Molecular sites of anesthetic action in postsynaptic nicotinic membranes

Structural Basis of Alcohol Inhibition of the Pentameric Ligand-Gated Ion Channel ELIC

The structural basis for alcohol modulation of neuronal pentameric ligand-gated ion channels (pLGICs) remains elusive. We determined an inhibitory mechanism of alcohol on the pLGIC Erwinia chrysanthemi (ELIC) through direct binding to the pore. X-ray structures of ELIC co-crystallized with 2-bromoethanol, in both the absence and presence of agonist, reveal 2-bromoethanol binding in the pore near T237(6') and the extracellular domain (ECD) of each subunit at three different locations. Binding to the ECD does not appear to contribute to the inhibitory action of 2-bromoethanol and ethanol as indicated by the same functional responses of wild-type ELIC and mutants. In contrast, the ELIC-α1β3GABA_AR chimera, replacing the ELIC transmembrane domain (TMD) with the TMD of α1β3GABA_AR, is potentiated by 2-bromoethanol and ethanol. The results suggest a dominant role of the TMD in modulating alcohol effects. The X-ray structures and functional measurements support a pore-blocking mechanism for inhibitory action of short-chain alcohols.

QSAR Studies on the Anesthetic Action of Some Polyhalogenated Ethers

There has been an on-going debate about the mode of action of general anesthetics and until now, many sites have been postulated as target site for action of these compounds. Here, some quantum chemical-based quantitative structure-activity relationship (QSAR) models were developed for a set of polyhalogenated ethers in order to investigate the aspects of their anesthetic action, which is not completely defined yet, although some hypotheses have been suggested. A data set including 25 polyhalogenated methyl ethyl ethers were selected, and different descriptors were calculated for each molecule using density functional theory calculations, and subsequently some multilinear QSAR models were built by using different sets of the calculated molecular descriptors. The result showed that polar (polarizability) and non-polar (log P) parameters have mixed role on the anesthetic activity i.e. models with high statistical quality were obtained in combination with these two parameters. Also a good model between anesthetic action and electrostatic potentials was obtained, which may imply the important role of electronic interactions in the anesthetic activity of the compounds. Finally, a four-parametric QSAR model containing log P, molecular polarizability, most positive charge and an electrostatic potential parameters was obtained, which indicated that the anesthetic action of the polyhaloganted ethers may be proceeded through lipophilic, steric and columbic interactions.

Low dose acute alcohol effects on GABA A receptor subtypes

GABA(A) receptors (GABA(A)Rs) are the main inhibitory neurotransmitter receptors and have long been implicated in mediating at least part of the acute actions of ethanol. For example, ethanol and GABAergic drugs including barbiturates and benzodiazepines share many pharmacological properties. Besides the prototypical synaptic GABA(A)R subtypes, nonsynaptic GABA(A)Rs have recently emerged as important regulators of neuronal excitability. While high doses (> or =100 mM) of ethanol have been reported to enhance activity of most GABA(A)R subtypes, most abundant synaptic GABA(A)Rs are essentially insensitive to ethanol concentrations that occur during social ethanol consumption (< 30 mM). However, extrasynaptic delta and beta3 subunit-containing GABA(A)Rs, associated in the brain with alpha4 or alpha6 subunits, are sensitive to low millimolar ethanol concentrations, as produced by drinking half a glass of wine. Additionally, we found that a mutation in the cerebellar alpha6 subunit (alpha6R100Q), initially reported in rats selectively bred for increased alcohol sensitivity, is sufficient to produce increased alcohol-induced motor impairment and further increases of alcohol sensitivity in recombinant alpha6beta3delta receptors. Furthermore, the behavioral alcohol antagonist Ro15-4513 blocks the low dose alcohol enhancement on alpha4/6/beta3delta receptors, without reducing GABA-induced currents. In binding assays alpha4beta3delta GABA(A)Rs bind [(3)H]Ro15-4513 with high affinity, and this binding is inhibited, in an apparently competitive fashion, by low ethanol concentrations, as well as analogs of Ro15-4513 that are active to antagonize ethanol or Ro15-4513's block of ethanol. We conclude that most low to moderate dose alcohol effects are mediated by alcohol actions on alcohol/Ro15-4513 binding sites on GABA(A)R subtypes.

