Additive and non-additive effects of mixtures of short-acting intravenous anaesthetic agents and their significance for theories of anaesthesia

Binding site location on GABAA receptors determines whether mixtures of intravenous general anaesthetics interact synergistically or additively in vivo

BACKGROUND AND PURPOSE

General anaesthetics can act on synaptic GABA_A receptors by binding to one of three classes of general anaesthetic sites. Canonical drugs that bind selectively to only one class of site are etomidate, alphaxalone, and the mephobarbital derivative, R-mTFD-MPAB. We tested the hypothesis that the general anaesthetic potencies of mixtures of such site-selective agents binding to the same or to different sites would combine additively or synergistically respectively.

EXPERIMENTAL APPROACH

The potency of general anaesthetics individually or in combinations to cause loss of righting reflexes in tadpoles was determined, and the results were analysed using isobolographic methods.

KEY RESULTS

The potencies of combinations of two or three site-selective anaesthetics that all acted on a single class of site were strictly additive, regardless of which single site was involved. Combinations of two or three site-selective anaesthetics that all bound selectively to different sites always interacted synergistically. The strength of the synergy increased with the number of separate sites involved such that the percentage of each agent's EC₅₀ required to cause anaesthesia was just 35% and 14% for two or three sites respectively. Propofol, which binds non-selectively to the etomidate and R-mTFD-MPAB sites, interacted synergistically with each of these agents.

CONCLUSIONS AND IMPLICATIONS

The established pharmacology of the three anaesthetic binding sites on synaptic GABA_A receptors was sufficient to predict whether a mixture of anaesthetics interacted additively or synergistically to cause loss of righting reflexes in vivo. The principles established here have implications for clinical practice.

Enhanced GABAergic actions resulting from the coapplication of the steroid 3α-hydroxy-5α-pregnane-11,20-dione (alfaxalone) with propofol or diazepam

Many GABAergic drugs are in clinical use as anesthetics, sedatives, or anxiolytics. We have investigated the actions of the combinations of the neuroactive steroid 3α-hydroxy-5α-pregnane-11,20-dione (alfaxalone) with the intravenous anesthetic propofol or the benzodiazepine diazepam. The goal of the study was to determine whether coapplication of alfaxalone reduces the effective doses and concentrations of propofol and diazepam. Behavioral effects of alfaxalone, propofol, diazepam, and the combinations of the drugs were evaluated during a 30-min activity test in mice. Functional effects of the individual drugs and drug combinations were tested by measuring the decay times of spontaneous inhibitory postsynaptic currents in rat hippocampal neurons, and peak current responses from heterologously expressed concatemeric α1β2γ2L GABA_A receptors. Co-administration of alfaxalone increased the sedative actions of propofol and diazepam in mice. The combination of alfaxalone with propofol or diazepam increased the decay times of sIPSCs and shifted the concentration-response relationships for GABA-activated receptors to lower transmitter concentrations. We infer that alfaxalone acts as a co-agonist to enhance the GABAergic effects of propofol and diazepam. We propose that co-administration of alfaxalone, and possibly other neuroactive steroids, can be employed to reduce dosage requirements for propofol and diazepam.

The neurosteroid 5β-pregnan-3α-ol-20-one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABAA receptors

BACKGROUND AND PURPOSE

Neurosteroids potentiate responses of the GABAA receptor to the endogenous agonist GABA. Here, we examined the ability of neurosteroids to potentiate responses to the allosteric activators etomidate, pentobarbital and propofol.

EXPERIMENTAL APPROACH

Electrophysiological assays were conducted on rat α1β2γ2L GABAA receptors expressed in HEK 293 cells. The sedative activity of etomidate was studied in Xenopus tadpoles and mice. Effects of neurosteroids on etomidate-elicited inhibition of cortisol synthesis were determined in human adrenocortical cells.

KEY RESULTS

The neurosteroid 5β-pregnan-3α-ol-20-one (3α5βP) potentiated activation of GABAA receptors by GABA and allosteric activators. Co-application of 1 μM 3α5βP induced a leftward shift (almost 100-fold) of the whole-cell macroscopic concentration-response relationship for gating by etomidate. Co-application of 100 nM 3α5βP reduced the EC50 for potentiation by etomidate of currents elicited by 0.5 μM GABA by about three-fold. In vivo, 3α5βP (1mg kg(-1) ) reduced the dose of etomidate required to produce loss of righting in mice (ED50 ) by almost 10-fold. In tadpoles, the presence of 50 or 100 nM 3α5βP shifted the EC50 for loss of righting about three- or ten-fold respectively. Exposure to 3α5βP did not influence inhibition of cortisol synthesis by etomidate.

