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

Propofol selectively alters GluA1 AMPA receptor phosphorylation in the hippocampus but not prefrontal cortex in young and aged mice

Propofol is a commonly used general anesthetic agent which has been previously shown to enhance the inhibitory GABAergic transmission in the central nervous system. In addition to the GABAergic element, the excitatory transmission may be another central molecular site impacted by propofol. Increasing evidence implies that the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor represents an excitatory amino acid receptor subtype subjected to the regulation by propofol. Indeed, in this study, we found that a single injection of propofol at an anesthetic dose increased AMPA receptor GluA1 subunit phosphorylation in young (2-3 months old) and aged (20-21 months old) mice in vivo. Propofol caused an increase in GluA1 phosphorylation in the hippocampus but not in the prefrontal cortex. The propofol effect was also site-selective as the drug elevated GluA1 phosphorylation at serine 831 (S831) but not serine 845. Interestingly, while propofol induced a moderate and transient increase in S831 phosphorylation in young mice, the drug caused a substantial and sustained S831 phosphorylation in aged animals. Total GluA1 abundance remained stable in the hippocampus and prefrontal cortex in both young and aged mice in response to propofol. These results provide evidence supporting the sensitivity of GluA1 AMPA receptors to propofol. A single dose of propofol was able to upregulate GluA1 phosphorylation in the confined hippocampus in an age-dependent manner.

A comparison of two different ketamine and diazepam combinations with an alphaxalone and medetomidine combination for induction of anaesthesia in sheep

AIMS

To investigate the perceived adverse effects of a particular batch of ketamine during induction of anaesthesia in sheep and to assess if any adverse effects would make intubation more difficult for the veterinary students.

METHODS

Thirty adult sheep (mean bodyweight 74.5 (SD 9.4) kg) were randomly assigned to one of six groups of five sheep. Sheep in Groups A and B received I/V 0.5 mg/kg diazepam and 10 mg/kg ketamine (Ketamine Injection; Parnell Laboratories NZ Ltd, of the suspect batch); those in Groups C and D received I/V 0.5 mg/kg diazepam and 10 mg/kg ketamine (Ketalar; Hospira NZ Ltd.), and those in Groups E and F received I/V 2 μg/kg medetomidine and 2 mg/kg alphaxalone. In Groups A, C and E, intubation was by an experienced anaesthetist, and in Groups B, D and F intubation was by a veterinary student. Time from injection to successful intubation, the ease of intubation, saturation of haemoglobin with oxygen (SpO₂) and partial pressure of oxygen in arterial blood (PaO₂) were measured before the sheep were connected to an anaesthetic machine and allowed to breath oxygen. Times to extubation, holding its head up and standing, maximum and minimum heart rates, respiratory rates, maximal end tidal CO₂, and the quality of recovery were then recorded.

RESULTS

There were no measurable differences in outcomes between sheep in Groups A and B compared with C and D. Time to intubation was slightly shorter for the experienced anaesthetist than the student, but the difference was not significant. The sheep in Groups E and F took less time to recover than those in Groups A-D (p<0.05), but there were no significant differences between the groups in either the ease of induction or quality of recovery. Most sheep in Groups E and F showed minor excitatory effects, mainly at induction, which did not interfere with induction. Respiratory rates were lower in Groups E and F than Groups A-D (p<0.01), but SpO₂ was higher in Groups E and F than A and B (p<0.05).

CONCLUSIONS

The clinical impression that the batch of Parnell ketamine produced unexpected effects was shown to be incorrect. All the combinations produced anaesthesia that allowed intubation by the veterinary student.

CLINICAL RELEVANCE

All the drug combinations produced satisfactory anaesthesia in sheep, but the alphaxaloneand medetomidine combination resulted in faster recovery.

Development of pharmacophoric maps for cardiovascular depression by intravenous anaesthetic agents: comparison with maps for immobilizing activity

BACKGROUND

The molecular basis of the cardiovascular effects of i.v. anaesthetics was investigated using comparative molecular field analysis (CoMFA).

METHODS

The cardiovascular effects, measured as changes in mean arterial pressure (MAP), compared with awake values of continuous infusions of 13 structurally diverse i.v. anaesthetics were compared at EC(50) plasma concentrations, and by determination of plasma-free drug concentrations associated with a 20% decrease in MAP (dMAP(20)). Data were obtained both from the literature and from unpublished data of the author. The results were fitted to a CoMFA activity model using field-fit minimization techniques to maximize similarities in molecular bulk and electrostatic potential to the lead compound, eltanolone.