2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate: a derivative of the stereoselective general anesthetic etomidate for photolabeling ligand-gated ion channels

To locate general anesthetic binding sites on ligand-gated ion channels, a diazirine derivative of the potent intravenous anesthetic, R-(+)-etomidate (2-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate), has been synthesized and characterized. R-(+)-Azietomidate [2-(3-methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate] anesthetizes tadpoles with an EC(50) of 2.2 microM, identical to that of R-(+)-etomidate. At this concentration both agents equally enhanced GABA-induced currents and decreased binding of the caged-convulsant [(35)S]TBPS to GABA(A) receptors. In all of the above actions R-(+)-azietomidate is about an order of magnitude more potent than S-(-)-azietomidate, an enantioselectivity comparable to etomidate's. R-(+)-Azietomidate also inhibits acetylcholine-induced currents in nicotinic acetylcholine receptors, with about twice the potency of the parent compound. [(3)H]Azietomidate photoincorporated into Torpedo nicotinic acetylcholine receptor-rich membranes. Desensitization decreased photoincorporation into the delta-subunit and increased that into the alpha-subunit. The latter increase was confined to a proteolytic fragment containing the first three transmembrane segments. Thus, R-(+)-azietomidate is a potent stereoselective general anesthetic and an effective photolabel.

Butanol effects on gamma-amino butyric acid concentration-responses in human alpha1beta2gamma2L gamma-amino butyric acid type A receptors with a mutation at alpha1S270

Alcohol enhancement of gamma-amino butyric acid type A receptor (GABA(A)R) gating at low GABA is reduced by a serine-to-isoleucine mutation at residue alphaS270, suggesting that alphaS270 forms an enhancement site. However, whether the alphaS270I mutation strengthens alcohol inhibition of GABA(A)Rs remains unexplored. Furthermore, alphaS270 mutations have not been studied in the most prevalent form of mammalian GABA(A)Rs consisting of alpha1, beta2, and gamma2 subunits. In voltage-clamped Xenopus oocytes expressing recombinant alpha1beta2gamma2L GABA(A)Rs, electrophysiological analysis of GABA concentration-responses demonstrates that the alpha1(S270I) mutation increases apparent GABA affinity and significantly reduces the Hill coefficient of GABA(A)R activation. Butanol-induced leftward-shifts in GABA concentration-responses for both wild-type alpha1beta2gamma2L and alpha1(S270I)beta2gamma2L GABA(A)Rs are equal. At high GABA, butanol neither enhances nor inhibits alpha1(S270I)beta2gamma2L responses. Thus, in the dominant mammalian GABA(A)R isoform, the alphaS270I mutation affects neither enhancement nor inhibition by butanol, but alters the gating mechanism by reducing cooperativity, producing an apparent reduction in alcohol enhancement at low GABA.

Inhibition of 5-HT3 receptors by propofol: equilibrium and kinetic measurements

Patch-clamp/rapid solution exchange experiments as well as tracer ([14C]-guanidinium) influx measurements were applied to investigate effects of propofol on 5-HT3 receptor channels and compare the results with those obtained with pentobarbital. Currents induced by 30 microM 5-HT were recorded in outside-out patches from N1E-115 cells. Application of propofol 45 s before and during 5-HT application inhibited peak-currents and integrated current responses in a concentration-dependent manner (IC50 values=14.5 and 10.5 microM; Hill coefficients -1.5 and -1.3, respectively). The inhibitory effect of propofol in the current measurements was similar to the propofol-induced inhibition in tracer influx experiments in whole N1E-115 cells (Barann et al., 1993. Naunyn-Schmiedeberg's Archives of Pharmacology 347, 125-132). Pentobarbital-induced inhibition of 5-HT3 receptors in both patch-clamp (Barann et al., 1997. Neuropharmacology 36, 655-664) and tracer influx measurements indicated a lower potency and lower slope (IC50 values=130 and 55 microM; Hill coefficients -0.8 and -0.7, respectively) compared to propofol. Propofol, in contrast to pentobarbital, showed nearly the full potency when applied to the patches exclusively 45 s before 5-HT. Propofol was least effective when administered exclusively during 5-HT. The onset of inhibition of 5-HT-induced peak currents by propofol had a time constant of 220 ms, similar to the kinetics of 5-HT-induced desensitization.

