The partial molar volumes of anesthetics in lipid bilayers

An In Silico Investigation of the Molecular Interactions between Volatile Anesthetics and Actin

Volatile anesthetics (VAs) are medicinal chemistry compounds commonly used to enable surgical procedures for patients who undergo painful treatments and can be partially or fully sedated, remaining in an unconscious state during the operation. The specific molecular mechanism of anesthesia is still an open issue, but scientific evidence supports the hypothesis of the involvement of both putative hydrophobic cavities in membrane receptors as binding pockets and interactions between anesthetics and cytoplasmic proteins. Previous studies demonstrated the binding of VAs to tubulin. Since actin is the other major component of the cytoskeleton, this study involves an investigation of its interactions with four major anesthetics: halothane, isoflurane, sevoflurane, and desflurane. Molecular docking was implemented using the Molecular Operating Environment (MOE) software (version 2022.02) and applied to a G-actin monomer, extrapolating the relative binding affinities and root-mean-square deviation (RMSD) values. A comparison with the F-actin was also made to assess if the generally accepted idea about the enhanced F-to-G-actin transformation during anesthesia is warranted. Overall, our results confirm the solvent-like behavior of anesthetics, as evidenced by Van der Waals interactions as well as the relevant hydrogen bonds formed in the case of isoflurane and sevoflurane. Also, a comparison of the interactions of anesthetics with tubulin was made. Finally, the short- and long-term effects of anesthetics are discussed for their possible impact on the occurrence of mental disorders.

A potent photoreactive general anesthetic with novel binding site selectivity for GABAA receptors

The pentameric γ-aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory ligand-gated ion channels in the central nervous system. They mediate diverse physiological functions, mutations in them are associated with mental disorders and they are the target of many drugs such as general anesthetics, anxiolytics and anti-convulsants. The five subunits of synaptic GABA_ARs are arranged around a central pore in the order β-α-β-α-γ. In the outer third of the transmembrane domain (TMD) drugs may bind to five homologous intersubunit binding sites. Etomidate binds between the pair of β - α subunit interfaces (designated as β⁺/α^-) and R-mTFD-MPAB binds to an α⁺/β^- and an γ⁺/β^- subunit interface (a β^- selective ligand). Ligands that bind selectively to other homologous sites have not been characterized. We have synthesized a novel photolabel, (2,6-diisopropyl-4-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenyl)methanol or pTFD-di-iPr-BnOH). It is a potent general anesthetic that positively modulates agonist and benzodiazepine binding. It enhances GABA-induced currents, shifting the GABA concentration-response curve to lower concentrations. Photolabeling-protection studies show that it has negligible affinity for the etomidate sites and high affinity for only one of the two R-mTFD-MPAB sites. Exploratory site-directed mutagenesis studies confirm the latter conclusions and hint that pTFD-di-iPr-BnOH may bind between the α⁺/β^- and α⁺/γ^- subunits in the TMD, making it an α⁺ ligand. The latter α⁺/γ^- site has not previously been implicated in ligand binding. Thus, pTFD-di-iPr-BnOH is a promising new photolabel that may open up a new pharmacology for synaptic GABA_ARs.

Ligand partitioning into lipid bilayer membranes under high pressure: Implication of variation in phase-transition temperatures

The variation in phase-transition temperatures of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane by adding two membrane-active ligands, a long-chain fatty acid (palmitic acid (PA)) and an inhalation anesthetic (halothane (HAL)), was investigated by light-transmittance measurements and fluorometry. By assuming the thermodynamic colligative property for the bilayer membrane at low ligand concentrations, the partitioning behavior of these ligands into the DPPC bilayer membrane was considered. It was proved from the differential partition coefficients between two phases that PA has strong affinity with the gel (lamellar gel) phase in a micro-molal concentration range and makes the bilayer membrane more ordered, while HAL has strong affinity with the liquid crystalline phase in a milli-molal concentration range and does the bilayer membrane more disordered. The transfer volumes of both ligands from the aqueous solution to each phase of the DPPC bilayer membrane showed that the preferential partitioning of the PA molecule into the gel (lamellar gel) produces about 20% decrease in transfer volume as compared with the liquid crystalline phase, whereas that of the HAL molecule into the liquid crystalline phase does about twice increase in transfer volume as compared with the gel (ripple gel) phase. Furthermore, changes in thermotropic and barotropic phase behavior of the DPPC bilayer membrane by adding the ligand was discussed from the viewpoint of the ligand partitioning. Reflecting the contrastive partitioning of PA and HAL into the pressure-induced interdigitated gel phase among the gel phases, it was revealed that PA suppresses the formation of the interdigitated gel phase under high pressure while HAL promotes it. These results clearly indicate that each phase of the DPPC bilayer membrane has a potential to recognize various ligand molecules.