CONCLUSIONS AND IMPLICATIONS

Potentiating neurosteroids act similarly on orthosterically and allosterically activated GABAA receptors. Co-application of neurosteroids with etomidate can significantly reduce dosage requirements for the anaesthetic, and is a potentially beneficial combination to reduce undesired side effects.

The molecular features of membrane perturbation by anaesthetic steroids: a study using differential scanning calorimetry, small angle X-ray diffraction and solid state 2H NMR

We have studied the interactions of the anaesthetic steroid alphaxalone and its inactive isomer delta 16-alphaxalone with model membrane bilayers using differential scanning calorimetry, small angle X-ray diffraction and solid state NMR. Our data show that the anaesthetic steroid broadens the membrane phase transition and increases the ratio of gauche to trans conformers in the membrane. Delta 16-Alphaxalone has only small effects on membrane and incorporates to a limited degree in the bilayer. The amphipathic anaesthetic steroid alphaxalone is located near the membrane interface (the junction of the polar and hydrophobic regions of the phospholipids forming the bilayer). It orients with its long axis parallel to the chains of the lipid membranes and its 3 alpha-hydroxyl group near the sn-2 carbonyl. Anchoring of the steroid at the membrane interface and imperfect packing with the bilayer chains may be involved in membrane perturbation and eventually lead to anaesthesia.

Can molecular similarity-activity models for intravenous general anaesthetics help explain their mechanism of action?

BACKGROUND

The importance of molecular shape and electrostatic potential in determining the activities of 11 structurally-diverse i.v. general anaesthetics was investigated using computational chemistry techniques.

METHODS

The free plasma anaesthetic concentrations that abolished the response to noxious stimulation were obtained from the literature. The similarities in the molecular shapes and electrostatic potentials of the agents to eltanolone (the most potent anaesthetic agent in the group) were calculated using Carbo indices, and correlated with in vivo potency.

RESULTS

The best model obtained was based on the similarities of the anaesthetics to two eltanolone conformers (r2=0.820). This model correctly predicted the potencies of the R- and S-enantiomers of ketamine, but identified alphaxalone as an outlier. Exclusion of alphaxalone substantially improved the activity correlation (r2=0.972). A bench mark model based on octanol/water partition coefficients (r2=0.647) failed to predict the potency order of the ketamine enantiomers.

CONCLUSIONS

The results demonstrate that a single activity model can be formulated for chiral and non-chiral i.v. anaesthetic agents using molecular similarity indices.

The action of anaesthetics on stimulus-secretion coupling and synaptic activity

1. Anaesthetics are known to depress excitatory synaptic transmission and the mechanism of this inhibition has been investigated using bovine adrenal chromaffin cells as an experimental model. 2. These cells are homologous with post-ganglionic sympathetic neurons and have well characterized receptor and secretory mechanisms. They are amenable both to the direct measurement of evoked secretion with its associated ion fluxes, and to electrophysiological investigation using the patch clamp technique. 3. These approaches have been used to study the influence of anaesthetics on pre- and post-synaptic mechanisms involved in stimulus-secretion coupling. 4. A variety of agents inhibited secretion evoked by direct depolarization, and this was shown to be due to a reduction in calcium influx. 5. Direct inhibition of voltage-gated calcium currents was confirmed by whole-cell patch clamp measurements. 6. In addition, anaesthetics powerfully modulated nicotinic receptor mediated events: carbachol-evoked secretion was more sensitive to anaesthetics than that stimulated by high potassium. 7. The mechanism of anaesthetic action on the nAChR was examined in more detail with patch-clamp experiments. 8. These showed that anaesthetics reduced the probability of channels being in the open state, largely as a result of reduction in mean channel open time. 9. The data are discussed in relation to excitatory synaptic transmission.

