Pharmacological mechanisms of local anesthetics are still unknown. To clarify unknown mechanisms of local anesthetics, we analyzed physical property changes of artificial bio-mimetic membrane “liposome” induced by addition of local anesthetics. We observed for (a) thermo-response of liposomes. (b) effect of local anesthetic on phase separation on liposomes. (c) fluidity change of membrane. According to results, direct interactions between local anesthetics and physical property changes of bio-mimetic membrane. These results indicated relation between membrane interactions and pharmacological action of local anesthetics.
Local anesthetics (LAs) are essential drugs for modern medicine. Although they are often used as painkiller, the mechanisms how they suppress the pain are still unclear.
LAs are known to target voltage-dependent sodium channels which located on cellular membrane. Until recently, the mechanism how LAs shield pain signals was believed that LAs interact with the channel protein directly. However, because of the relation between hydrophobicity and strength of LA, the indirect effect on channel proteins through a change in physical properties of the cellular membrane had been suggested[1].
Cell membranes which are composed of lipid bilayer structure are known to relate with signal transductions via membrane dynamics such as vesicular formation. Moreover, “lipid raft model” was proposed by Simon and Ikonen in 1997 [2]. Lipid raft is the region where the large amount of saturated lipids and cholesterols are included and some functional molecules such as ion channels and membrane receptors can be localized, and they are speculated as a platform for signal transduction.
In order to reveal the mechanism of raft formation, “liposome” which is mainly composed of phospholipids is used as a model of biomembrane system. Liposome has similar structure and component to actual biomembranes, so we can directly observe similar phenomena on liposome with optical microscopes. Especially, raft formation can be regarded as the phase separation between saturated and cholesterol-rich and unsaturated lipid-rich phases in model membrane system.[3]
An objective of this thesis is to observe physical property changes of liposomes that are derived from interactions between lipid membranes and LAs, and discuss relation with physiological action of LAs. First, to confirm the direct interaction between phospholipid membranes and LAs, we observe the membrane fluctuation and analyze the thermo-responses of liposomes. Next, the phase separation in membranes containing LAs is observed and we discuss how LAs affect on the domain formation. Finally, the membrane fluidity change by addition of LAs is evaluated using Laurdan which is sensitive to the membrane fluidity. Based on these experimental results, we discuss how LAs interact with lipid membranes.

Author: Ko Sugahara, Naofumi Shimokawa, Masahiro Takagi