. Halothane also induced the strongest DNA damage of the liver cells, while the combination with cisplatin increased its genotoxicity even more. The genotoxicity of cisplatin and isoflurane on kidney cells were nearly at the same level, but halothane caused a significantly lower effect. The combinations of inhalation anaesthetics with cisplatin had stronger effects on kidney cells than inhalation anaesthetics alone. The observed drugs and their combinations induced strong genotoxicity on all of the mentioned cells.

Advancing environmental toxicology through chemical dosimetry: external exposures versus tissue residues

The tissue residue dose concept has been used, although in a limited manner, in environmental toxicology for more than 100 y. This review outlines the history of this approach and the technical background for organic chemicals and metals. Although the toxicity of both can be explained in tissue residue terms, the relationship between external exposure concentration, body and/or tissues dose surrogates, and the effective internal dose at the sites of toxic action tends to be more complex for metals. Various issues and current limitations related to research and regulatory applications are also examined. It is clear that the tissue residue approach (TRA) should be an integral component in future efforts to enhance the generation, understanding, and utility of toxicity testing data, both in the laboratory and in the field. To accomplish these goals, several key areas need to be addressed: 1) development of a risk-based interpretive framework linking toxicology and ecology at multiple levels of biological organization and incorporating organism-based dose metrics; 2) a broadly applicable, generally accepted classification scheme for modes/mechanisms of toxic action with explicit consideration of residue information to improve both single chemical and mixture toxicity data interpretation and regulatory risk assessment; 3) toxicity testing protocols updated to ensure collection of adequate residue information, along with toxicokinetics and toxicodynamics information, based on explicitly defined toxicological models accompanied by toxicological model validation; 4) continued development of residue-effect databases is needed ensure their ongoing utility; and 5) regulatory guidance incorporating residue-based testing and interpretation approaches, essential in various jurisdictions.

The role of structured water in mediating general anesthetic action on alpha4beta2 nAChR

Water is an essential component for many biological processes. Pauling proposed that water might play a critical role in general anesthesia by forming water clathrates around anesthetic molecules. To examine potential involvement of water in general anesthesia, we analyzed water within alpha4beta2 nAChR, a putative protein target hypersensitive to volatile anesthetics. Experimental structure-derived closed- and open-channel nAChR systems in a fully hydrated lipid bilayer were examined using all-atom molecular dynamics simulations. At the majority of binding sites in alpha4beta2 nAChR, halothane replaced the slow-exchanging water molecules and caused a regional water population decrease. Only two binding sites had an increased quantity of water in the presence of halothane, where water arrangements resemble clathrate-like structures. The small number of such clathrate-like water clusters suggests that the formation of water clathrates is unlikely to be a primary cause for anesthesia. Despite the decrease in water population at most of the halothane binding sites, the number of sites that can be occupied transiently by water is actually increased in the presence of halothane. Many of these water sites were located between two subunits or in regions containing agonist binding sites or critical structural elements for transducing agonist binding to channel gating. Changes in water sites in the presence of halothane affected water-mediated protein-protein interactions and the protein dynamics, which can have direct impact on protein function. Collectively, water contributes to the action of anesthetics in proteins by mediating interactions between protein subunits and altering protein dynamics, instead of forming water clathrates around anesthetics.

[Propofol-induced myocardial depression: possible role of atrial muscarinic cholinergic receptors]

OBJECTIVE

To investigate the possible role of muscarinic cholinergic receptors (MCRs) in the depression of myocardial function induced by propofol, an intravenous anesthetic chemically unrelated to other drugs. Although adverse effects are rare, bradycardia has been reported and this can lead to cardiac arrest in some patients. The mechanism behind this effect is still unknown but a possible role for MCRs has been suggested.

MATERIAL AND METHODS

The interaction of propofol with human atrial MCRs was determined by means of inhibition tests using [3H] quinuclidinyl benzilate ([3H] QNB).

RESULTS

The displacement of [3H] QNB binding to human atrial MCRs by propofol was concentration dependent but the observed effect was not consistent with a model of simple competition between propofol and [3H] QNB.

CONCLUSION

Propofol appears to have the ability to modify the activity of human atrial MCRs and this effect may be related to its ability to induce bradycardia.

n-Alcohols inhibit voltage-gated Na+ channels expressed in Xenopus oocytes

Voltage-gated sodium channels are essential for the initiation and propagation of action potentials in excitable cells and are known as a target of local anesthetics. In addition, inhibition of sodium channels by volatile anesthetics has been proposed as a mechanism of general anesthesia. The n-alcohols produce anesthesia, and their potency increases with carbon number until a "cut-off" is reached. In this study, we examined effects of a range of n-alcohols on Na(v)1.2 subunits to determine the alcohol cut-off for this channel. We also studied the effect of a short-chain alcohol (ethanol) and a long-chain alcohol (octanol) on Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.8 subunits, and we investigated the effects of alcohol on channel kinetics. Ethanol and octanol inhibited sodium currents of all subunits, and the inhibition of the Na(v)1.2 channel by n-alcohols indicated a cut-off at nonanol. Ethanol and octanol produced open-channel block, which was more pronounced for Na(v)1.8 than for the other sodium channels. Inhibition of Na(v)1.2 was due to decreased activation and increased inactivation. These results suggest that sodium channels may have a hydrophobic binding site for n-alcohols and demonstrate the differences in the kinetic mechanisms of inhibition for n-alcohols and inhaled anesthetics.