Mechanism of action of a structural analog of alphaxalone on myelinated nerve fibre

The action of alphadolone acetate (0.05-5 mM), a steroid anaesthetic and structural analog of alphaxalone, was investigated on frog myelinated axons under voltage-clamp conditions. When applied externally, alphadolone acetate reduced K and Na currents, with apparent dissociation constants of 0.70 and 1.74 mM, respectively, and without noticeable modification in their time course. In addition, Na conductance-voltage and steady-state inactivation-voltage curves were shifted towards negative voltages. This effect was more pronounced on the steady-state inactivation-voltage relationship. These results suggest that alphadolone acetate blocked K channels indifferently in their resting or open state, and Na channels preferentially in their inactivated state. Alphaxalone has been shown to preferentially block open K and inactivated Na channels (Benoit et al., 1988, Br. J. Pharmacol. 94, 635). Thus, a structural change of a steroid molecule can lead to differences in its mechanism of action. This supports the hypothesis of direct interactions between steroid molecules and target membrane proteins with resulting anaesthetic activity.

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids

1. The modulation of the gamma-aminobutyric acidA (GABAA) receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2. The progesterone metabolites 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one (greater than or equal to 30 nM) reversibly and dose-dependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 microM), and over the concentration range 30 nM-100 microM stimulated the binding of [3H]-muscimol. In contrast, 5 alpha- and 5 beta-pregnan-3 beta-ol-20-one (30 nM-100 microM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3. Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers. 4. GABA-evoked currents were also potentiated by androsterone (1 microM) and the deoxycorticosterone metabolite 5 alpha-pregnane-3 alpha,21-diol-20-one (100 nM). 5. Secobarbitone (10-100 microM), pentobarbitone (10-300 microM) and phenobarbitone (100-500 microM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6. At relatively high concentrations (greater than or equal to 30 microM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (approximately 0 mV), are reversibly antagonized by bicuculline (3 microM), and potentiated by either diazepam (1 microM) or 5 beta-pregnan-3 alpha-ol-20-one (500 nM). 7. Secobarbitone (1 microM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8. A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5 beta-pregnan-3 alpha-ol-20-one (30 nM-100 microM). Similarly the steroid, at a concentration of 100 microM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 microM-1 mM). In all combinations of concentrations tested, the effects of secobarbitone and 5#-pregnan-3a-ol-20-one on [3H]-muscimol binding were additive. 9. In conjunction with previously published observations, the present data indicate close similarities in the GABA-mimetic and potentiating actions of barbiturates and steroids. However, the results obtained with combinations of steroids and barbiturates in the ligand binding assay appear inconsistent with the two classes of compound interacting with a common site to modulate the GABAA receptor activity.

Interactions between molecules of a steroid anaesthetic (alphaxalone) and ionic channels of nodal membrane in voltage-clamped myelinated nerve fibre

1. The effects of the anaesthetic alphaxalone (0.05 to 1 mM) on the node of Ranvier of isolated myelinated nerve fibres of the frog were studied under voltage-clamp conditions. 2. When added to the solution bathing voltage-clamped nodes, alphaxalone modified neither linear leakage nor capacitative currents but rapidly and reversibly blocked K and Na currents. The blocking effects of the anaesthetic on both types of current were not dependent on the frequency of stimulation of the nerve fibres between 0.7 and 10 Hz. 3. The kinetics of the Na current were not modified by alphaxalone but, in the presence of the drug, the K current showed an apparent fast inactivation. 4. Alphaxalone rapidly and reversibly shifted towards negative voltages both the steady-state K conductance-voltage and the peak Na steady-state inactivation-voltage relationships, without noticeable modification of their shape. In contrast, the anaesthetic reversibly decreased the slope of the peak Na conductance-voltage curve. 5. The reduction of the K current induced by alphaxalone was voltage-dependent with an apparent dissociation constant first decreasing from about 0.25 to 0.08 mM between -20 mV and +20 mV and then remaining constant above +20 mV. In contrast, the apparent dissociation constant for the Na current was almost constant with increasing voltages and equalled about 0.30 mM. Hill coefficient values for both K and Na currents were noticeably less than one. 6. It is concluded that, at higher concentrations than those attainable in the brain or in the plasma during surgical anaesthesia in man, alphaxalone has a 'local anaesthetic-like' action on the peripheral nervous system in that it specifically and differentially interacts with K and Na channel gating systems: it is suggested that the anaesthetic would preferentially modify open K and inactivated Na channels.