Neural Synchrony, Axonal Path Lengths, and General Anesthesia: A Hypothesis

Despite decades of research, the mechanism by which general anesthetics produce loss of consciousness remains mysterious. A clue may be provided by the evidence that synchronous firing of cortical neurons underlies higher forms of neural processing. In order for these synchrony codes to be precise, transmission time must be independent of path length over all the connected sites between any two cortical areas. Because path lengths vary, developmental mechanisms must compensate for the resulting delay variations. Delay variations could be detected by spike-timing-dependent cues and compensation implemented by systematic changes in axon diameter, myelin thickness, or internodal distance. Anesthetics have been shown to increase conduction velocity in myelinated fibers and may therefore disrupt path-length compensation by changing velocities by different amounts in different types of axon. This simple and testable theory explains why anesthetics interfere selectively with higher cognitive functions but leave those dominated by rate-based firing relatively intact.

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'.

Packing properties of 1-alkanols and alkanes in a phospholipid membrane

We have used vibrating tube densitometry to investigate the packing properties of four alkanes and a homologous series of ten alcohols in fluid-phase membranes of dimyristoyl phosphatidylcholine (DMPC). It was found that the volume change of transferring these compounds from their pure states into the membrane, DeltaV(m)(pure-->mem), was positive for small (C4-C6) 1-alkanols while it was negative for larger alcohols and all alkanes. The magnitude of DeltaV(m)(pure-->mem) ranged from about +4 cm3/mol for alcohols with an alkyl chain about half the length of the fatty acids of DMPC, to -10 to -15 cm3/mol for the alkanes and long chain alcohols. On the basis of these observations, previously published information on the structure of the membrane-solute complexes and the free volume properties of (pure) phospholipid membranes, we suggest that two effects dominate the packing properties of hydrophobic solutes in DMPC. First, perturbation of the tightly packed interfacial zone around the ester bonds and first few methylene groups of DMPC brings about a positive contribution to DeltaV(m)(pure-->mem). This effect dominates the volume behavior for alcohols like 1-butanol, 1-pentanol and 1-hexanol. More hydrophobic solutes penetrate into the membrane core, which is loosely packed. In this region, they partially occupy interstitial (or free-) volume, which bring about a denser molecular packing and generate a negative contribution to DeltaV(m)(pure-->mem).

Distribution of halothane in a dipalmitoylphosphatidylcholine bilayer from molecular dynamics calculations

We report a 2-ns constant pressure molecular dynamics simulation of halothane, at a mol fraction of 50%, in the hydrated liquid crystal bilayer phase of dipalmitoylphosphatidylcholine. Halothane molecules are found to preferentially segregate to the upper part of the lipid acyl chains, with a maximum probability near the C(5) methylene groups. However, a finite probability is also observed along the tail region and across the methyl trough. Over 95% of the halothane molecules are located below the lipid carbonyl carbons, in agreement with photolabeling experiments. Halothane induces lateral expansion and a concomitant contraction in the bilayer thickness. A decrease in the acyl chain segment order parameters, S(CD), for the tail portion, and a slight increase for the upper portion compared to neat bilayers, are in agreement with several NMR studies on related systems. The decrease in S(CD) is attributed to a larger accessible volume per lipid in the tail region. Significant changes in the electric properties of the lipid bilayer result from the structural changes, which include a shift and broadening of the choline headgroup dipole (P-N) orientation distribution. Our findings reconcile apparent controversial conclusions from experiments on diverse lipid systems.

