Stochastic resonance in the system of membrane ion channels
Igor Vodyanoy
(NIH, Bethesda; ONR Europe), London, UK
Biological amplifiers are unique in their ability to detect small signals in a noisy environment. As was shown lately, many nonlinear systems can use noise to enhance their performance, and this phenomenon, called stochastic resonance, may underline the extraordinary ability of some biological systems to detect and amplify small signals. Previous work has demonstrated stochastic resonance in complex systems of biological transducers and neural signal pathways, but the possibility that it could occur at the membrane level has remained open. Here we report the observation of noise-enhanced electrical signal transfer in a simple system of voltage-dependent ion channels formed by the peptide alamethicin in a lipid bilayer footnote S.M.Bezrukov and I.Vodyanoy, Nature (London), November 1995. Channels are expressed in a stochastic manner as "current bursts" rising from the background, and their formation is highly voltage-sensitive. An average alamethicin- induced conductance increases e-fold every 4 or 5 mV, depending on bilayer lipid composition. Alamethicin channel transitions between nonconducting and conducting aggregates can be described by a quasi-bistable energy diagram, where the probability distribution along the reaction coordinate is sensitive to the transmembrane voltage mostly at the level of the transition between two main energy wells. To study the interaction between external noise and signal transfer, we measure amplitude of output signal and the signal-to-noise ratio at the system output as a function of external noise intensity. We show that a hundred-fold increase in signal transduction induced by external noise is accompanied by a growth in the output signal-to-noise ratio.