The lateral pressure profile in membranes: a physical mechanism of general anesthesia

1. A lipid-mediated mechanism of general anesthesia is suggested and investigated using lattice statistical thermodynamics. 2. Anesthetics are predicted to shift the distribution of lateral pressure within a lipid bilayer, and thus alter the mechanical work required to open ion channel proteins, if channel opening is accompanied by a non-uniform change in cross-sectional area of the protein. 3. Calculations based on this mechanical thermodynamic hypothesis yield qualitative agreement with anesthetic potency at clinical anesthetic membrane concentrations, and predict the alkanol cutoff and anomalously low potencies of strongly hydrophobic molecules with little attraction for the aqueous interface, such as perfluorocarbons.

Direct interactions of anesthetics and nonanesthetics with the nicotinic acetylcholine receptor pore

(1) We review evidence that anesthetics inhibit peripheral nAChR cation translocation by binding directly to a protein site in the transmembrane pore. (2) This site is near the middle of the pore-forming M2 domains on alpha and beta subunits, but further from the homologous portions of gamma and delta subunits. (3) Interactions between both anesthetics and nonanesthetics with the nAChR pore site are determined primarily by hydrophobic forces rather than steric factors. (4) Anesthetics and nonanesthetics display different state-dependent accessibility to this site, suggesting a mechanism for the different in vivo actions of these two classes of drugs.

Anesthetic-induced structural changes in the nicotinic acetylcholine receptor

The difference between infrared spectra of the nicotinic acetylcholine receptor (nAChR) recorded in the absence and presence of the agonist carbamylcholine (Carb) reveals a complex pattern of positive and negative bands that provides a spectral map of Carb-induced structural change. This spectral map is affected by the presence of either the local anesthetic, dibucaine, or the short chain alcohol, propanol. Both antagonists alter the intensities of difference bands in a manner consistent with the stabilization of a desensitized state. Spectral variations are also observed that are indicative of both the displacement of the anesthetics from the nAChR upon the addition of Carb and physical interactions that occur between the anesthetics and binding site residues.

5-HT3 receptors in outside-out patches of N1E-115 neuroblastoma cells: basic properties and effects of pentobarbital

A fast solution exchange system (Dilger and Brett, 1990; Biophysics Journal 57: 723-731) with an exchange rate < 1 msec was used to study 5-HT3 (5-HT; 5-hydroxytryptamine) receptor-mediated currents in superfused outside-out patches of N1E-115 mouse neuroblastoma cells. At negative membrane potentials, 5-HT induced inward currents in a concentration-dependent manner (IC50 = 3.8 microM, Hill coefficient = 1.8). The mean peak current at a near-maximally effective 5-HT concentration of 30 microM was 20.6 pA. The 5-HT3 receptor antagonist ondansetron (0.3 nM) reversibly inhibited the 5-HT (30 microM) signal by approximately 50%. The currents induced during application of 30 microM 5-HT for 2 sec were characterized by inward rectification, a monophasic onset (tau ON = 37.5 msec) and, after reaching a peak, a monophasic decay (desensitization; tau OFF = 391 msec). Onset and decay were slower at lower 5-HT concentrations. The recovery of fully desensitized patches required a washout period of 45 sec. Pentobarbital inhibited 5-HT-induced (30 microM) currents in a concentration-dependent manner. The maximally obtainable inhibition with a given pentobarbital concentration was reached already when it was exclusively coapplied with 5-HT (IC50 = 135 microM. Hill coefficient = -0.7), since additional preexposure for at least 45 sec did not alter the concentration-response curve of pentobarbital. In conclusion, outside-out patches of N1E-115 cells are suitable to study the kinetic properties of 5-HT3 receptor channels. The results obtained in this model with pentobarbital are compatible with the suggestion that the inhibitory action of pentobarbital on 5-HT3 receptors is dependent on the agonist-activated (open) channel.

Amphiphilic sites for general anesthetic action? Evidence from 129Xe-[1H] intermolecular nuclear Overhauser effects

Because a strong correlation exists between the potency of general anesthetics and their ability to dissolve in oil, a lipophilic site of action is often assumed. We show here that a lipophilic molecule may preferentially target less lipophilic sites after interaction with a membrane takes place. Xenon, a chemically inert and structureless general anesthetic, was chosen as an unbiased molecular probe for assessment of its dynamic distribution. Site-selective intermolecular 129Xe-[1H] nuclear Overhauser effects were used to measure the specific interaction between xenon and protons in different regions in a phosphatidylcholine lipid membrane. It was evident that xenon-membrane interaction was directed toward the amphiphilic head region, with significant involvement of interfacial water, despite xenon's apolar and highly lipophilic nature in the gas phase. This result may suggest the importance of amphiphilicity in association with anesthetic action.