RESULTS

The final model for cardiovascular depression based on free drug concentrations associated with dMAP(20) explained 95.8% of the variance in observed activities, with a cross-validated q(2) of 0.824 (n=12). A second model based on change in MAP at EC(50) plasma concentrations explained 98.3% of the variance in arterial pressure, but performed poorly at cross-validation (q(2) 0.526). The comparative model for immobilizing potency had an r(2) value of 0.987 and q(2) 0.823. Comparison of pharmacophoric maps showed several key electrostatic and steric regions common to both models when isocontours were constructed linking lattice grid points, making the greatest 40% contributions (87.57% for electrostatic fields and 86.16% for steric fields).

CONCLUSIONS

Comparison of activity models for cardiovascular depression and immobilizing potency for i.v. anaesthetics shows significant commonality, suggesting that it may not be possible to separate those molecular features associated with each of these effects.

What makes a molecule an anaesthetic? Studies on the mechanisms of anaesthesia using a physicochemical approach

Recent studies of mechanisms of anaesthesia have been mainly 'target orientated', investigating the activity of both volatile and i.v. agents at putative sites of action. An alternative approach is one that is 'ligand orientated', focusing on the properties of molecules that define their immobilizing ability and secondly define their potency. The use of conventional descriptors (such as non-polar solubility or the octanol-water partition coefficient [Log P]) are limited in their utility as predictors of potency as they represent three-dimensional molecular properties as a one-dimensional parameter. Using different computer-based molecular modelling methods (molecular similarity studies and comparative molecular field analysis [CoMFA]), we have identified the molecular bases of the activity of structurally diverse anaesthetics, such that they can be described as a single model based on the spatial distribution of molecular bulk and electrostatic potential. The same approach can also be used to model other properties of anaesthetic agents, such as cardiovascular depression. The present data suggest that, for the i.v. agents, it may be difficult to separate immobilizing (anaesthetic) activity and cardiovascular depression within a single molecule.

Propofol causes neurite retraction in neurones

BACKGROUND

The mechanism by which anaesthetic agents produce general anaesthesia is not yet fully understood. Retraction of neurites is an important function of individual neurones and neural plexuses during normal and pathological conditions, and it has been shown that such a retraction pathway exists in developing and mature neurones. We hypothesized that propofol decreases neuronal activity by causing retraction of neuronal neurites.

METHODS

Primary cultures of rat cortical neurones were exposed in concentration- and time-response experiments to 0.02, 0.2, 2, and 20 microM propofol or lipid vehicle. Neurones were pretreated with the GABA(A) receptor (GABA(A)R) antagonist, bicuculline, the myosin II ATPase activity inhibitor, blebbistatin, and the F-actin stabilizing agent, phalloidin, followed by administration of propofol (20 microM). Changes in neurite retraction were evaluated using time-lapse light microscopy.

RESULTS

Propofol caused a concentration- and time-dependent reversible retraction of cultured cortical neurone neurites. Bicuculline, blebbistatin, and phalloidin completely inhibited propofol-induced neurite retraction. Images of retracted neurites were characterized by a retraction bulb and a thin trailing membrane remnant.

CONCLUSIONS

Cultured cortical rat neurones retract their neurites after exposure to propofol in a concentration- and time-dependent manner. This retraction is GABA(A)R mediated, reversible, and dependent on actin and myosin II. Furthermore, the concentrations and times to full retraction and recovery correspond to those observed during propofol anaesthesia.

Modulation of AMPA receptor GluR1 subunit phosphorylation in neurons by the intravenous anaesthetic propofol

BACKGROUND

The ionotropic glutamate receptor is a potential molecular site in the central nervous system that general anaesthetics may interact with to produce some of their biological actions. Protein phosphorylation has been well documented to occur in the intracellular C-terminal domain of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors, which represents a pivotal mechanism for the post-translational modulation of AMPA receptor functions. In this study, we investigated a possible influence of an i.v. anaesthetic agent propofol on the phosphorylation of AMPA receptor GluR1 subunits in cultured neurons.

METHODS

The effect of propofol on phosphorylation of GluR1 subunits at serine 831 and 845 was assayed in cultured rat striatal and cortical neurons by western blot with phospho- and site-specific antibodies.

RESULTS

Propofol consistently elevated phosphorylation of GluR1 subunits at the C-terminal serine 845 site in both striatal and cortical neurons. The elevation in phosphorylation was concentration-dependent and started at a low concentration (3 microM). This increase in serine 845 phosphorylation was rapid and sustained during the entire course of propofol exposure. In contrast to serine 845, phosphorylation of GluR1 at serine 831 was not altered by propofol in striatal and cortical neurons. Total GluR1 abundance remained unchanged in response to propofol incubation.

CONCLUSIONS

These data indicate that propofol possesses the ability to upregulate AMPA receptor GluR1 subunit phosphorylation at a specific serine 845 site in neurons and provide evidence supporting the AMPA receptor as a molecular target for general anaesthetics.

