Investigation of the effects of anesthetic compounds on lipid bilayer organization

The mechanism of action of general anesthetics has been under investigation since 1899 when Meyer and Overton independently developed what has come to be known as the Meyer - Overton hypothesis. The Meyer-Overton hypothesis states that general anesthetic potency is proportional to the solubility of the anesthetic in a hydrophobic solvent, (olive oil) which was in contact with hydrophilic region (water) and the oil - water interface. The present thinking in this field has evolved into two hypotheses, both of which involve the cellular plasma membrane. The Modern Lipid Hypothesis holds that the anesthetic interacts with the lipid membrane directly, and The Membrane Protein Hypothesis of General Anesthetic Action hypothesis states that anesthetics interact with transmembrane protein(s) embedded in the membrane. The investigations presented in this thesis focused on understanding interactions between model lipid bilayer structures and three general anesthetics (pentobarbital, isoflurane and halothane) to determine the effect(s) of direct interactions between bilayers and anesthetics. The synthetic vesicles used in this work were intended to mimic qualitatively the neuronal membrane in lipid composition, with 63 - mole % 1, 2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), 33 - mole % cholesterol and 4 - mole % 1,2-dihexadecanoyl-sn-glycerol-3-phosphate (sodium salt) (DPPA). For spectroscopic investigations, 0.5 - mole % perylene was added to the vesicle mixture prior to vesicle formation. Vesicles were formed by extrusion from mixtures containing phospholipid, sterol and chromophore species in predetermined amounts. The formation of vesicles from mixtures of these components is driven entropically to produce structures where interactions between polar solution and nonpolar organic constituents are minimized. Fluorescence anisotropy decay measurements were used to investigate the local environment formed by the lipid bilayer acyl chain region in the vesicles. The acyl chain region of the vesicles was seen to undergo a change in the extent of the organization observed with the control when exposed to selected anesthetics. These changes were observed with the change in acyl chain viscosity. The change in local organization sensed by perylene rotational diffusion was seen to be similar for all anesthetics used despite the absence of structural similarities among the anesthetics. In all cases, a change in the organization of the lipid bilayer was seen for anesthetic concentrations of ca. 4 mM. In all cases, the change in lipid organization was observed to be an increase in the rate of rotational motion of perylene about the axis perpendicular to its pi-system plane. Rotation of the chromophore about its in-plane axis was affected much less by exposure to anesthetics, consistent with limited dilation of the bilayer structure. These findings support the Modern Lipid Hypothesis, which asserts that anesthetics interact directly with the lipid membrane however we cannot make a claim about whether or not they interact with transmembrane proteins imbedded within the membrane since proteins were not incorporated in the membrane in this study. This finding does not exclude the possibility that the anesthetics may also act on transmembrane proteins. Interactions between anesthetics and the plasma membrane gives rise to structural changes in the membrane that lead to increased motional freedom for a chromophore imbedded within the membrane acyl chain region. This change in structure implies increased disorder in the lipid acyl chain region, a factor that may have a significant effect on the ability of transmembrane proteins to fold into their functional forms. Any resulting changes in the function of transmembrane proteins in the membrane due to induced changes in the membrane from anesthetic interactions may account for data suggesting increased or decreased neurotransmitter release upon anesthetic administration in model organisms. There remains, however, much to be learned about the mode of anesthetic action in the CNS, but this body of data has provided some insight into the molecular-scale interactions between anesthetics and lipid bilayer structures.