Allosteric modulation of Torpedo nicotinic acetylcholine receptor ion channel activity by noncompetitive agonists

Similar to other neuroreceptors of the vertebrate central nervous system, the nicotinic acetylcholine receptor (nAChR) is subject to modulatory control by allosterically acting ligands. Of particular interest in this regard are allosteric ligands that enhance the sensitivity of the receptor to its natural agonist acetylcholine (ACh), as such ligands could be useful as drugs in diseases associated with impaired nicotinic neurotransmission. Here we discuss the action of a novel class of nAChR ligands which act as allosterically potentiating ligands (APL) on the nicotinic responses induced by ACh and competitive agonists. In addition, APLs also act as noncompetitive agonists of very low efficacy, and as direct blockers of ACh-activated channels. These actions are observed with nAChRs from brain, muscle and electric tissue, and they depend on the structure of the APL and the concentration range applied. We focus here on Torpedo nAChR because (i) the unusual pharmacology of these ligands was first discovered with this system, and (ii) large quantities of this receptor are readily available for biochemical studies.

Biphasic effect of pentobarbitone on chick myotube nicotinic receptor channel kinetics

1. The modulatory action of pentobarbitone on chick myotube nicotinic acetylcholine receptor kinetics was studied by the patch clamp technique, particularly focussing on effects at low concentrations. 2. Open time sojourn distributions of foetal-type receptors recorded at room temperature (22-24 degrees C) in cell-attached mode in the presence of 0.2 microM acetylcholine were well described by two exponentials, with fast and slow time constants of 0.53 +/- 0.12 and 16.7 +/- 2.2 ms (means +/- s.e. mean) respectively. 3. The duration of the slow open time constant was increased by low concentrations of pentobarbitone (up to 1 microM), and thereafter decreased with higher concentrations (10-50 microM) in a concentration-dependent manner. 4. Complementary evidence for the stimulatory effect of pentobarbitone on open time was obtained (i) by using a backfill technique where drug concentration at the patch gradually increases over time, and (ii) through use of perfused outside-out preparations where receptors in the same patch were successively exposed to acetylcholine in the absence and presence of pentobarbitone. 5. The dual action of pentobarbitone on channel kinetics probably, indicates that an allosteric interaction mechanism is involved rather than simple steric channel blockade.

Effects of halothane on the nicotinic acetylcholine receptor from Torpedo californica

To determine whether the binding of anesthetics to key membrane receptors is a plausible mode of action, we modeled the effect of the general anesthetic halothane in the nicotinic acetylcholine receptor membrane system isolated from Torpedo californica. Our results demonstrated that halothane inhibits the binding of [3H]phencyclidine ([3H]PCP) to the acetylcholine receptor. The inhibition was reversible, concentration dependent, and had an equilibrium dissociation constant (Kd) of 2.2% atm halothane at 25 degrees. Double-reciprocal plots of the halothane effects at various phencyclidine (PCP) concentrations imply that, under equilibrium conditions, halothane inhibits [3H]PCP binding competitively. In contrast, results from kinetic studies showed that the rate of PCP dissociation is highly sensitive to halothane with EC50 = 0.8% atm halothane in nitrogen. Several possible interpretations are discussed; however, the basic observation was that the kinetics of [3H]PCP binding to the nicotinic acetylcholine receptor was affected by halothane at low concentrations in this model system.

Cardiovascular effects of different schedules of nicotine administration on spinal rats: influence of pentobarbital

This work examined the cardiovascular effects of different schedules of i.v. nicotine administration with respect to doses and timing in non-anesthetized and pentobarbital-treated spinal rats. For this purpose, complete nicotine dose-response curves were made for mean arterial pressure and heart rate. In non-anesthetized spinal rats, tachyphylaxis was not found for the pressor effects; moreover, consecutive doses, given at short intervals, produced additive actions. Furthermore, nicotine produced a biphasic heart rate response: an initial and brief bradycardia followed by a longer lasting tachycardia. In pentobarbital-treated rats, the sensitivity of the cardiovascular system to nicotine was decreased; in these rats, consecutive nicotine doses did show tachyphylaxis for the pressor and tachycardiac responses. The present series of experiments, using different schedules of administration for single and consecutive nicotine doses, demonstrated opposite tachyphylactic effects in non-anesthetized and in pentobarbital-treated spinal rats.