Thermodynamics of alcohol-lipid bilayer interactions: application of a binding model

Several recent reports have provided evidence that interactions of small alcohols with lipid bilayer membranes are dominated by adsorption to the membrane-water interface. This mode of interaction is better modeled by binding models than solution theories. In the present study, alcohol-membrane interactions are examined by applying the 'solvent exchange model' [J.A. Schellmann, Biophys. Chem. 37 (1990) 121] to calorimetric measurements. Binding constants (in mole fraction units) for small alcohols to unilamellar liposomes of dimyristoyl phosphatidylcholine were found to be close to unity, and in contrast to partitioning coefficients they decrease through the sequence ethanol, 1-propanol, 1-butanol. Thus, the direct (intrinsic) affinity of the bilayer for these alcohols is lower the longer the acyl chain. A distinction between binding and partitioning is discussed, and it is demonstrated that a high concentration of solute in the bilayer (large partitioning coefficients) can be obtained even in cases of weak binding. Other results from the model suggest that the number of binding sites on the lipid bilayer interface is 1-3 times the number of lipid molecules and that the binding is endothermic with an enthalpy change of 10-15 kJ/mol. Close to the main phase transition of the lipid bilayer the results suggest the presence of two distinct classes of binding sites: 'normal' sites similar to those observed at higher temperatures, and a lower number of high-affinity sites with binding constants larger by one or two orders of magnitude. The occurrence of high-affinity sites is discussed with respect to fluctuating gel and fluid domains in bilayer membranes close to the main phase transition.

The internal critical level concept of nonspecific toxicity

The internal lethal concentration (ILC) can be an effective approach in describing the toxicity of a chemical to aquatic organisms that can complement the use of the external toxic concentration characteristic of the LC50. The ILC is an estimate of the toxicant concentration close to the target site and can be estimated from bioconcentration relationships and acute toxicity data. The observed ILC values were found to be consistent for organic compounds exerting the same mode of toxic action. The nonspecific toxicants have the lowest toxicity and the highest ILC values, whereas the chemicals exhibiting specific modes of action have lower concentrations and higher toxicity. There are some reports that the ILC value decreases with increasing exposure periods for various organic chemicals with aquatic organisms. The nonspecific toxicants possibly exhibit their toxic action at the target site by at least two different mechanisms depending on the toxicant concentrations. First, the toxicants bind directly to membrane proteins at relatively low concentrations, resulting in reversible toxic effect. Second, the toxicants inhibit the membrane proteins, and alterations in the lipid bilayers occur at toxicant concentrations sufficient to produce mortality of the organisms. The nonspecific toxicity expressed as acute and chronic toxicity measures are found to correlate well with log Kow. However, the relationship between the ILC and log Kow is less satisfactory because the values of ILC are relatively consistent compared to those of LC50.

The effects of temperature and pressure on the thermodynamic activity of anesthetics

Anesthetic potency is often expressed by volume percent or partial pressure in the gas phase, concentrations in the aqueous phase, etc. However. these values do not represent the anesthetic activity at the action sites. Because the activity at the action sites is difficult to obtain. Ferguson (Ferguson, J., 1939. Proc. R. Soc. Lond. B 127 387-404) defined the thermodynamic activity, which is the ratio between the anesthetizing partial pressure and the vapor pressure of the pure anesthetic at the same temperature. This paper discusses the effects of temperature and pressure on the thermodynamic activity of anesthetics. It also discusses the limitations of the Meyer-Overton rule.

Actions of general anaesthetics on a neuronal nicotinic acetylcholine receptor in isolated identified neurones of Lymnaea stagnalis