Pressure reversal of alphaxalone/alphadolone and methohexitone in tadpoles: evidence for different molecular sites for general anaesthesia

Tadpoles were used to study quantitative interactions between high pressure and two intravenous anaesthetics, alphaxalone/alphadolone and methohexitone. The potencies of the two agents were decreased by high pressure but to different extents. The maximum effect was seen in the pressure range 70-130 atmospheres absolute (ATA). The increases in the normobaric anaesthetizing concentration (ED50) required at 100 ATA were alphaxalone/alphadolone:405 +/- 5 (s.d.)%; methohexitone:658 +/- 80 (s.d.)%. For both alphaxalone/alphadalone and methohexitone, the curves obtained when the increase in ED50 was plotted against increasing pressure showed plateaux at pressures above 70 ATA. These data support the concept of the two intravenous drugs causing general anaesthesia by the occupation of separate molecular 'sites' with different but finite capacities.

Interactions between hexobarbital and thiopental in male rats evaluated with an anaesthesia threshold

The anaesthetic interaction between hexobarbital and thiopental in different combinations were investigated in male rats by using the isobolografic method. Data on dose, serum or brain concentrations at an EEG-criterion induced with a threshold method were utilized. The criterion was defined by a burst suppression in the EEG of 1 sec. or more (the "silent second"). With rats more than 115 days of age a synergistic interaction (potentiation) was obtained when a low dose of hexobarbital and a high dose of thiopental were used. The relationship between serum and brain concentrations of hexobarbital and thiopental in the different tested combinations gave no evidence for a pharmacokinetic interaction. Thus the synergistic interaction was located in the brain and probably related to the mechanisms of action. The same tests performed on rats at an age of approximately 90 days gave not only a potentiation when a low dose of hexobarbital and a high dose of thiopental were used, but also a potentiation when a high dose of hexobarbital and a low dose of thiopental were used. These results indicate an age-related change in sensitivity in the CNS between 90 and 115 days of age. Further investigations of this age-related change showed that this change from a potentiation to an additive interaction occurred between the age of 85 and 96 days.

Molecular mechanisms of action of general anesthetics

While a plethora of information exists describing particular changes caused by anesthetics on the molecular architecture of membranes, it is clear that models for anesthetic action remain unproven by rigid scientific criteria. This article describes historical and contemporary theories of how anesthetics act on a molecular level, and examines the discrepancies between these hypotheses and current data.

The mechanism of steroid anaesthetic (alphaxalone) block of acetylcholine-induced ionic currents

The effects of the steroid anaesthetic alphaxalone on acetylcholine (ACh)-induced ionic channels were studied in voltage clamped 'myoballs' in culture. Alphaxalone produced a reversible blockade of the ACh-evoked inward current, ED50 = 6.0 microM. The ACh reversal potential (-5.0 mV), the single channel conductance (13.5 pS) and mean open time (3.6 ms) were unchanged by the drug. Thus, alphaxalone produced an 'all or none' block of the ionic channel. In double pulse conditioning experiments, alphaxalone produced an additional inhibition with a time constant of recovery (550 ms) much longer than the time constant of recovery of the normal desensitization (250 ms). It was concluded that alphaxalone blocks active (open) ionic channels.

Anesthetics block excitation with various effects on inhibition in MRF neurons

The effects of isoflurane and halothane on the behavior of mesencephalic reticular neurons in the rat were studied by long-term extracellular microelectrode recording. Both anesthetics always suppressed the excitatory responses with simultaneous blocking or augmenting effects on the inhibitory responses of the reticular neurons. The blocking effects on the inhibitory responses were more frequently noticed during light anesthesia than during the deep stage.

Temperature-dependence of the action of nerve blocking agents and its relationship to membrane-buffer partition coefficients: thermodynamic implications for the site of action of local anaesthetics

The local anaesthetic action of a series of nerve blocking agents was examined at temperatures between 4 degrees C and 20 degrees C in isolated sciatic nerves from cold-adapted frogs. Cooling alone had little effect on the amplitude of the action potential but the conduction velocity was decreased and the duration increased. Cooling had little effect on the local anaesthetic action of the short chain alkanols but that of the long chain alkanols, benzyl alcohol, benzocaine and pentobarbitone was markedly enhanced. The partition of both short and long chain alkanols and of pentobarbitone into a liposome suspension of similar composition to axonal membrane lipids was reduced by cooling from 40 degrees C to 4 degrees C. The results are not compatible with the lipid hypotheses of anaesthetic action. The implications for the nature of the site of action are discussed.

