Evidence for long-term potentiation in lipid membranes.

Abstract

Biological supramolecular assemblies, such as phospholipid bilayer membranes, have been used to demonstrate signal processing via short-term synaptic plasticity (STP) in the form of paired pulse facilitation (PPF) and depression (PPD), thus showing that model membranes have the potential to emulate the brain's efficiency and flexible cognitive capabilities. However, STP memory states in lipid bilayers are volatile and cannot be stored or accessed over relevant periods of time, a central requirement for learning. Using droplet interface bilayers (DIBs) composed of lipids and hexadecane, we apply an electrical stimulation protocol featuring repetitive voltage cycling in the form of a sinusoidal waveform. We show that DIBs displaying memcapacitance can also exhibit long-term synaptic plasticity in the form of long-term potentiation (LTP) associated with capacitive energy storage in the phospholipid bilayer. The time scales for the physical changes associated with LTP range between minutes to hours and are substantially longer than previous STP studies, where stored energy dissipated after only a few seconds. While STP behavior is the result of changes in bilayer geometry associated with reversible changes in bilayer area and thickness, LTP is the result of molecular and structural changes due to the zwitterionic lipid headgroups and the dielectric properties of the lipid bilayer that result from the buildup of an increasingly asymmetric charge distribution between the bilayer interfaces.