1. Completely isolated identified neurones from the right parietal ganglion of the pond snail Lymnaea stagnalis were studied under two-electrode voltage-clamp. Neuronal nicotinic acetylcholine receptor currents were studied at low acetylcholine (ACh) concentrations (< or = 200 nM). At these levels, control currents were non-desensitizing and proportional to the square of the ACh concentration. 2. IC50 concentrations were determined for the steady-state inhibition of the ACh-activated current by 31 general anaesthetics plus the non-anaesthetic alcohol n-tridecanol. The general anaesthetics included inhalational agents, n-alcohols, n-alkane-(alpha,omega)-diols, cycloalcohols and an n-alkane. 3. Anaesthetic inhibition was independent of voltage and consistent with two anaesthetic-binding sites on the receptor. 4. IC50 concentrations for inhibiting the neuronal nicotinic ACh receptor correlated well (r = 0.97) with EC50 concentrations for general anaesthesia. The maximum deviation from the line of identity was less than fourfold. The inhalational agents tended to be more potent as inhibitors of the ACh receptor than as general anaesthetics, while the alcohols and diols were less potent. 5. The inhibition of the ACh-induced current by the homologous series of n-alcohols exhibited a cutoff at the same position (just after dodecanol) as found for the induction of general anaesthesia in tadpoles. 6. Polarity profile maps of the anaesthetic-binding sites on the neuronal nicotinic ACh receptor were calculated from IC50 concentrations for the homologous series of n-alcohols and n-alkane-(alpha,omega)-diols. They reveal amphiphilic sites with apolar regions capable of accommodating the hydrocarbon chains of n-alcohols as large as decanol. A striking resemblance was found to profiles previously calculated from data for tadpole general anaesthesia.

Molecular dynamics simulation of anesthetic-phospholipid bilayer interactions

To probe the hypothesis of a lipid-mediated mechanism of general anesthetic action on a molecular level, and to help elucidate the nature of the interactions of bioactive compounds with membranes, the effects of trichloroethylene (TCE), an inhalational general anesthetic, on a dioleoylphosphatidylcholine (DOPC) lipid bilayer have been investigated by molecular dynamics (MD) simulations at 37 degrees C and 1 atm and the results compared with 31P and 2H NMR experimental studies (Ref 1). The model used included a single TCE molecule embedded in a lipid bilayer consisting of 24 DOPC molecules and an 8 A layer of explicit water of solvation in each polar head group region of the bilayer, together with constant-pressure periodic boundary conditions in three dimensions. A comparison of the bilayer properties calculated in the presence and absence of the anesthetic led to the detection of three major perturbations of the bilayer caused by the anesthetic at 1 atm: i) an increase in the ratio of the effective areas of hydrocarbon tails and the head group per lipid, predicting the tendency of lipids near the anesthetic site of action to form a hexagonal phase (HII); ii) a slight increase in the frequency of chain dihedral angles found in the gauche conformation; and iii) a significant increase in the lateral mean-square displacement of lipid molecules, an indication of increased lipid lateral diffusion and membrane fluidity. The pressure antagonism of these effects was also studied by MD simulations at pressures of 200 and 400 atm. The study of the pressure reversibility of these effects at 200 and 400 atm indicated that they were partially prevented at 200 atm and essentially blocked at 400 atm, suggesting their probable relevance to the pressure reversal effect seen with general anesthesia. These results may thus provide insights into the interaction between general anesthetics and similar small organic molecules with membranes.

Narcosis due to environmental pollutants in aquatic organisms: residue-based toxicity, mechanisms, and membrane burdens

The well-known correlation between the hydrophobicity of narcotic chemicals and the exposure concentration needed to produce an effect indicates that a lipid phase in the aquatic organism is the most likely target. The molar concentration in aquatic organisms at death is found to be approximately constant for different narcotic chemicals, varying from 2 to 8 mmol/kg organism. Because the proportion of lipid is known, the lethal in vivo membrane burden can be calculated to be 40 to 160 mmol/kg lipid. The exact mechanism underlying narcosis is still unknown. However, disturbance by narcotic chemicals in model membrane systems has been investigated, attention having been paid to disturbance of phospholipids and proteins, and of the interaction between the two groups. Model membrane burdens of different chemicals have been shown to be approximately constant for a particular effect. Different effects are found at different membrane concentrations. In the present review, the toxicity of narcotic chemicals to aquatic organisms is discussed, the possible mechanisms underlying narcosis are reviewed, and a comparison is made between membrane burdens that are lethal in vivo and membrane burdens that cause an effect in in vitro systems.