Inhibition of the nicotinic acetylcholine receptor by barbiturates and by procaine: do they act at different sites?

1. The effects of three barbiturates and the local anesthetic procaine on the ion channel function of mouse nicotinic acetylcholine receptor (nAChR) muscle subtype expressed in Xenopus laevis oocytes were examined by whole-cell voltage-clamp technique. 2. A concentration-response curve for the specific nicotinic agonist dimethylphenylpiperazinium iodide (DMPP) was first determined. This agonist produced increasing whole-cell currents up to a concentration of 100 microM (EC50 = 13 microM), then decreased responses at higher concentrations. 3. The barbiturates (amobarbital, secobarbital, pentobarbital) and procaine produced reversible inhibition of DMPP-induced currents at clinically used concentrations. The two classes of drugs differed in the voltage dependence of the inhibition: procaine-induced inhibition was increased at more negative transmembrane holding potentials (-90 vs. -45 mV); whereas amobarbital-induced inhibition did not vary at different transmembrane potentials. 4. Mutant forms of the nAChR, containing single amino acid changes in the M2 regions of alpha and beta subunits, showed increased sensitivity to procaine but no change in sensitivity to amobarbital-induced inhibition. 5. These electrophysiologic studies provide further evidence that barbiturates and local anesthetics produce inhibition of the nAChR at different sites.

Inhibition by anaesthetics of 14C-guanidinium flux through the voltage-gated sodium channel and the cation channel of the 5-HT3 receptor of N1E-115 neuroblastoma cells

The influence of local and general anaesthetics on cation influx through the fast, voltage-dependent sodium channel and the 5-HT3 receptor cation channel was studied in N1E-115 mouse neuroblastoma cells by measuring 2-min influx of the organic cation 14C-guanidinium induced by either veratridine (1 mmol/l) or 5-HT (100 mumol/l). The veratridine-induced influx of 14C-guanidinium was potentiated by scorpion toxin and inhibited by tetrodotoxin. The 5-HT-induced 14C-guanidinium influx was not affected by tetrodotoxin but it was inhibited by nanomolar concentrations of the selective 5-HT3 receptor antagonists ondansetron and ICS 205-930; at high micromolar concentrations these compounds also inhibited the veratridine-induced influx of 14C-guanidinium. The 14C-guanidinium influx through both channels was inhibited by local and general anaesthetics. The rank order of potency for inhibition of veratridine-induced influx by local anaesthetics was tetracaine > bupivacaine > cocaine > lidocaine > procaine and that for inhibition of the 5-HT3 receptor channel was tetracaine > bupivacaine > cocaine > procaine > lidocaine. With the exception of procaine and cocaine, which were equipotent at both channels, the local anaesthetics were 4.4-fold (lidocaine) to 25-fold (tetracaine) more potent at the fast sodium channel than at the 5-HT3 receptor channel. The rank order of potency for general anaesthetics was propofol > etomidate = alfaxalone = ketamine > thiopental = methohexital at the fast sodium channel, and propofol > or = etomidate > alfaxalone = methohexital > thiopental > ketamine at the 5-HT3 receptor channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Enflurane is a potent inhibitor of high conductance Ca(2+)-activated K+ channels of Chara australis

The volatile anaesthetic, enflurane, is commonly used in surgery for inducing the state of general anaesthesia. It is assumed, that general anaesthetics act on ion channels, but little is known of how they do so and what kinds of channels are sensitive. We found, that enflurane inhibits a large conductance Ca(2+)-activated K+ channel of the green alga, Chara australis. Effects occur at clinically relevant concentrations are fully reversible. The actions of enflurane are distinct from those of charybdotoxin and tetraethylammonium, which are well known blockers of this channel type. Kinetic analysis of single-channel data demonstrates multiple effects of enflurane on the channel protein.