A comparison of pharmacokinetic/pharmacodynamic versus mass-balance measurement of brain concentrations of intravenous anesthetics in sheep

BACKGROUND

There are two recognized methods of estimating the brain concentrations of IV anesthetic drugs: (i) use of pharmacokinetic/pharmacodynamic (PK/PD) modeling of drug effect, from arterial concentrations and electroencephalogram changes, and (ii) direct measurement of the uptake of drug in the brain, by simultaneously measuring arterial and jugular concentrations and cerebral blood flow (mass-balance method). These two methods have not been directly compared. Because an accurate estimate of the time taken for transfer of anesthetic drug from arterial blood to its effect-compartment in the brain is critical for accurate effect-compartment dosing in IV anesthesia, we compared the PK/PD and mass-balance methods for propofol, methohexital, and ketamine in a sheep model.

METHODS

After instrumentation with arterial and sagittal-sinus cannulae, electrocorticogram, and sagittal sinus Doppler flow measurement seven adult sheep were given a random sequence of short anesthetic infusions with methohexital, ketamine, and propofol. Multiple blood samples were taken for measurement of the time course of the drug concentrations, and the electrocorticogram processed (approximate entropy, for propofol and methohexital and percentage high frequency time, for ketamine) to numerically quantify drug effect.

RESULTS

Using the PK/PD method the t(1/2)K(eo) was 2.0 +/- 0.4 min for ketamine, 2.7 +/- 1.1 min for propofol, and was significantly shorter (0.3 +/- 0.1 min) for methohexital. PK/PD and the mass-balance methods did not differ in the times to peak effect.

CONCLUSIONS

Both methods of calculating the delay in transfer of drug from arterial blood to brain give similar values. Methohexital crosses into the brain much faster than either propofol or ketamine.

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.

Determinants of volatile general anesthetic potency: a preliminary three-dimensional pharmacophore for halogenated anesthetics

We investigated the molecular basis for the immobilizing activity of halogenated volatile anesthetics using comparative molecular field analysis. In vivo potency data (expressed as minimum alveolar concentrations) for 69 structurally diverse anesthetics were obtained from the literature. The drugs were randomly divided into a training set (n = 52) used to derive the activity model and a test set (n = 17) used to independently assess the model's predictive power. The anesthetic structures were aligned so as to maximize their similarity in molecular shape and electrostatic potential to the most potent drug in the group, CF2H-(CF2)3-CH2OH. The conformers and alignments of the anesthetics with maximum similarity (calculated as Carbo indices) were retained and used to derive the comparative molecular field analysis models. The final model explained 94.2% of the variance in the observed activities of the training set compounds. The model showed good predictive capability for both the training set (cross-validated r2 = 0.705) and randomly excluded test set anesthetics (r2 = 0.837). Three-dimensional pharmacophoric maps were derived to identify the spatial distribution of key areas where steric and electrostatic interactions are important in determining immobilizing activity of the halogenated drugs and were compared with our previously published maps obtained for nonhalogenated volatile anesthetics.

Derivation of preliminary three-dimensional pharmacophoric maps for chemically diverse intravenous general anaesthetics † †This work was supported in part by a project grant from the British Journal of Anaesthesia. It was presented in part at the Anaesthetic Research Society meeting, Cardiff, July 2002 and published in abstract form in the Br J Anaesth 2002; 89: 672–673P

BACKGROUND

The molecular basis of i.v. general anaesthetic activity was investigated using comparative molecular field analysis (CoMFA).

METHODS

The free plasma concentrations that abolish movement to a noxious stimulus for 14 structurally diverse i.v. anaesthetics were obtained from the literature. The compounds were randomly divided into a training set (n=10) to derive the activity model, and a separate test set (n=4) used to assess its predictive capability. The anaesthetic structures were aligned so as to maximize their similarities in molecular shape and electrostatic potential to conformers of the most active agent in the group, eltanolone. The conformers and alignments that showed the maximum similarity (calculated using combined Carbo indices) were retained, and used to derive the CoMFA models.

RESULTS

The final model explained 94.0% of the variance in the observed activities of the training set (n=10, P<0.0001) and was a good predictor of test set activity (n=4, r(2)=0.799). In contrast, a model based on non-polar solubility (LogP) explained only 78.3% of the variance in the observed activities of the training set (n=10, P=0.0007) and was a poor predictor for the test set (n=4, r(2)=0.272). Further analysis of the CoMFA results identified the spatial distribution of key areas where steric and electrostatic interactions are important in determining the activity of the 14 anaesthetics considered.

CONCLUSIONS

A single activity model can be formulated for i.v. general anaesthetics and preliminary pharmacophoric maps derived, which describe the molecular basis of their in vivo potency.