Halothane's effects on GABA-gated chloride flux in mice selectively bred for sensitivity or resistance to diazepam

The DS (diazepam-sensitive) and DR (diazepam-resistant) lines of mice, selected on the basis of their ataxic response to diazepam, also diverge in the physiologic response of their brain gamma-aminobutyric acidA (GABAA) receptors to benzodiazepines, as indicated by augmentation of GABA-mediated chloride flux. Cross-sensitivity and -resistance to other sedatives known to interact with the GABAA-receptor have also been demonstrated in DS and DR mice. Based on the finding that these mice also show cross-sensitivity and -resistance to obtundation by halothane, we predicted that their GABAA-receptors would also exhibit a differential response to halothane as assayed by an in vitro 36Cl- influx assay using purified brain microvesicles. Consistent with this prediction, therapeutic concentrations of halothane enhanced 1 mumol/l GABA-gated flux with significantly greater potency in DS than in DR mice (halothane EC50 336 +/- 64 mumol/l (S.E.M.) vs. 605 +/- 110 mumol/l, respectively, P = 0.03), but there was no difference in maximal flux enhancement between the two lines (DS 4.7 +/- 0.4 nmol.mg-1 x 3 s-1, vs. DR 4.7 +/- 0.5 nmol.mg-1 x 3 s-1). Halothane (500 mumol/l) also shifted the entire GABA concentration-flux relationship significantly to the left, decreasing the EC50 for GABA in both the DS and DR lines. Importantly, the shift in the GABA concentration-flux response in the presence of halothane was more pronounced in the DS mice (GABA EC50 1.8 +/- 0.4 mumol/l vs. 14.7 +/- 0.9 mumol/l without halothane) than in the DR mice (GABA EC50 4.7 +/- 0.6 mumol/l vs. 14.7 +/- 0.9 mumol/l without halothane).(ABSTRACT TRUNCATED AT 250 WORDS)

High pressure antagonism of alcohol effects on the main phase-transition temperature of phospholipid membranes: biphasic response

The combined effects of high pressure (up to 300 bar) and a homologous series of 1-alkanols (ethanol C2 to 1-tridecanol C13) were studied on the main phase-transition temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. It is known that short-chain alkanols depress and long-chain alkanols elevate the main transition temperature. The crossover from depression to elevation occurs at the carbon-chain length about C10-C12 in DPPC vesicle membranes coinciding with the cutoff chain-length where anesthetic potency suddenly disappears. Alkanols shorter than C8 linearly decreased the transition temperature and high pressure antagonized the temperature depression. Alkanols longer than C10 showed biphasic dose-response curves. High pressure enhanced the biphasic response. In addition, alkanols longer than the cutoff length depressed the transition temperature under high pressure at the low concentration range. These non-anesthetic alkanols may manifest anesthetic potency under high pressure. At higher concentrations, the temperature elevatory effect was accentuated by pressure. This biphasic effect of long-chain alkanols is not related to the 'interdigitation' associated with short-chain alkanols. The increment of the transition temperature by pressure was 0.0242 K bar-1 in the absence of alkanols. The volume change of the transition was estimated to be 27.7 cm3 mol-1. This value stayed constant to the limit of the present study of 300 bar.

High pressure and anesthesia: compressibility, molal volume, and partial molal volume of volatile anesthetics

This study was undertaken to provide volume data of volatile anesthetics under high pressure. Molal volumes of liquid halothane, enflurane, and isoflurane and their partial molal volumes in water and in 1-octanol were determined by high-precision solution densitometry at 25.000 +/- 0.0005 degrees C over the pressure range from ambient to 34.56 MPa (341 atm). The isothermal compressibilities of the pure anesthetics and their isothermal partial molal compressibilities at their infinite dilution in water and in 1-octanol have also been calculated at 0.1013 MPa (1 atm).