The effect of aliphatic alcohols on the transient potassium current in hippocampal neurones

1. The transient potassium current was recorded in single hippocampal CA1 neurones from the rat by use of the whole-cell patch clamp technique. The effects on this current of a homologous series of aliphatic alcohols, ranging from butanol to octanol, were investigated. 2. The predominant effect of octanol (and the other alcohols) was to cause an increase in the initial rate of decay of the transient potassium current together with a slight decrease in the rate of decay of later phases of the current, such that the current decay became markedly non-monotonic. The alcohols also caused a decrease in peak current amplitude which could not be accounted for solely by the change in current decay kinetics. 3. The effect of the alcohols was concentration-dependent and readily reversible. Increasing chain length increased the potency of each alcohol by about 3 fold for each methylene group added. Other than a difference in potency, there appeared to be little difference in the action of aliphatic alcohols of different chain length on the transient current. 4. The alcohols did not appreciably change the voltage-dependence of steady state inactivation or activation of the transient potassium current. 5. The rate of inactivation of the transient current in these cells was only weakly voltage-dependent. This weak voltage-dependence was not changed by the presence of aliphatic alcohols, neither was the effect of the alcohols themselves voltage-dependent. 6. The potencies of each of the aliphatic alcohols were well correlated with their respective membrane/buffer partition coefficients, a finding which implies a hydrophobic locus of action.

Neurobiology of alcohol abuse

Excessive consumption of beverage alcohol (ethanol) is a major health concern worldwide. Understanding the mechanisms by which ethanol affects neural functioning, after both acute and chronic exposure, has become a major goal in the study of alcoholism. With such an understanding, we should be able to institute more effective treatments and preventative measures for alcohol abuse problems. Recent studies have found, contrary to earlier assumptions, that ethanol has selective, dose-dependent effects on various neurotransmitter systems within the CNS. These effects are observed at all levels of analysis, from molecular to behavioral. This review by Herman Samson and Adron Harris covers these recent findings, with the intent of generating questions that will focus further research efforts.

K+ channels in PC12 cells are affected by propofol

The effect on K+ currents (IK) of the general anaesthetic propofol (PR) (2,6-diisopropylphenol) was tested in undifferentiated clonal pheochromocytoma (PC 12) cells using the patch-clamp technique in whole-cell and single-channel configurations. PR decreased macroscopic IK amplitudes in a concentration-dependent way from 50 microM to 1 mM. The blocking effect was unchanged by repetitive depolarizing pulses and it was independent of the holding potential. Whereas activation of IK in control conditions was fitted by sigmoidal plus exponential time courses, only the sigmoidal time course gave an adequate fit with PR in the bath. The above effects were reversible. PR concentrations below 140 microM decreased single-channel activity for K+ channels with unitary conductance of 22 pS, in the voltage range between -40 and 60 mV from a holding potential of -50 mV. In contrast, the anaesthetic had nearly no effect on the opening probability of a channel with conductance of 10 pS. The unitary current amplitudes were unaffected in both channel types. These results suggest that PR action on IK may depend on the different blocking mechanisms of the K+ channels.

Effets des anesthésiques intraveineux sur les neurones du système nerveux central : mécanismes d'action cellulaires et moléculaires

The mechanisms of action of intravenous anaesthetics are not yet completely elucidated. Until recently, most of the studies had focused on the interactions between anaesthetics and lipid bilayers. It has been proposed that loss of consciousness is produced by disorganization of the lipid phase of nerve membranes, which impairs the action potential propagation. However, new data obtained with sophisticated neuropharmacological tools such as the patch clamp technique have recently contributed to challenge this hypothesis. Indeed, several lines of evidence suggest that intravenous anaesthetics are thought to induce loss of consciousness by blocking the excitatory synaptic transmission. This can be achieved presynaptically, by inhibiting glutamate release from nerve endings via alterations in the gating properties of voltage-dependent calcium channels. Blockade of excitatory synaptic transmission can also occur at the postsynaptic level by antagonizing the glutamate receptors of the N-methyl D-aspartate subtype. Some anaesthetic agents including ketamine also block the nicotinic receptors, however the relevance of this finding with respect to clinical anaesthesia requires further investigation. Preliminary data also suggest that propofol and etomidate elicit uncoupling of gap junctions between astrocytes, which represent a major nonneuronal cell population in the central nervous system. This phenomenon might indirectly contribute to the hypnotic action of these compounds. Whether loss of consciousness involves preferential target structures within the brain remains to be delineated. A better understanding of the mechanisms of action of general anaesthetics might contribute to generate new agents with more pharmacological selectivity and less undesirable side-effects.