Anaesthesia: the role of adrenergic mechanisms

The beta-blocker propranolol administered intraperitoneally to rats prior to the barbiturate anaesthetic thiopentone caused a dose-dependent increase in anaesthesia duration. Sotalol, which only poorly crosses the blood-brain barrier, had no such effect, implying a central site of action. The selective beta 1-blockers, metoprolol and atenolol, did not alter thiopentone anaesthesia duration; implying that the effect of propranolol was mediated by a beta 2-receptor. The selective alpha 1-blocker prazocin increased thiopentone anaesthesia duration, while the alpha 1-agonist ST 587 decreased it. Since the alpha 1-agonist methoxamine, which only poorly crosses the blood-brain barrier, was ineffective, a central site of action is indicated. The alpha 2-agonist clonidine markedly increased thiopentone-anaesthesia duration, while the alpha 2-blocker yohimbine, shortened the duration. These effects were shown to be noradrenergic since they were blocked by prior depletion of brain noradrenaline using 6-hydroxydopamine. A model is proposed in which drug-induced alterations in the firing of locus coeruleus cells, or drug-induced changes in the postsynaptic effect of released noradrenaline, may be responsible for modulation of cortical arousal, wakefulness and the processing of sensory stimuli; thus affecting the duration of barbiturate anaesthesia.

Dissimilar influences of some injectable anaesthetics on the responses of reticulo-spinal neurones to inhibitory transmitters in the lamprey

1 Intracellular recordings were made from identified bulbar reticulo-spinal neurones in the medulla of lamprey ammocoetes. Responses to iontophoretically applied inhibitory transmitters were measured as changes in membrane potential and input resistance. 2 Dose-dependent alterations in the responses to gamma-aminobutyric acid (GABA) and glycine during bath application of injectable anaesthetic drugs were measured; the compounds used were pentobarbitone, ketamine, metomidate and the steroid mixture alphaxalone/alphadolone (Saffan). 3 GABA responses were potentiated by pentobarbitone (1-3 X 10(-4) M) and prolonged by ketamine (3.7 X 10(-5) M); but depressed by high concentrations (10(-3) M) of all drugs, as well as by anaesthetic concentrations of alphaxalone (1-3 X 10(-5) M). 4 Glycine responses were depressed by alphaxalone (1-3 X 10(-5) M) and by supra-anaesthetic concentrations of ketamine (3.7 X 10(-4) M) and metomidate (1.8 X 10(-3) M). No drug potentiated the glycine responses. 5 In the absence of an effect common to the 4 anaesthetics, it is concluded that neither potentiation nor inhibition of all GABA or glycine responses is an essential feature of anaesthesia. However, effects comparable to those described here may contribute to the overall clinical picture during anaesthesia of higher vertebrates. The findings do not support the notion that all anaesthetic agents act on biological membranes by a single mechanism.

The effect of anaesthetic agents on cerebral cortical responses in the rat

1. In rats, surgically anaesthetized with Urethane, an increase in the depth of anaesthesia upon administration of ethyl carbamate (Urethane), pentobarbitone sodium (Nembutal), thiopentone sodium (Intraval), althesin, ketamine, trichloroethylene, halothane, methoxyflurane, diethyl ether, ethyl-vinyl ether, cyclopropane, enflurane or chloroform resulted in a dose-dependent increase in the latency, the decrease in the amplitudes of the initial positive and negative components of the short latency cortical response to electrical stimuli applied to the forepaw. 2. The same changes were seen when starting from initially unanaesthetized rats and anaesthetizing them with Urethane. 3. With all the inhalational agents used these changes lasted for as long as the administration except with nitrous oxide where the changes in the cortical response were transient. 4. The tranquilizing agents diazepam, chlordiazepoxide, and haloperidol showed no such action. Chloral hydrate and chlorpromazine, on the other hand, produced moderate changes in the evoked cortical response similar to those seen with the other anaesthetic agents used.