Fluidization of brain membranes by A2C does not produce anesthesia and does not augment muscimol-stimulated 36Cl- influx

Intravenous administration of 2-[2-methoxyethoxy]-ethyl 8-[cis-2-n-octylcyclopropyl]-octanoate (A2C) was found to disorder brain membranes but did not produce intoxication or anesthesia in mice. The abilities of A2C and an anesthetic (benzyl alcohol) to inhibit [35S]t-butylbicyclophosphorothionate (TBPS) binding, and modify gamma-aminobutyric acid (GABA) receptor-mediated 36Cl- influx into brain vesicles were then compared. Both of the perturbants inhibited [35S]TBPS binding at the same concentrations at which they reduced membrane order; however, the anesthetic was nearly 4 times more effective in reducing [35S]TBPS binding than was A2C. Muscimol-stimulated 36Cl- uptake was enhanced by benzyl alcohol at a concentration which produced little or no change in membrane order. Concentrations of both A2C and benzyl alcohol which reduced membrane order inhibited muscimol-stimulated 36Cl- influx. Similarly, membrane order and muscimol-activated 36Cl- uptake were reduced in brain vesicles prepared from mice which had received A2C in vivo. The effects of anesthetics on the GABAA receptor-chloride channel complex were analyzed by a two site model of action in which a 'perturbant' site is responsible for decreased 36Cl- uptake; but a distinct 'anesthetic' site is responsible for augmentation of chloride flux and anesthesia.

The pharmacology of simple molecules

The biological effects of simple molecules have traditionally been ascribed to their actions on the lipid portions of biological membranes. However, at the low concentrations of these molecules which induce general anaesthesia or have acute toxic effects in animals, changes in lipid bilayer properties are so small that they are unlikely to be relevant biologically. On the other hand, these molecules do inhibit the activity of a pure lipid-free protein, with ED50 concentrations which are essentially identical to the biological ED50 and LD50 concentrations. Moreover, the well-known but puzzling cutoffs in potencies in homologous series of compounds are also found with this enzyme. The accumulating evidence now suggests that the pharmacological effects of low concentrations of relatively inert agents are best explained in terms of their direct binding to amphiphilic pockets of circumscribed dimensions on proteins.

Applicability of Henry's law to hydrogen, helium, and nitrogen solubilities in water and olive oil at 37 degrees C and pressures up to 300 atmospheres

The solubilities of pure hydrogen, helium, and nitrogen in water and olive oil were measured at 37 degrees C at gas-saturation pressures from 25 to 300 atmospheres. Rigorous thermodynamic criteria were used to assess the applicability of Henry's law to the pressure dependence of the gas solubility in each system. The solubilities of the three gases in water and helium in olive oil followed Henry's law as given by the Krichevsky-Kasarnovsky equation. In contrast, hydrogen and nitrogen in olive oil each attained concentrations high enough to cause significant concentration-dependent variations of the dissolved gas activity coefficient and/or partial molal volume. The consequent deviations from Henry's law were greatest in the nitrogen-oil system, where mole fraction nitrogen solubilities calculated from the Krichevsky-Kasarnovsky equation exceeded measured values by 8, 14, and 23% at 50, 100, and 250 atm, respectively. Incorporation of results into the critical volume model of nitrogen anesthesia, using olive oil as a model of the physiological anesthetic site and literature data for the anesthetic potency of nitrogen in mice breathing high-pressure He-N2-O2 atmospheres, shows that nonideal solution behavior may become important for gases dissolved in physiological hydrophobic regions at biologically active concentrations, even if dissolved gas binding to proteins or other macromolecules is not involved.

Temperature dependence of ethanol depression: linear models in male and female mice

The relationship between body temperature and ethanol sensitivity was studied in male and female mice. Age-matched drug-naive mice of both sexes were injected with 3.6 g/kg ethanol (20% w/v) and placed into a chamber kept at one of 8 designated temperatures from 13 to 36 degrees C. In both sexes, wake-up rectal temperatures were significantly, positively correlated with chamber temperatures and sleep-times and were significantly, negatively correlated with wake-up brain and blood ethanol concentrations. Linear regression analyses indicated that wake-up temperature accounted for up to 71% of the variability in sleep-times and wake-up ethanol concentrations in these mice. Similar relationships were found when the change in body temperature from baseline (delta T) was substituted for wake-up rectal temperature. Adding body weights and baseline temperatures did not improve the predictive ability of linear models based on wake-up rectal temperature alone. The results support the contention that body temperature represents an important determinant of ethanol sensitivity in both sexes. These findings provide additional evidence that ethanol sensitivity varies with body temperature in accordance with membrane perturbation theories of anesthesia.

