Interaction between propofol and benzodiazepine site ligands to potentiate GABAA receptor activation in human recombinant receptors expressed in Xenopus oocytes

Maitra, Rajatavo (1995) Interaction between propofol and benzodiazepine site ligands to potentiate GABAA receptor activation in human recombinant receptors expressed in Xenopus oocytes. Masters thesis, Memorial University of Newfoundland.

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Abstract

Potentiation of inhibitory synaptic transmission mediated by γ-aminobutyric acid (GABA) acting at the GABA type A (GABAA) receptor is increasingly regarded as a primary site of action for anesthetic drugs. GABAA receptors across different brain regions can have a different protein subunit composition, which influences the pharmacological sensitivity of the receptor complex. The intravenous general anesthetic propofol is frequently combined with benzodiazepines, and there are reports of synergism in the production of anesthesia. However, the nature of this interaction is unknown. The purpose of this study was therefore to examine the interaction between propofol and the benzodiazepine receptor agonist flurazepam at the receptor level. In addition, preliminary studies on the influence of subunit composition on the potentiation of GABAA receptor function by propofol have been carried out. Two-electrode voltage-clamp recording of GABA-activated membrane currents were performed in Xenopus laevis oocytes expressing human recombinant GABAA receptors. -- Propofol was found to produce similar potentiation of GABA (3 μM) receptor-activated currents in receptors composed of α₁β₂γ₂L and α₂β₂γ₂L subunits. Propofol appears to increase the affinity of GABAA receptors for GABA without changing the maximal current. In contrast, propofol potentiation of GABA currents was influenced by the γ₂L subunit. Receptors composed of α₂β₂ subunits were potentiated to a greater extent than receptors composed of α₂β₂γ₂L subunits. The time course of GABAA receptor desensitization can be described by a double exponential function. Propofol was found to reduce receptor desensitization by significantly prolonging the slow time constant of current decay, resulting in a large increase in the steady-state current activated by a high GABA concentration. -- The interaction between propofol and flurazepam was examined in receptors composed of α₂β₂γ₂L (x = 1 or 2) subunits. Currents activated by GABA (3-30 μM) were potentiated by low concentrations of propofol (0.5-10 μM) plus flurazepam (0.25 and 0.5 μM) to levels which were significantly greater than expected from an additive response. At higher concentrations of propofol (20 μM) or flurazepam (1 μM) the potentiation of GABA receptor-activated currents was not different than expected from an additive response. The benzodiazepine receptor partial inverse agonist RO15-4513 was found to abolish the flurazepam potentiation of GABA-activated membrane currents without affecting the propofol response. Zopiclone, a cyclopyrrolone derivative which is classified as a full agonist at benzodiazepine receptors, did not produce the same greater than additive interaction with propofol. In fact, zopiclone (100 nM) combined with propofol (1-10 μM) produced an enhancement of the GABA current which was significantly less than the expected additive response. -- These results suggest that propofol has multiple effects on GABAA receptor function: An increase in the affinity of the receptor for GABA plus a decrease in receptor desensitization at higher GABA concentrations. The type of α subunit contained in the receptor does not appear to have a major influence on the ability of propofol to modulate GABAA receptor function. However, propofol was significantly less effective in the presence of a γ₂L subunit. Thus similarly to many other modulators, potentiation of GABAA receptor function by propofol does exhibit subunit dependence. Synergism observed clinically by the combination of propofol or propofol plus benzodiazepines can be explained by an interaction at the receptor level. In addition to increasing the affinity for GABA, flurazepam increased the affinity of the receptor for propofol, resulting in a greater than expected potentiation of GABAA receptor function. This interaction between propofol and benzodiazepines was not due to simple potentiation of GABAA receptor activity, since zopiclone failed to produce the supra-additive effect when combined with propofol.

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