Actions of n-alcohols on nicotinic acetylcholine receptor channels in cultured rat myotubes

1. The actions of the n-alcohols from pentanol to dodecanol on nicotinic acetylcholine receptor (nAChR) channels were investigated by recording single ACh-activated channel activity from inside-out membrane patches isolated from cultured rat myotubes. Alcohols were applied to the cytoplasmic side of the membrane; aqueous concentrations ranged from 11.7 mM-pentanol to 0.02 mM-dodecanol. 2. The intermediate-chain alcohols (pentanol to octanol) caused channel currents to fluctuate between the fully open and closed state level so that openings occurred in bursts interrupted by brief gaps. Closed time distributions were fitted well with two exponential components, the fast component representing the closures within a burst. The number of gaps within a burst was dependent on alcohol concentration whereas gap duration was independent of concentration but increased with increasing chain length of the alcohol up to octanol. 3. Nonanol and decanol reduced the mean duration of bursts of openings but did not cause an increase in the number of short closed intervals within a burst. Beyond decanol there was a decline in the ability of the n-alcohols to affect channel function. A saturated solution of undecanol (0.07 mM) reduced the mean open time by 33 +/- 17%, whereas a saturated solution of dodecanol had no significant effect. 4. The current integral per burst was reduced by all the n-alcohols between pentanol and undecanol. The IC50S were as follows: hexanol, 0.53 +/- 0.14 mM; heptanol, 0.097 +/- 0.02 mM; octanol, 0.04 mM and nonanol, 0.16 +/- 0.035 mM. 5. The results were analysed in terms of an open channel block model with a long-lived closed-blocked state beyond the blocked state. Over the range of concentrations tested this describes the effects of all the n-alcohols (C5 to C12) on channel gating reasonably well. 6. Blocking rate constants (k+B) for pentanol through to nonanol were calculated to be between 2.8 and 5.7 X 10(6) M-1 S-1. These values are based on the assumption that the concentration of the alcohols at their site(s) of action was equal to the aqueous concentration applied to the membrane. 7. Equilibrium dissociation constants (KD), calculated from the blocking and unblocking rate constants (KD = k-B/k+B), decreased with increasing chain length from 8 mM for pentanol to 0.15 mM for octanol. The standard free energy per methylene group for adsorption to the site of action was calculated to be about -3.3 kJ mol-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Actions of n-alcohols on nicotinic acetylcholine receptor ion channels in cultured rat muscle cells

The effects of the n-alcohols from pentanol to dodecanol on nAChR channel function were resolved at the single channel level. ACh-activated channel activity was recorded from isolated membrane patches using the patch clamp method. The intermediate-chain alcohols (C5-C8) had two main effects: (1) They caused channel openings to be interrupted by brief shut or blocked periods, the duration of which was dependent on chain length of the alcohol but independent of concentration. (2) They caused a reduction in the duration of bursts of openings. The long-chain alcohols (C9-C11) produced only the second effect, and there was a decline in activity beyond undecanol. Results were consistent with a mechanism of channel blockade and were analyzed in terms of an open channel block model with a long-lived closed-blocked state beyond the blocked state. Affinity for the binding site increased with chain length up to octanol. The standard free energy per methylene group for adsorption to the site was calculated to be -3.3 kJ/mol, indicating the very hydrophobic nature of the site.

The effects of short-chain phospholipids on the acetylcholine-activated ion channel

The effects of a homologous series of short-chain phospholipids, the phosphatidylcholines from dihexanoylglycerophosphocholine (Hxo2GroPCho) to didecanoylglycerophosphocholine, on the nicotinic acetylcholine-activated ion channel in cultured rat muscle cells were investigated. Standard patch-clamp techniques were used to measure single-channel currents in excised patches. All phospholipids investigated markedly reduced the frequency of channel opening in a concentration-dependent manner. Other parameters, such as the mean open time, the duration and frequency of brief closures within an opening, and channel amplitude, were not significantly affected. This effect was independent of the side of the membrane to which the phospholipid was added. Dose/response curves were obtained for Hxo2-, diheptanoyl(Hpo2)- and dinonanoyl(Nno2)GroPCho. The concentration leading to 50% reduction in channel activity decreased upon ascending the homologous series from 16.69 microM Hxo2GroPCho to 4.52 microM and 0.043 microM for Hpo2- and Nno2GroPCho, respectively. The more hydrophobic the molecule the more effective it was, and hence the higher its affinity to the binding site. Calculation of the standard free-energy change of adsorption into the site led to a value of -3.1 kJ/mol, which indicates a very hydrophobic binding site. In conclusion, the phospholipids interact in a non-specific way with the lipid membrane thereby disturbing proper channel function.