The physiological effects of hydrostatic pressure are not equivalent to those of helium pressure on Rana pipiens

The effects of helium pressure and hydrostatic pressure on Rana pipiens were compared. Both agents caused paralysis at pressures greater than 135 atmospheres (1 atm = 101.325 kPa), but the median pressure for hydrostatic-pressure-induced paralysis was 35 atm less than that for helium pressure. When the ability of both pressurizing agents to reverse urethane-induced anaesthesia was compared, it was found that hydrostatic pressure raised the median dose for anaesthesia 2.2-fold more per atmosphere than did helium pressure. Animals that were lightly anaesthetized by urethane at 110 atm hydrostatic pressure became more deeply anaesthetized when helium was admitted isobarically into the pressure chamber. This difference in depth of anaesthesia between hydrostatic pressure and helium pressure is consistent with helium possessing an inherent anaesthetic effect. The abilities of other gases to pressure-reverse urethane anaesthesia were also determined. The degree of attenuation of the full pressure reversal effect observed with hydrostatic pressure was proportional to the lipid solubility of the gases, increasing in the order helium, neon, hydrogen, nitrogen and argon. Our data on the difference between hydrostatic and helium pressure are consistent with the critical volume hypothesis.

Benzyl alcohol penetration into micelles, dielectric constant of the binding site, partition coefficient and high-pressure squeeze-out

The absorbance maximum, lambda max, of a local anesthetic, benzyl alcohol, is shifted to longer wavelengths when solvent polarity is decreased. The shift was approximately a linear function of the dielectric constant of the solvent. This transition in electronic spectra according to the microenvironmental polarity is used to analyze benzyl alcohol binding to surfactant micelles. A facile method is devised to estimate the micelle/water partition coefficient from the dependence of lambda max of benzyl alcohol on surfactant concentrations. The effective dielectric constants of the sodium decyl sulfate, dodecyl sulfate and tetradecyl sulfate micelles were 29, 31 and 33, respectively. The partition coefficient of benzyl alcohol between the micelles and the aqueous phase was 417, 610 and 1089, respectively, in the mole fraction unit. The pressure dependence of the partition coefficient was estimated from the depression of the critical micelle concentration of sodium dodecyl sulfate by benzyl alcohol under high pressure up to 200 MPa. High pressure squeezed out benzyl alcohol molecules from the micelle until about 120 MPa, then started to squeeze in when the pressure was further increased. The volume change of benzyl alcohol by transfer from the aqueous to the micellar phase was calculated from the pressure dependence of the partition coefficient. The volume change, estimated from the thermodynamic argument, was 3.5 +/- 1.1 cm3.mol-1 at 298.15 K, which was in reasonable agreement with the partial molal volume change determined directly from the solution density measurements, 3.1 +/- 0.2 cm3.mol-1. Benzyl alcohol apparently solvates into the micelles close to surface without losing contact with the aqueous phase.

Do general anaesthetics act by competitive binding to specific receptors?

Most proteins are insensitive to the presence of anaesthetics at concentrations which induce general anaesthesia, while some are inhibited by certain agents but not others. Here we show that, over a 100,000-fold range of potencies, the activity of a pure soluble protein (firefly luciferase) can be inhibited by 50% at anaesthetic concentrations which are essentially identical to those which anaesthetize animals. This identity holds for inhalational agents (such as halothane, methoxyflurane and chloroform), aliphatic and aromatic alcohols, ketones, ethers and alkanes. This finding is all the more striking in view of the fact that the inhibition is shown to be competitive in nature, with anaesthetic molecules competing with substrate (luciferin) molecules for binding to the protein. We show that the anaesthetic-binding site can accommodate only one large, but more than one small, anaesthetic molecule. The obvious mechanism suggested by our results is that general anaesthetics, despite their chemical and structural diversity, act by competing with endogenous ligands for binding to specific receptors.

