Inhibition of Membrane Effects of General Anesthetic Propofol by Benzodiazepine Inverse Agonist Tetrahydro-β-carboline

Propofol induces general anesthesia through binding to GABAA receptors. It is inhibited by interacting with benzodiazepine inverse agonist tetrahydro-β-carbolines present in the human body. Aside from acting on receptor proteins, both propofol and tetrahydro-β-carbolines act on membrane lipid bilayers. In this study, the hypothetical inhibitory relation on membrane biophysical modification was verified between propofol and tetrahydro-β-carbolines. Lipid bilayer membranes were prepared with phospholipids and cholesterol. The membrane preparations were reacted with propofol, 1-methyl-1,2,3,4-tetrahydro-β-carboline and its metabolites 6-hydroxy-1-methyl-1,2,3,4-tetrahydro-β-carboline and 7-hydroxy-1-methyl-1,2,3,4-tetrahydro-β-carboline, followed by measuring fluorescence polarization to determine their induced changes in membrane fluidity. In the pilot experiments using model membranes prepared with 100 mol% 1,2-dipalmitoyl phosphatidylcholine, 1-methyl-1,2,3,4-tetrahydro-β-carboline showed a concentration-dependent biphasic effect to increase membrane fluidity at high micromolar concentrations but decrease at <50 μM. In the lower concentration range of 0.01-0.5 μM, 1-methyl-1,2,3,4-tetrahydro-β-carboline was effective in rigidifying the membranes, whereas neither 6-hydroxyl nor 7-hydroxyl metabolite showed any membrane effects. Based on these results, biomimetic membranes consisting of 60 mol% 1-palmitoyl-2-oleoylphosphatidylcholine and 40 mol% cholesterol were pretreated with 1-methyl-1,2,3,4-tetrahydro-β-carboline of physiologically presumable concentrations and then reacted with propofol of clinically relevant concentrations. Propofol fluidized the membranes at 0.125-1 μM. However, the membrane-fluidizing effects of 0.125 and 0.25 μM propofol were inhibited by 0.01-5 nM 1-methyl-1,2,3,4-tetrahydro-β-carboline. In addition to their interaction at a receptor level, propofol and 1-methyl-1,2,3,4-tetrahydro-β-carboline show the inhibitory relation on membrane fluidity changes. Such a relation may be hypothetically associated with the anesthetic tolerance.