Dynamics of firefly luciferase inhibition by general anesthetics: Gaussian and anisotropic network analyses

The new crystal structures of the product-bound firefly luciferase combined with the previously determined substrate-free structures allow for a detailed analysis of the dynamics basis for the luciferase enzymatic activities. Using the Gaussian network model and the anisotropic network model, we show here that the superposition of the three slowest anisotropic network model modes, consisting of the bending, rotating, and rocking motions of the C-domain, accounts for large rearrangement of domains from the substrate-free (open) to product-bound (closed) conformation and thus constitutes a critical component of the enzyme's functions. The analysis also offers a unique platform to reexamine the molecular mechanism of the anesthetic inhibition of the firefly luciferase. Through perturbing the protein backbone network by introducing additional nodes to represent anesthetics, we found that the presence of two representative anesthetics, halothane and n-decanol, in different regions of luciferase had distinctively different effects on the protein's global motion. Only at the interface of the C- and N-domains did the anesthetics cause the most profound reduction in the overall flexibility of the C-domain and the concomitant increase in the flexibility of the loop, where the substitution of a conserved lysine residue was found experimentally to lead to >2-3 orders of magnitude reduction in activity. These anesthetic-induced dynamics changes can alter the normal function of the protein, appearing as an epiphenomenon of an "inhibition". The implication of the study is that a leading element for general anesthetic action on proteins is to disrupt the modes of motion essential to protein functions.

Anesthetic interaction with ketosteroid isomerase: insights from molecular dynamics simulations

The nature and the sites of interactions between anesthetic halothane and homodimeric Delta5-3-ketosteroid isomerase (KSI) are characterized by flexible ligand docking and confirmed by 1H-15N NMR. The dynamics consequence of halothane interaction and the implication of the dynamic changes to KSI function are studied by multiple 5-ns molecular dynamics simulations in the presence and absence of halothane. Both docking and MD simulations show that halothane prefer the amphiphilic dimeric interface to the hydrophobic active site of KSI. Halothane occupancy at the dimer interface disrupted the intersubunit hydrogen bonding formed either directly through side chains of polar residues or indirectly through the mediation of the interfacial water molecules. Moreover, in the presence of halothane, the exchange rate of the bound waters with bulk water was increased. Halothane perturbation to the dimer interface affected the overall flexibility of the active site. This action is likely to contribute to the halothane-induced reduction of the KSI activity. The allosteric halothane modulation of the dynamics-function relationship of KSI without direct competition at the enzymatic active sites may be generalized to offer a unifying explanation of anesthetic action on a diverse range of multidomain neuronal proteins that are potentially relevant to clinical general anesthesia.

Volatile organic compounds inhibit human and rat neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes

The relative sensitivity of rats and humans to volatile organic compounds (VOCs) such as toluene (TOL) and perchloroethylene (PERC) is unknown and adds to uncertainty in assessing risks for human exposures to VOCs. Recent studies have suggested that ion channels, including nicotinic acetylcholine receptors (nAChRs), are targets of TOL effects. However, studies comparing TOL effects on human and rat ligand-gated ion channels have not been conducted. To examine potential toxicodynamic differences between these species, the sensitivity of human and rat nAChRs to TOL was assessed. Since PERC has similar effects, in vivo, to TOL, effects of PERC on nAChR function were also examined. Two-electrode voltage-clamp techniques were utilized to measure acetylcholine-induced currents in neuronal nAChRs (alpha4beta2, alpha3beta2, and alpha7) expressed in Xenopus oocytes. PERC (0.065 mM) inhibited alpha7 nAChR currents by 60.1 +/- 4.0% (human, n = 7) and 40 +/- 3.5% (rat, n = 5), and inhibited alpha4beta2 nAChR currents by 42.0 +/- 5.2% (human, n = 6) and 52.2 +/- 5.5% (rat, n = 8). Likewise, alpha3beta2 nAChRs were significantly inhibited by 62.2 +/- 3.8% (human, n = 7) and 62.4 +/- 4.3% (rat, n = 8) in the presence of 0.065 mM PERC. TOL also inhibited both rat and human alpha7, alpha4beta2, and alpha3beta2 nAChRs. Statistical analysis indicated that although there was not a species (human vs. rat) difference with PERC (0.0015-0.065 mM) or TOL (0.03-0.9 mM) inhibition of alpha7, alpha4beta2, or alpha3beta2 nAChRs, all receptor types were more sensitive to PERC than TOL. These results demonstrate that human and rat nACh receptors represent a sensitive target for VOCs. This toxicodynamic information will help decrease the uncertainty associated with animal to human extrapolations in the risk assessment of VOCs.