Temperature dependence of ethanol depression in C57BL/6 and BALB/c mice

The relationship between environmental temperature, body temperature and brain sensitivity to ethanol was investigated in male C57BL/6S and BALB/cS mice. Age-matched, drug-naive mice of both strains were injected with 3.6 g/kg ethanol (20% w/v) and placed into a chamber kept at one of eight designated temperatures from 12 to 36 degrees C. Chamber temperature significantly affected wake-up rectal temperatures, sleep-times and wake-up brain ethanol concentrations in the intoxicated mice. Wake-up rectal temperatures were significantly, positively correlated with sleep-times and significantly, negatively correlated with wake-up brain ethanol concentrations in both strains. Linear regression analyses indicated that up to 47% of the variability in ethanol sensitivity of C57 mice and up to 31% of the variability in sensitivity of BALB mice could be accounted for by their wake-up rectal temperatures suggesting that the effects of ambient temperature on ethanol sensitivity were mediated, in part, by the resultant body temperatures. These results replicate and extend previous findings and demonstrate that temperature dependence of ethanol depression is not strain specific. The correlational and regression analyses provide additional evidence that brain sensitivity to ethanol depression varies with body temperature in accordance with membrane perturbation theories of anesthesia.

The effects of pressure on the molecular structure and physiological functions of cell membranes

The effects of high pressure on the phase state and molecular structure of pure lipid bilayers are discussed. The relations of delta H, delta S and delta V in phase transitions are straightforward and are discernible in heterogeneous bilayers in natural membranes. The effects of pressure on the dynamic properties of bilayer constituents are less clearly understood, but order parameters obtained at pressure by different techniques show agreement. The extent and significance of hydration is poorly understood. Four physiological functions are discussed: passive permeability, active transport, membrane excitability and synaptic transmission. It is shown that a full interpretation of the kinetic effects of pressure on these processes requires much more detailed molecular information than is available at present.

Temperature dependence of ethanol lethality in mice

The present study provides systematic evidence indicating a direct relationship between environmental temperature, rectal temperature and ethanol lethality. Male, C57 BL/6J mice, previously housed at room temperature (23 +/- 1 degree C), were injected intraperitoneally with 4.8 to 9.2 g kg-1 ethanol and then exposed for 24 h to ambient temperatures that did not appreciably exceed the thermally neutral range for sober mice (20 to 35 degrees C). There was a direct relationship between temperature and ethanol lethality at 8 and 24 h after injection. The 8 h LD50 increased by 64%, from 5.3 to 8.7 g kg-1, as environmental temperature decreased from 35 to 20 degrees C. The 24 h LD50 increased by 51%, from 5.3 to 8.0 g kg-1, across this temperature range. Each 5 degrees C reduction in ambient temperature induced a significant decrease in the rectal temperature of ethanol-injected mice. Mean rectal temperature ranged from 2.2 degrees C above baseline at an ambient temperature of 35 to 15 degrees C below baseline in the 20 degrees C environment. Ethanol induced a significant dose-related hypothermia in mice exposed to the 20, 25 and 30 degrees C environments but did not produce hypothermia in animals kept in the 35 degrees C environment. These findings indicate that the potency of potentially lethal ethanol doses varies with body temperature in accordance with partition and membrane expansion-fluidization theories of anaesthesia.

Molecular mechanisms of general anaesthesia

Important constraints on possible molecular mechanisms of general anaesthesia are derived from a quantitative reappraisal of data on the potency of general anaesthetics on whole animals. Despite their popularity, theories that invoke lipids as the prime target do not look at all promising, and available data point much more plausibly to a direct effect on particularly sensitive proteins. Structural changes of proteins on binding general anaesthetics are probably small but may be sufficient to perturb normal function; alternatively, anaesthetics may compete with an endogenous ligand. The phenomenon of pressure reversal of anaesthesia may simply be due to anaesthetics being squeezed away from their target sites.

Correlation of general anesthetic potency with solubility in membranes

Recently (Franks, N.P. and Lieb, W.R. (1978) Nature 274, 339-342) it has been claimed that the traditional correlation between anesthetic potency and vegetable oil solubility breaks down when the alkanols are compared to other volatile anesthetics. Lately, however, new information on the partitioning of anesthetics into lipid bilayers has become available. In this report the potency of twenty-one structurally diverse anesthetic agents is shown to correlate well with their ability to partition into phosphatidylcholine bilayers. Thus the original Meyer-Overton oil solubility hypothesis accommodates a wider range of anesthetics, including alkanols, volatile and gaseous agents, and barbiturates, when lipid bilayer solubility is substituted for oil solubility.